A web service returns a huge XML and I need to access deeply nested fields of it. For example:
return wsObject.getFoo().getBar().getBaz().getInt()
The problem is that getFoo(), getBar(), getBaz() may all return null.
However, if I check for null in all cases, the code becomes very verbose and hard to read. Moreover, I may miss the checks for some of the fields.
if (wsObject.getFoo() == null) return -1;
if (wsObject.getFoo().getBar() == null) return -1;
// maybe also do something with wsObject.getFoo().getBar()
if (wsObject.getFoo().getBar().getBaz() == null) return -1;
return wsObject.getFoo().getBar().getBaz().getInt();
Is it acceptable to write
try {
return wsObject.getFoo().getBar().getBaz().getInt();
} catch (NullPointerException ignored) {
return -1;
}
or would that be considered an antipattern?
Catching NullPointerException is a really problematic thing to do since they can happen almost anywhere. It's very easy to get one from a bug, catch it by accident and continue as if everything is normal, thus hiding a real problem. It's so tricky to deal with so it's best to avoid altogether. (For example, think about auto-unboxing of a null Integer.)
I suggest that you use the Optional class instead. This is often the best approach when you want to work with values that are either present or absent.
Using that you could write your code like this:
public Optional<Integer> m(Ws wsObject) {
return Optional.ofNullable(wsObject.getFoo()) // Here you get Optional.empty() if the Foo is null
.map(f -> f.getBar()) // Here you transform the optional or get empty if the Bar is null
.map(b -> b.getBaz())
.map(b -> b.getInt());
// Add this if you want to return null instead of an empty optional if any is null
// .orElse(null);
// Or this if you want to throw an exception instead
// .orElseThrow(SomeApplicationException::new);
}
Why optional?
Using Optionals instead of null for values that might be absent makes that fact very visible and clear to readers, and the type system will make sure you don't accidentally forget about it.
You also get access to methods for working with such values more conveniently, like map and orElse.
Is absence valid or error?
But also think about if it is a valid result for the intermediate methods to return null or if that is a sign of an error. If it is always an error then it's probably better throw an exception than to return a special value, or for the intermediate methods themselves to throw an exception.
Maybe more optionals?
If on the other hand absent values from the intermediate methods are valid, maybe you can switch to Optionals for them also?
Then you could use them like this:
public Optional<Integer> mo(Ws wsObject) {
return wsObject.getFoo()
.flatMap(f -> f.getBar())
.flatMap(b -> b.getBaz())
.flatMap(b -> b.getInt());
}
Why not optional?
The only reason I can think of for not using Optional is if this is in a really performance critical part of the code, and if garbage collection overhead turns out to be a problem. This is because a few Optional objects are allocated each time the code is executed, and the VM might not be able to optimize those away. In that case your original if-tests might be better.
I suggest considering Objects.requireNonNull(T obj, String message). You might build chains with a detailed message for each exception, like
requireNonNull(requireNonNull(requireNonNull(
wsObject, "wsObject is null")
.getFoo(), "getFoo() is null")
.getBar(), "getBar() is null");
I would suggest you not to use special return-values, like -1. That's not a Java style. Java has designed the mechanism of exceptions to avoid this old-fashioned way which came from the C language.
Throwing NullPointerException is not the best option too. You could provide your own exception (making it checked to guarantee that it will be handled by a user or unchecked to process it in an easier way) or use a specific exception from XML parser you are using.
Assuming the class structure is indeed out of our control, as seems to be the case, I think catching the NPE as suggested in the question is indeed a reasonable solution, unless performance is a major concern. One small improvement might be to wrap the throw/catch logic to avoid clutter:
static <T> T get(Supplier<T> supplier, T defaultValue) {
try {
return supplier.get();
} catch (NullPointerException e) {
return defaultValue;
}
}
Now you can simply do:
return get(() -> wsObject.getFoo().getBar().getBaz().getInt(), -1);
As already pointed out by Tom in the comment,
Following statement disobeys the Law of Demeter,
wsObject.getFoo().getBar().getBaz().getInt()
What you want is int and you can get it from Foo. Law of Demeter says, never talk to the strangers. For your case you can hide the actual implementation under the hood of Foo and Bar.
Now, you can create method in Foo to fetch int from Baz. Ultimately, Foo will have Bar and in Bar we can access Int without exposing Baz directly to Foo. So, null checks are probably divided to different classes and only required attributes will be shared among the classes.
My answer goes almost in the same line as #janki, but I would like to modify the code snippet slightly as below:
if (wsObject.getFoo() != null && wsObject.getFoo().getBar() != null && wsObject.getFoo().getBar().getBaz() != null)
return wsObject.getFoo().getBar().getBaz().getInt();
else
return something or throw exception;
You can add a null check for wsObject as well, if there's any chance of that object being null.
You say that some methods "may return null" but do not say in what circumstances they return null. You say you catch the NullPointerException but you do not say why you catch it. This lack of information suggests you do not have a clear understanding of what exceptions are for and why they are superior to the alternative.
Consider a class method that is meant to perform an action, but the method can not guarantee it will perform the action, because of circumstances beyond its control (which is in fact the case for all methods in Java). We call that method and it returns. The code that calls that method needs to know whether it was successful. How can it know? How can it be structured to cope with the two possibilities, of success or failure?
