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How is using libraries checking nulls better than getting NPE? [duplicate]

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Java: How to check for null pointers efficiently
(14 answers)
Closed 8 years ago.
Sometimes I see developers using libraries like Guava’s Preconditions to validate the parameters for nulls in the beginning of a method. How is that different than getting NPE during runtime since a runtime exception occurs in either way?
EDIT: If there are good reasons, then shouldn't developers use libraries for null-check on all methods?

Some reasons are:
Prevents code from being run prior to the first time the variable (which could be null) is de-referenced.
You can have custom error messages, such as "myVar was null, I can't proceed further", or any relevant message which will look better in logs and more easily traceable. A normal NPE would be less readable in logs.
Readability of code is better (arguably) because a programmer reading this code will realize immediately that these values are expected to be non-null.
Ultimately it's a matter of taste IMO. I have seen programs that are inherently null safe and don't require pre-conditions at all.

There are various reasons. One big one, as mentioned in the comments, is to get the checking done up front before any work is done. It's also not uncommon for a method to have a code path in which a specific parameter is never dereferenced, in which case without upfront checking, invalid calls to the method may sometimes not produce an exception. The goal is to ensure they always produce an exception so that the bug is caught immediately. That said, I don't necessarily use checkNotNull if I'm going to immediately dereference the parameter on the next line or something.
There's also the case of constructors, where you want to checkNotNull before assigning a parameter to a field, but I don't think that's what you're talking about since you're talking about methods where the parameter will be used anyway.

Java : "xx".equals(variable) better than variable.equals("xx") , TRUE?

I'm reviewing a manual of best practices and recommendation coding java I think is doubtful.
Recomendation:
String variable;
"xx".equals(variable) // OK
variable.equals("xx") //Not recomended
Because prevents appearance of NullPointerException that are not controlled
Is this true?

This is a very common technique that causes the test to return false if the variable is null instead of throwing a NullPointerException. But I guess I'll be different and say that I wouldn't regard this as a recommendation that you always should follow.
I definitely think it is something that all Java programmers should be aware of as it is a common idiom.
It's also a useful technique to make code more concise (you can handle the null and not null case at the same time).
But:
It makes your code harder to read: "If blue is the sky..."
If you have just checked that your argument is not null on the previous line then it is unnecessary.
If you forgot to test for null and someone does come with a null argument that you weren't expecting it then a NullPointerException is not necessarily the worst possible outcome. Pretending everything is OK and carrying until it eventually fails later is not really a better alternative. Failing fast is good.
Personally I don't think usage of this technique should be required in all cases. I think it should be left to the programmer's judgement on a case-by-case basis. The important thing is to make sure you've handled the null case in an appropriate manner and how you do that depends on the situation. Checking correct handling of null values could be part of the testing / code review guidelines.

It is true. If variable is null in your example,
variable.equals("xx");
will throw a NPE because you can't call a method (equals) on a null object. But
"xx".equals(variable);
will just return false without error.

Actually, I think that the original recommendation is true. If you use variable.equals("xx"), then you will get a NullPointerException if variable is null. Putting the constant string on the left hand side avoids this possibility.
It's up to you whether this defense is worth the pain of what many people consider an unnatural idiom.

This is a common technique used in Java (and C#) programs. The first form avoids the null pointer exception because the .equals() method is called on the constant string "xx", which is never null. A non-null string compared to a null is false.
If you know that variable will never be null (and your program is incorrect in some other way if it is ever null), then using variable.equals("xx") is fine.

It's true that using any propertie of an object that way helps you to avoid the NPE.
But that's why we have Exceptions, to handle those kind of thing.
Maybe if you use "xx".equals(variable) you would never know if the value of variable is null or just isn't equal to "xx". IMO it's best to know that you are getting a null value in your variable, so you can reasign it, rather than just ignore it.

You are correct about the order of the check--if the variable is null, calling .equals on the string constant will prevent an NPE--but I'm not sure I consider this a good idea; Personally I call it "slop".
Slop is when you don't detect an abnormal condition but in fact create habits to personally avoid it's detection. Passing around a null as a string for an extended period of time will eventually lead to errors that may be obscure and hard to find.
Coding for slop is the opposite of "Fail fast fail hard".
Using a null as a string can occasionally make a great "Special" value, but the fact that you are trying to compare it to something indicates that your understanding of the system is incomplete (at best)--the sooner you find this fact out, the better.
On the other hand, making all variables final by default, using Generics and minimizing visibility of all objects/methods are habits that reduce slop.

If you need to check for null, I find this better readable than
if (variable != null && variable.equals("xx")). It's more a matter of personal preference.