Using exceptions, we can write methods that have success as a post condition. If the method returns, it was successful. If it throws an exception, it had failed. This is a big win for clarity. We can write code that clearly processes the normal, success case, and move all the error handling code into catch clauses. It often transpires that the details of how or why a method was unsuccessful are not important to the caller, so the same catch clause can be used for handling several types of failure. And it often happens that a method does not need to catch exceptions at all, but can just allow them to propagate to its caller. Exceptions due to program bugs are in that latter class; few methods can react appropriately when there is a bug.
So, those methods that return null.
Does a null value indicate a bug in your code? If it does, you should not be catching the exception at all. And your code should not be trying to second guess itself. Just write what is clear and concise on the assumption that it will work. Is a chain of method calls clear and concise? Then just use them.
Does a null value indicate invalid input to your program? If it does, a NullPointerException is not an appropriate exception to throw, because conventionally it is reserved for indicating bugs. You probably want to throw a custom exception derived from IllegalArgumentException (if you want an unchecked exception) or IOException (if you want a checked exception). Is your program required to provide detailed syntax error messages when there is invalid input? If so, checking each method for a null return value then throwing an appropriate diagnostic exception is the only thing you can do. If your program need not provide detailed diagnostics, chaining the method calls together, catching any NullPointerException and then throwing your custom exception is clearest and most concise.
One of the answers claims that the chained method calls violate the Law of Demeter and thus are bad. That claim is mistaken.
When it comes to program design, there are not really any absolute rules about what is good and what is bad. There are only heuristics: rules that are right much (even almost all) of the time. Part of the skill of programming is knowing when it is OK to break those kinds of rules. So a terse assertion that "this is against rule X" is not really an answer at all. Is this one of the situations where the rule should be broken?
The Law of Demeter is really a rule about API or class interface design. When designing classes, it is useful to have a hierarchy of abstractions. You have low level classes that uses the language primitives to directly perform operations and represent objects in an abstraction that is higher level than the language primitives. You have medium level classes that delegate to the low level classes, and implement operations and representations at a higher level than the low level classes. You have high level classes that delegate to the medium level classes, and implement still higher level operations and abstractions. (I've talked about just three levels of abstraction here, but more are possible). This allows your code to express itself in terms of appropriate abstractions at each level, thereby hiding complexity. The rationale for the Law of Demeter is that if you have a chain of method calls, that suggests you have a high level class reaching in through a medium level class to deal directly with low level details, and therefore that your medium level class has not provided a medium-level abstract operation that the high level class needs. But it seems that is not the situation you have here: you did not design the classes in the chain of method calls, they are the result of some auto-generated XML serialization code (right?), and the chain of calls is not descending through an abstraction hierarchy because the des-serialized XML is all at the same level of the abstraction hierarchy (right?)?
As others have said, respecting the Law of Demeter is definitely part of the solution. Another part, wherever possible, is to change those chained methods so they cannot return null. You can avoid returning null by instead returning an empty String, an empty Collection, or some other dummy object that means or does whatever the caller would do with null.
To improve readability, you may want to use multiple variables, like
Foo theFoo;
Bar theBar;
Baz theBaz;
theFoo = wsObject.getFoo();
if ( theFoo == null ) {
// Exit.
}
theBar = theFoo.getBar();
if ( theBar == null ) {
// Exit.
}
theBaz = theBar.getBaz();
if ( theBaz == null ) {
// Exit.
}
return theBaz.getInt();
Don't catch NullPointerException. You don't know where it is coming from (I know it is not probable in your case but maybe something else threw it) and it is slow.
You want to access the specified field and for this every other field has to be not null. This is a perfect valid reason to check every field. I would probably check it in one if and then create a method for readability. As others pointed out already returning -1 is very oldschool but I don't know if you have a reason for it or not (e.g. talking to another system).
public int callService() {
...
if(isValid(wsObject)){
return wsObject.getFoo().getBar().getBaz().getInt();
}
return -1;
}
public boolean isValid(WsObject wsObject) {
if(wsObject.getFoo() != null &&
wsObject.getFoo().getBar() != null &&
wsObject.getFoo().getBar().getBaz() != null) {
return true;
}
return false;
}
Edit: It is debatable if it's disobeyes the Law Of Demeter since the WsObject is probably only a data structure (check https://stackoverflow.com/a/26021695/1528880).
If you don't want to refactor the code and you can use Java 8, it is possible to use Method references.
A simple demo first (excuse the static inner classes)
public class JavaApplication14
{
static class Baz
{
private final int _int;
public Baz(int value){ _int = value; }
public int getInt(){ return _int; }
}
static class Bar
{
private final Baz _baz;
public Bar(Baz baz){ _baz = baz; }
public Baz getBar(){ return _baz; }
}
static class Foo
{
private final Bar _bar;
public Foo(Bar bar){ _bar = bar; }
public Bar getBar(){ return _bar; }
}
static class WSObject
{
private final Foo _foo;
public WSObject(Foo foo){ _foo = foo; }
public Foo getFoo(){ return _foo; }
}
interface Getter<T, R>
{
R get(T value);
}
static class GetterResult<R>
{
public R result;
public int lastIndex;
}
/**
* #param args the command line arguments
*/
public static void main(String[] args)
{
WSObject wsObject = new WSObject(new Foo(new Bar(new Baz(241))));
WSObject wsObjectNull = new WSObject(new Foo(null));
GetterResult<Integer> intResult
= getterChain(wsObject, WSObject::getFoo, Foo::getBar, Bar::getBar, Baz::getInt);
GetterResult<Integer> intResult2
= getterChain(wsObjectNull, WSObject::getFoo, Foo::getBar, Bar::getBar, Baz::getInt);
System.out.println(intResult.result);
System.out.println(intResult.lastIndex);
System.out.println();
System.out.println(intResult2.result);
System.out.println(intResult2.lastIndex);
// TODO code application logic here
}
public static <R, V1, V2, V3, V4> GetterResult<R>
getterChain(V1 value, Getter<V1, V2> g1, Getter<V2, V3> g2, Getter<V3, V4> g3, Getter<V4, R> g4)
{
GetterResult result = new GetterResult<>();
Object tmp = value;
if (tmp == null)
return result;
tmp = g1.get((V1)tmp);
result.lastIndex++;
if (tmp == null)
return result;
tmp = g2.get((V2)tmp);
result.lastIndex++;
if (tmp == null)
return result;
tmp = g3.get((V3)tmp);
result.lastIndex++;
if (tmp == null)
return result;
tmp = g4.get((V4)tmp);
result.lastIndex++;
result.result = (R)tmp;
return result;
}
}
Output
241
4
null
2
The interface Getter is just a functional interface, you may use any equivalent.
GetterResult class, accessors stripped out for clarity, hold the result of the getter chain, if any, or the index of the last getter called.
The method getterChain is a simple, boilerplate piece of code, that can be generated automatically (or manually when needed).
I structured the code so that the repeating block is self evident.
This is not a perfect solution as you still need to define one overload of getterChain per number of getters.
I would refactor the code instead, but if can't and you find your self using long getter chains often you may consider building a class with the overloads that take from 2 to, say, 10, getters.
I'd like to add an answer which focus on the meaning of the error. Null exception in itself doesn't provide any meaning full error. So I'd advise to avoid dealing with them directly.
There is a thousands cases where your code can go wrong: cannot connect to database, IO Exception, Network error... If you deal with them one by one (like the null check here), it would be too much of a hassle.
In the code:
wsObject.getFoo().getBar().getBaz().getInt();
Even when you know which field is null, you have no idea about what goes wrong. Maybe Bar is null, but is it expected? Or is it a data error? Think about people who read your code
Like in xenteros's answer, I'd propose using custom unchecked exception. For example, in this situation: Foo can be null (valid data), but Bar and Baz should never be null (invalid data)
The code can be re-written:
void myFunction()
{
try
{
if (wsObject.getFoo() == null)
{
throw new FooNotExistException();
}
return wsObject.getFoo().getBar().getBaz().getInt();
}
catch (Exception ex)
{
log.error(ex.Message, ex); // Write log to track whatever exception happening
throw new OperationFailedException("The requested operation failed")
}
}
void Main()
{
try
{
myFunction();
}
catch(FooNotExistException)
{
// Show error: "Your foo does not exist, please check"
}
catch(OperationFailedException)
{
// Show error: "Operation failed, please contact our support"
}
}
NullPointerException is a run-time exception, so generally speaking is not recommended to catch it, but to avoid it.
You will have to catch the exception wherever you want to call the method (or it will propagate up the stack). Nevertheless, if in your case you can keep working with that result with value -1 and you are sure that it won't propagate because you are not using any of the "pieces" that may be null, then it seems right to me to catch it
Edit:
I agree with the later answer from #xenteros, it wil be better to launch your own exception instead returning -1 you can call it InvalidXMLException for instance.
Have been following this post since yesterday.
I have been commenting/voting the comments which says, catching NPE is bad. Here is why I have been doing that.
package com.todelete;
public class Test {
public static void main(String[] args) {
Address address = new Address();
address.setSomeCrap(null);
Person person = new Person();
person.setAddress(address);
long startTime = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
try {
System.out.println(person.getAddress().getSomeCrap().getCrap());
} catch (NullPointerException npe) {
}
}
long endTime = System.currentTimeMillis();
System.out.println((endTime - startTime) / 1000F);
long startTime1 = System.currentTimeMillis();
for (int i = 0; i < 1000000; i++) {
if (person != null) {
Address address1 = person.getAddress();
if (address1 != null) {
SomeCrap someCrap2 = address1.getSomeCrap();
if (someCrap2 != null) {
System.out.println(someCrap2.getCrap());
}
}
}
}
long endTime1 = System.currentTimeMillis();
System.out.println((endTime1 - startTime1) / 1000F);
}
}
public class Person {
private Address address;
public Address getAddress() {
return address;
}
public void setAddress(Address address) {
this.address = address;
}
}
package com.todelete;
public class Address {
private SomeCrap someCrap;
public SomeCrap getSomeCrap() {
return someCrap;
}
public void setSomeCrap(SomeCrap someCrap) {
this.someCrap = someCrap;
}
}
package com.todelete;
public class SomeCrap {
private String crap;
public String getCrap() {
return crap;
}
public void setCrap(String crap) {
this.crap = crap;
}
}
Output
3.216
0.002
I see a clear winner here. Having if checks is way too less expensive than catch an exception. I have seen that Java-8 way of doing. Considering that 70% of the current applications still run on Java-7 I am adding this answer.
Bottom Line For any mission critical applications, handling NPE is costly.
If efficiency is an issue then the 'catch' option should be considered.
If 'catch' cannot be used because it would propagate (as mentioned by 'SCouto') then use local variables to avoid multiple calls to methods getFoo(), getBar() and getBaz().
It's worth considering to create your own Exception. Let's call it MyOperationFailedException. You can throw it instead returning a value. The result will be the same - you'll quit the function, but you won't return hard-coded value -1 which is Java anti-pattern. In Java we use Exceptions.
try {
return wsObject.getFoo().getBar().getBaz().getInt();
} catch (NullPointerException ignored) {
throw new MyOperationFailedException();
}
EDIT:
According to the discussion in comments let me add something to my previous thoughts. In this code there are two possibilities. One is that you accept null and the other one is, that it is an error.
If it's an error and it occurs, You can debug your code using other structures for debugging purposes when breakpoints aren't enough.
If it's acceptable, you don't care about where this null appeared. If you do, you definitely shouldn't chain those requests.
The method you have is lengthy, but very readable. If I were a new developer coming to your code base I could see what you were doing fairly quickly. Most of the other answers (including catching the exception) don't seem to be making things more readable and some are making it less readable in my opinion.
Given that you likely don't have control over the generated source and assuming you truly just need to access a few deeply nested fields here and there then I would recommend wrapping each deeply nested access with a method.
private int getFooBarBazInt() {
if (wsObject.getFoo() == null) return -1;
if (wsObject.getFoo().getBar() == null) return -1;
if (wsObject.getFoo().getBar().getBaz() == null) return -1;
return wsObject.getFoo().getBar().getBaz().getInt();
}
If you find yourself writing a lot of these methods or if you find yourself tempted to make these public static methods then I would create a separate object model, nested how you would like, with only the fields you care about, and convert from the web services object model to your object model.
When you are communicating with a remote web service it is very typical to have a "remote domain" and an "application domain" and switch between the two. The remote domain is often limited by the web protocol (for example, you can't send helper methods back and forth in a pure RESTful service and deeply nested object models are common to avoid multiple API calls) and so not ideal for direct use in your client.
For example:
public static class MyFoo {
private int barBazInt;
public MyFoo(Foo foo) {
this.barBazInt = parseBarBazInt();
}
public int getBarBazInt() {
return barBazInt;
}
private int parseFooBarBazInt(Foo foo) {
if (foo() == null) return -1;
if (foo().getBar() == null) return -1;
if (foo().getBar().getBaz() == null) return -1;
return foo().getBar().getBaz().getInt();
}
}
return wsObject.getFooBarBazInt();
by applying the the Law of Demeter,
class WsObject
{
FooObject foo;
..
Integer getFooBarBazInt()
{
if(foo != null) return foo.getBarBazInt();
else return null;
}
}
class FooObject
{
BarObject bar;
..
Integer getBarBazInt()
{
if(bar != null) return bar.getBazInt();
else return null;
}
}
class BarObject
{
BazObject baz;
..
Integer getBazInt()
{
if(baz != null) return baz.getInt();
else return null;
}
}
class BazObject
{
Integer myInt;
..
Integer getInt()
{
return myInt;
}
}
Giving answer which seems different from all others.
I recommend you to check for NULL in ifs.
Reason :
We should not leave a single chance for our program to be crashed.
NullPointer is generated by system. The behaviour of System
generated exceptions can not be predicted. You should not leave your
program in the hands of System when you already have a way of handling
it by your own. And put the Exception handling mechanism for the extra safety.!!
For making your code easy to read try this for checking the conditions :
if (wsObject.getFoo() == null || wsObject.getFoo().getBar() == null || wsObject.getFoo().getBar().getBaz() == null)
return -1;
else
return wsObject.getFoo().getBar().getBaz().getInt();
EDIT :
Here you need to store these values wsObject.getFoo(),
wsObject.getFoo().getBar(), wsObject.getFoo().getBar().getBaz() in
some variables. I am not doing it because i don't know the return
types of that functions.
Any suggestions will be appreciated..!!
I wrote a class called Snag which lets you define a path to navigate through a tree of objects. Here is an example of its use:
Snag<Car, String> ENGINE_NAME = Snag.createForAndReturn(Car.class, String.class).toGet("engine.name").andReturnNullIfMissing();
Meaning that the instance ENGINE_NAME would effectively call Car?.getEngine()?.getName() on the instance passed to it, and return null if any reference returned null:
final String name = ENGINE_NAME.get(firstCar);
It's not published on Maven but if anyone finds this useful it's here (with no warranty of course!)
It's a bit basic but it seems to do the job. Obviously it's more obsolete with more recent versions of Java and other JVM languages that support safe navigation or Optional.
Related
So our project back-end is a Java 8 Springboot application, springboot allows you to do some stuff really easily. ex, request validation:
class ProjectRequestDto {
#NotNull(message = "{NotNull.DotProjectRequest.id}")
#NotEmpty(message = "{NotEmpty.DotProjectRequest.id}")
private String id;
}
When this constraint is not meet, spring (springboot?) actually throws a validation exception, as such, we catch it somewhere in the application and construct a 404 (Bad Request) response for our application.
Now, given this fact, we kinda followed the same philosophy throughout our application, that is, on a deeper layer of the application we might have something like:
class ProjectService throws NotFoundException {
DbProject getProject(String id) {
DbProject p = ... // some hibernate code
if(p == null) {
Throw new NotFoundException();
}
return p;
}
}
And again we catch this exception on a higher level, and construct another 404 for the client.
Now, this is causing a few problems:
The most important one: Our error tracing stops being useful, we cannot differentiate (easily) when the exception is important, because they happen ALL the time, so if the service suddenly starts throwing errors we would not notice until it is too late.
Big amount of useless logging, on login requests for example, user might mistyped his password, and we log this and as a minor point: our analytics cannot help us determine what we are actually doing wrong, we see a lot of 4xx's but that is what we expect.
Exceptions are costly, gathering the stack trace is a resource intensive task, minor point at this moment, as the service scales up with would become more of a problem.
I think the solution is quite clear, we need to make an architectural change to not make exceptions part of our normal data flow, however this is a big change and we are short on time, so we plan to migrate over time, yet the problem remains for the short term.
Now, to my actual question: when I asked one of our architects, he suggested the use of monads (as a temporal solution ofc), so we don't modify our architecture, but tackle the most contaminating endpoints (ex. wrong login) in the short term, however I'm struggling with the monad paradigm overall and even more in java, I really have no idea on how to apply it to our project, could you help me with this? some code snippets would be really good.
TL:DR: If you take a generic spring boot application that throws errors as a part of its data flow, how can you apply the monad pattern to avoid login unnecessary amount of data and temporarily fix this Error as part of data flow architecture.
The standard monadic approach to exception handling is essentially to wrap your result in a type that is either a successful result or an error. It's similar to the Optional type, though here you have an error value instead of an empty value.
In Java the simplest possible implementation is something like the following:
public interface Try<T> {
<U> Try<U> flatMap(Function<T, Try<U>> f);
class Success<T> implements Try<T> {
public final T value;
public Success(T value) {
this.value = value;
}
#Override
public <U> Try<U> flatMap(Function<T, Try<U>> f) {
return f.apply(value);
}
}
class Fail<T> implements Try<T> {
// Alternatively use Exception or Throwable instead of String.
public final String error;
public Fail(String error) {
this.error = error;
}
#Override
public <U> Try<U> flatMap(Function<T, Try<U>> f) {
return (Try<U>)this;
}
}
}
(with obvious implementations for equals, hashCode, toString)
Where you previously had operations that would either return a result of type T or throw an exception, they would return a result of Try<T> (which would either be a Success<T> or a Fail<T>), and would not throw, e.g.:
class Test {
public static void main(String[] args) {
Try<String> r = ratio(2.0, 3.0).flatMap(Test::asString);
}
static Try<Double> ratio(double a, double b) {
if (b == 0) {
return new Try.Fail<Double>("Divide by zero");
} else {
return new Try.Success<Double>(a / b);
}
}
static Try<String> asString(double d) {
if (Double.isNaN(d)) {
return new Try.Fail<String>("NaN");
} else {
return new Try.Success<String>(Double.toString(d));
}
}
}
I.e. instead of throwing an exception you return a Fail<T> value which wraps the error. You can then compose operations which might fail using the flatMap method. It should be clear that once an error occurs it will short-circuit any subsequent operations - in the above example if ratio returns a Fail then asString doesn't get called and the error propagates directly through to the final result r.
Taking your example, under this approach it would look like this:
class ProjectService throws NotFoundException {
Try<DbProject> getProject(String id) {
DbProject p = ... // some hibernate code
if(p == null) {
return new Try.Fail<DbProject>("Failed to create DbProject");
}
return new Try.Succeed<DbProject>(p);
}
}
The advantage over raw exceptions is it's a bit more composable and allows, for example, for you to map (e.g. Stream.map) a fail-able function over a collection of values and end up with a collection of Fails and Successes. If you were using exceptions then the first exception would fail the entire operation and you would lose all results.
One downside is that you have to use Try return types all the way down your call stack (somewhat like checked exceptions). Another is that since Java doesn't have built-in monad support (al la Haskell & Scala) then the flatMap'ing can get slightly verbose. For example something like:
try {
A a = f(x);
B b = g(a);
C c = h(b);
} catch (...
where f, g, h might throw, becomes instead:
Try<C> c = f(x).flatMap(a -> g(a))
.flatMap(b -> h(b));
You can generalise the above implementation by making the error type an generic parameter E (instead of String), so it then becomes Try<T, E>. whether this is useful depends on your requirements - I've never needed it.
I have a more fully-implemented version here, alternatively the Javaslang and FunctionalJava libraries offer their own variants.
In code we have got a lot of chain methods, for example obj.getA().getB().getC().getD(). I want to create helper class which will check if method getD() isn't null, but before that I need to check all previous getters. I can do it in this way:
try {
obj.getA().getB().getC().getD();
}
catch (NullPointerException e) {
// some getter is null
}
or (which is "silly")
if (obj!null && obj.getA()!=null && obj.getA().getB()!=null && ...) {
obj.getA().getB().getC().getD();
}
else {
// some getter is null
}
I don't want to check it every time using try{} catch() in my code. What is the best solution for this purpose?
I think that the best will be:
obj.getA().getB().getC().getD().isNull() - for this purpose I will need to change all of my getters, for example implement some interface which contains isNull() method.
NullObjectHelper.isNull(obj.getA().getB().getC().getD()); - this will be the best (I think so) but how to implement this?
As of Java 8 you can use methods like Optional.isPresent and Optional.orElse to handle null in getter chains:
boolean dNotNull = Optional.ofNullable(obj)
.map(Obj::getA)
.map(A::getB)
.map(B::getC)
.map(C::getD)
.isPresent();
While this is preferable to catching NullPointerException the downside of this approach is the object allocations for Optional instances.
It is possible to write your own static methods that perform similar operations without this overhead:
boolean dNotNull = Nulls.isNotNull(obj, Obj::getA, A::getB, B::getC, C::getD);
For a sample implementation, see the Nullifier type here.
No approach is likely to have greater runtime efficiency than nested if-not-null checks.
You can achieve the desired result with Option pattern. This enforces you to change a method signature, but basically if your method returns some type T, it guarantees it has some non-null value, and if it returnsOption<T> it means it either has value T, or null.
Java 7 had some feature called null safety, but it was removed from the final release. You could do:
obj?.getA()?.getB()?.getC()?.getD()
Moreover, Java 8 will add a feature called Optional so you would do it safely.
In fact, if you really want to use that now, try Null Object pattern. It means that instead of returning plain null you can return some sort of default value, which won't trigger NullPointerException. Though, you need add some changes to your getters
class Object {
A getA() {
// ...
return a == null ? A.NULL : a;
}
}
class A {
static A NULL = new A(); // some default behaviour
B getB() {
if (this == NULL) return B.NULL;
// ...
return b == null ? B.NULL : b;
}
}
EDIT: If you want utility to do it you can wrap it in some functional interface and then call it.
static boolean isNullResult(Callable call) throws Exception {
try {
return call.call() == null;
} catch (NullPointerException npe) {
return true;
}
}
Usage will be the following:
isNullResult(new Callable<Integer>() {
#Override
public Integer call() throws Exception {
return new A().getB().getC().getInt();
}
});
It won't require you to change existing functionality
As already stated, the true solution is refactoring.
In the meantime, you could just wrap your first workaround in a function:
static D getD(MyClass obj) {
try {
return obj.getA().getB().getC().getD();
}
catch (NullPointerException e) {
return null; // Or even better, some default D
}
}
At the caller site:
D d = getD(obj);
At least you don't have to trash the caller with try-catch blocks. You still need to handle the errors somehow, when some of the intermediate getX() call returns a null and so d becomes null. The best would be to return some default D in the wrapper function.
I don't see how the two options you list at the end of your question would help if any of the intermediate getX() returns a null; you will get a NullPointerException.
Suppose we have a class Graph and another class called GraphWrapper.
Graph class has a method called returnAdjVertices.
public List returnAdjVertices(int vertex) {
if (vertex < 0 || vertex >= maxVertexCount) {
throw exception
}
}
Now we have a wrapper function in GraphWrapper which would calculate degree of a vertex
public static int getDegree(Graph graph, int vertex) {
if (graph == null) throw new NullPointerException();
if (vertext < 0 || vertex >= graph.maxVertexCount) throw exception // REDUNDANT
return graph.returnAdjVertices(vertex).size();
}
Now a call to find degree is checking for vertex bound condition twice.
This means we are doing a redundant check. What is the best practice recommended for exception handling in such a case ?
You can either rethrow an exception (it will be done automatically if you don't catch it in your wrapper)
public static int getDegree(Graph graph, int vertex) throws VertexOutOfBoundsException {
return graph.returnAdjVertices(vertex).size();
}
or catch it in your wrapper and retranslate to another
public static int getDegree(Graph graph, int vertex) throws WrapperException {
int result;
try {
result = graph.returnAdjVertices(vertex).size();
} catch (VertexOutOfBoundsException e) {
throw new WrapperException();
}
return result;
}
or even catch it and try to fix it
public static int getDegree(Graph graph, int vertex) {
int result;
try {
result = graph.returnAdjVertices(vertex).size();
} catch (VertexOutOfBoundsException e) {
result = fixGraphAndReturnAdjVertices(graph, vertex);
}
return result;
}
You shouldn't do your check again, because it can be really hard to maintain.
Selection of variant is always situative.
First variant (automatic rethrow) is good when your wrapper is on the same abstraction level with wrapped object.
Second variant is good when wrapper and wrapped objects are on different abstraction levels, as an example I can propose that "FileNotFoundException" of an object working with HDD can be translated to something like "CannotLoadDataException" of an object that try to load something if it makes no sence for caller what exactly goes wrong.
Third variant is good when your wrapper can fix things :)
So it's all up to you. Hope my answer helps.
As you note, the check is redundant. So you can simply drop it from the wrapper. The exception will move up the stack of calls.
I would make it clear in the documentation of both methods that an exception is thrown when the vertex is not between the accepted bounds. Or at least make it clear in the documentation of the wrapper method that it calls the wrapped method internally, and thus throws the same exceptions.
In this particular case? It's redundant, and should be eliminated–at least if you'd be throwing the same exception, and there are no other possible side-effects in the code.
In the general case? It depends entirely on your needs, the code, etc.
One issue is that if your explicit intent is to perform the exact same check you'd need to monitor the wrapped class to ensure the guard clause hasn't changed, or extract the guard logic into a separate method that can be called independently. This is leaky encapsulation, though.
Also, I'd throw an IllegalArgumentException with a message instead of an NPE.
What are the best practices if you have a class which accepts some parameters but none of them are allowed to be null?
The following is obvious but the exception is a little unspecific:
public class SomeClass
{
public SomeClass(Object one, Object two)
{
if (one == null || two == null)
{
throw new IllegalArgumentException("Parameters can't be null");
}
//...
}
}
Here the exceptions let you know which parameter is null, but the constructor is now pretty ugly:
public class SomeClass
{
public SomeClass(Object one, Object two)
{
if (one == null)
{
throw new IllegalArgumentException("one can't be null");
}
if (two == null)
{
throw new IllegalArgumentException("two can't be null");
}
//...
}
Here the constructor is neater, but now the constructor code isn't really in the constructor:
public class SomeClass
{
public SomeClass(Object one, Object two)
{
setOne(one);
setTwo(two);
}
public void setOne(Object one)
{
if (one == null)
{
throw new IllegalArgumentException("one can't be null");
}
//...
}
public void setTwo(Object two)
{
if (two == null)
{
throw new IllegalArgumentException("two can't be null");
}
//...
}
}
Which of these styles is best?
Or is there an alternative which is more widely accepted?
The second or the third.
Because it tells the user of your API what exactly went wrong.
For less verbosity use Validate.notNull(obj, message) from commons-lang. Thus your constructor will look like:
public SomeClass(Object one, Object two) {
Validate.notNull(one, "one can't be null");
Validate.notNull(two, "two can't be null");
...
}
Placing the check in the setter is also acceptable, with the same verbosity comment. If your setters also have the role of preserving object consistency, you can choose the third as well.
Java 7 added java.util.Objects.requireNonNull() to the APIs everybody can use. So checking all arguments for null boils down to a short list like:
this.arg1 = Objects.requireNonNull(arg1, "arg1 must not be null");
this.arg2 = Objects.requireNonNull(arg2, "arg2 must not be null");
Side notes:
make sure to not reverse the two arguments - the second one is the message that will be used for the NPE which is thrown if the first argument is null (if you reverse them, well, then your check will never fail)
another best practice: if possible, make all your class members final (so you can be sure: when some object has been created successfully, all its members are not null; and they won't change over time)
You can use one of the many libraries designed to facilitate precondition checks. Many code in Google Guava uses com.google.common.base.Preconditions
Simple static methods to be called at the start of your own methods to verify correct arguments and state. This allows constructs such as
if (count <= 0) {
throw new IllegalArgumentException("must be positive: " + count);
}
to be replaced with the more compact
checkArgument(count > 0, "must be positive: %s", count);
It has checkNotNull that is used extensively within Guava. You can then write:
import static com.google.common.base.Preconditions.checkNotNull;
//...
public SomeClass(Object one, Object two) {
this.one = checkNotNull(one);
this.two = checkNotNull(two, "two can't be null!");
//...
}
Most methods are overloaded to either take no error message, a fixed error message, or a templatized error message with varargs.
On IllegalArgumentException vs NullPointerException
While your original code throws IllegalArgumentException on null arguments, Guava's Preconditions.checkNotNull throws NullPointerException instead.
Here's a quote from Effective Java 2nd Edition: Item 60: Favor the use of standard exceptions:
Arguably, all erroneous method invocations boil down to an illegal argument or an illegal state, but other exceptions are standardly used for certain kinds of illegal argument and states. If a caller passes null in some parameter for which null values are prohibited, convention dictates NullPointerException be thrown rather than IllegalArgumentException.
A NullPointerException isn't reserved for just when you access members of a null reference; it's pretty standard to throw them when an argument is null when that's an illegal value.
System.out.println("some string".split(null));
// throws NullPointerException
I would have a utility method:
public static <T> T checkNull(String message, T object) {
if(object == null) {
throw new NullPointerException(message);
}
return object;
}
I would have it return the object so that you can use it in assignments like this:
public Constructor(Object param) {
this.param = checkNull("Param not allowed to be null", param);
}
EDIT: Regarding the suggestions to use a third party library, the Google Preconditions in particular does the above even better than my code. However, if this is the only reasons to include the library in your project, I'd be hesitant. The method is too simple.
Apart from the answers given above which are all valid and reasonable, I think it's good to point out that maybe checking for null isn't necessary "good practice". (Assuming readers other than the OP might take the question as dogmatic)
From Misko Hevery blog on testability:
To Assert or Not To Assert
An alternative to throwing an unchecked exception would be the usage of assert. Otherwise I´d throw checked exceptions to make the caller aware of the fact, that the constructor will not work with illegal values.
The difference between your first two solutions - do you need a detailed error message, do you need to know which parameter failed or is it enough to know, that the instance couldn't have been created due to illegal arguments?
Note, that the second and third example can't report correctly that both parameters have been null.
BTW - I vote for a variation of (1):
if (one == null || two == null) {
throw new IllegalArgumentException(
String.format("Parameters can't be null: one=%s, two=%s", one, two));
}
Comparison of Ways to Check Preconditions in Java - Guava vs. Apache Commons vs. Spring Framework vs. Plain Java Asserts
public static void fooSpringFrameworkAssert(String name, int start, int end) {
// Preconditions
Assert.notNull(name, "Name must not be null");
Assert.isTrue(start < end, "Start (" + start + ") must be smaller than end (" + end + ")");
// Do something here ...
}
public static void fooApacheCommonsValidate(String name, int start, int end) {
// Preconditions
Validate.notNull(name, "Name must not be null");
Validate.isTrue(start < end, "Start (%s) must be smaller than end (%s)", start, end);
// Do something here ...
}
public static void fooGuavaPreconditions(String name, int start, int end) {
// Preconditions
Preconditions.checkNotNull(name, "Name must not be null");
Preconditions.checkArgument(start < end, "Start (%s) must be smaller than end (%s)", start, end);
// Do something here ...
}
public static void fooPlainJavaAsserts(String name, int start, int end) {
// Preconditions
assert null != name : "Name must not be null";
assert start < end : "Start (" + start + ") must be smaller than end (" + end + ")";
// Do something here ...
}
this is summary of this article:
http://www.sw-engineering-candies.com/blog-1/comparison-of-ways-to-check-preconditions-in-java
Annotations for static analysis are also useful, either in-addition-to or in-place-of the run-time checks.
FindBugs, for example, provides an #NonNull annotation.
public SomeClass( #NonNull Object one, #NonNull Object two) {
You can simply have a method which takes all the constructor arguments that you need to validate. This method throws exception with specific message depending on which argument is not valid.
Your constructor calls this method, and if it passes, it initialize values.
I assume that you talk about the built in assert in Java. In my opinion it's not a really good idea to use it. Since it can be turned on/off using command line parameters. Therefore some says it is only acceptable to use in private methods.
My mentors are telling me not to re-invent the wheel! Their advice is to use libraries. They are (probably) well designed and tested. Of course it is your responsibility to make sure you grab a good-quality library.
Others are telling me that Enterprise ppl - in some terms - are wrong and you introduce more dependency - for simple tasks - than required. I can accept that point too... But here is my latest experience:
First I wrote my own private method to check null parameters. It's boring and redundant. I know I should put it into a Utility class. But why should I write it at the first place, when someone has already has done it? I can save time not writing unit test and design an existing stuff. Unless you want to exercise or learn I wouldn't recommend to do so.
I recently started to use google's guava and I find that - along with apache commons - once you start to use them, you won't use just for that one single method. You'll discover and use it more and more. At the end, this'll make your code shorter, more readable, more consistent and more maintainable.
BTW.: Depending on your aims I would go with 2 or 3 using one of the mentioned library above...
I am implementing an interface which defines a method that can throw an exception if the parameters are not valid. What constitutes valid parameters depends on the implementing class. The interface also defines an isValid() method which can be used to check the parameters but returns a boolean rather than throwing an exception. I have found that implementing both methods will cause a lot of duplication. Consider this made up example:
public class Something implements SomeInterface {
// Other class stuff
public void doTheThing(SomeParameter sp) throws SpecificRuntimeException {
if(sp == null) throw new ParameterCannotBeNullException();
if(sp.getNonZeroInt() == 0) throw new ShouldBeNonZeroException();
if(!sp.someOtherCondition()) throw new SomeConditionNotMetException();
...
}
public boolean isValid(SomeParameter sp) {
if(sp == null) return false;
if(sp.getNonZeroInt() == 0) return false;
if(!sp.someOtherCondition()) return false;
...
return true;
}
}
The problem is the checks in both methods must be consistent, and are essentially duplicated logic. I've been trying to consolidate the checks so that both methods use the same checks but the behaviour is still retained. Some things I considered:
in doTheThing() have the line if(!isValid(sp) throw new RuntimeException();
separate the exception throwing part into a separate, private method, say checkParameter() and in isValid() do: try { checkParameter(sp); return true; } catch (SpecificRunTimeException e) { return false; }
The problem with 1. is that the specific exception is lost, and I want to provide as detailed an exception as possible. The problem with 2. is using the exception mechanism seems... wrong somehow. This part of the code may be performance sensitive, and I don't want to depend on something that's fundamentally slower than it needs to be (if I have to do it this way and profiling doesn't show a problem, fair enough... but what if is a problem?). Or has the performance hit of using exceptions this way been shown to be negligible?
What is the best way to refactor this code to use the same validity checking logic?
What if your create a isValidParam method that returns a bean like:
class ValidBean {
private boolean isValid;
private Exception exceptionOnInvalid;
public ValidBean(boolean isValid, Exception exceptionOnInvalid) {...}
// accessors
}
With the method being:
private ValidBean isValidParam(SomeParameter sp) {
if (sp == null) {
return new ValidBean(false, new ParameterCannotBeNullException());
}
if (sp.getNonZeroInt() == 0) {
return new ValidBean(false, new ShouldBeNonZeroException());
}
if (!sp.someOtherCondition()) {
return new ValidBean(false, new SomeConditionNotMetException());
}
…
return new ValidBean(true, null);
}
And then you do:
public boolean isValid(SomeParameter sp) {
return isValidParam(sp).isValid();
}
And
public void doTheThing(SomeParameter sp) throws SpecificRuntimeException {
ValidBean validbean = isValidParam(sp);
if (! validBean.isValid()) {
throw validBean.getExceptionOnInvalid();
}
...
}
I don't know if it's the best way, but a way is to have an internal method like this:
private boolean isValidInternal(SomeParameter sp, boolean throwIfInvalid)
throws SpecificRuntimeException
Then you call isValidInternal with true from the doTheThing method, and false from the public isValid method.
Edit: And then to implement isValidInternal, you have the logic for testing validity once, but either return false or throw an exception as per the throwIfInvalid flag.
This can be a murky issue; when it comes to exceptions in APIs I've never found a really clear, logical, satisfactory answer to every case.
Do you expect every user of isValid() to be using it to guard a subsequent call to doTheThing()? If so, the client code would be clearer to use the standard try/catch idiom instead, and you may not need an isValid() method at all.
(You'd still have to ponder whether the exception should be checked or unchecked; the line can be a fine one and I won't go into it here.)
And if you don't see isValid() being used this way, then just implement it with a try/catch of your own; I know it feels dirty, but it's better than the alternatives!
Your urge to "say it once and only once" is laudable. Good thinking; it'll be worth the effort.
Maybe you can create an abstract class that implements the interface and provides the default implementation for isValid().
If your object is mutable and has setters, perhaps you could move the check that throws IllegalArgumentException into each one and then call them. You'll have a more fine-grained view of what went wrong that way.
The fact that your code checks the arguments for sanity and returns runtime exceptions is a strong indication that you want to report programming error, i.e. your API is used/called in a way it should not be used/called. Since you could expect that your API is called with valid arguments, I wouldn't expect that your second solution to be problematic.
P.S.: You should not use runtime exception types in a method's throws declaration. But you should state those runtime exceptions in the method's Javadoc #throws documentation.