As a side note, here is a design pattern where this code recommendation might not make any difference, since the String (i.e. Optional<String>) is never null because of the .isPresent() call from the design pattern:
Optional<String> gender = Optional.of("MALE");
if (gender.isPresent()) {
System.out.println("Value available.");
} else {
System.out.println("Value not available.");
}
gender.ifPresent(g -> System.out.println("Consumer: equals: " + g.equals("whatever")));

Best practice with respect to NPE and multiple expressions on single line

I'm wondering if it is an accepted practice or not to avoid multiple calls on the same line with respect to possible NPEs, and if so in what circumstances. For example:
anObj.doThatWith(myObj.getThis());
vs
Object o = myObj.getThis();
anObj.doThatWith(o);
The latter is more verbose, but if there is an NPE, you immediately know what is null. However, it also requires creating a name for the variable and more import statements.
So my questions around this are:
Is this problem something worth
designing around? Is it better to go
for the first or second possibility?
Is the creation of a variable name something that would have an effect performance-wise?
Is there a proposal to change the exception
message to be able to determine what
object is null in future versions of
Java ?

Is this problem something worth designing around? Is it better to go for the first or second possibility?
IMO, no. Go for the version of the code that is most readable.
If you get an NPE that you cannot diagnose then modify the code as required. Alternatively, run it using the debugger and use breakpoints and single stepping to find out where the null pointer is coming from.
Is the creation of a variable name something that would have an effect performance-wise?
Adding an extra variable may increase the stack frame size, or may extend the time that some objects remain reachable. But both effects are unlikely to be significant.
Is there a proposal to change the exception message to be able to determine what object is null in future versions of Java ?
Not that I am aware of. Implementing such a feature would probably have significant performance downsides.

The Law of Demeter explicitly says not to do this at all.

If you are sure that getThis() cannot return a null value, the first variant is ok. You can use contract annotations in your code to check such conditions. For instance Parasoft JTest uses an annotation like #post $result != null and flags all methods without the annotation that use the return value without checking.
If the method can return null your code should always use the second variant, and check the return value. Only you can decide what to do if the return value is null, it might be ok, or you might want to log an error:
Object o = getThis();
if (null == o) {
log.error("mymethod: Could not retrieve this");
} else {
o.doThat();
}

Personally I dislike the one-liner code "design pattern", so I side by all those who say to keep your code readable. Although I saw much worse lines of code in existing projects similar to this:
someMap.put(
someObject.getSomeThing().getSomeOtherThing().getKey(),
someObject.getSomeThing().getSomeOtherThing())
I think that no one would argue that this is not the way to write maintainable code.
As for using annotations - unfortunately not all developers use the same IDE and Eclipse users would not benefit from the #Nullable and #NotNull annotations. And without the IDE integration these do not have much benefit (apart from some extra documentation). However I do recommend the assert ability. While it only helps during run-time, it does help to find most NPE causes and has no performance effect, and makes the assumptions your code makes clearer.

If it were me I would change the code to your latter version but I would also add logging (maybe print) statements with a framework like log4j so if something did go wrong I could check the log files to see what was null.

Smarter setter? Good or Bad Idea?

In a GWT solution. (so this is java code that is then compiled to javascript).
There are of course some classes.
Is it a good idea to make the setter check for Null on a String field?
something like this
public void setSomeField(String someField){
if (null != someField)
this.someField = someField;
else
this.someField = String.Empty;
}
Is this a good or bad idea? On the one had it will make coding easier as i wont have to check for null , on the other hand it would make me probably forget that I have to do this for other strings.
Thoughts?
Thanks

I say if such a logic is needed in your application, the setter is the place to put it. The main reason to have a get/set wrap around a private var is to be able to put logic around the access.
To answer the question of to default or not to default:
In my application it made sence to have a set of properties fall back to string.empty for display reasons. Although people could argue that the view should then cover these possibilities and check for nulls and display nothing, it was a lot of bloat all over my pages to do a check on every property.
That's why I started implementing SafeXX properties. So say I had 'myObj.Name' that could possibly have a null value, there would also be a property 'myObj.SafeName' that caught the null in the getter and returned a string.empty in stead. The little naming convention gives away that it is not the regular getter.

Here's something to consider. Would you expect this unit test to pass or fail?:
yourClass.setSomeField(null);
assertNull(yourClass.getSomeField());
If you're changing the null value to an empty string and returning that in getSomeField, then the client now has to check two conditions when testing...a String and a null String. Not a big deal, but what happens if you've got twenty String properties in the class...you'd probably better try to be consistent amongst all of the setters, and if you're not then the reason should probably be more obvious than just the documentation saying so.
There are certain conventions around getters and setters; certain expectations. If I call a setter on an object, then I usually expect the getter to return what I set. I don't expect it to return some representation of what I passed in that is more convenient for the class to work with internally. I don't care about the internals of the class, and don't want to.

If null really should be changed to "" for a valid reason (for example, it might mean "I don't care" and the default could be ""), go for it(but document it).
Otherwise, like if you just caught a NullPointerException and are trying to fix it this way, don't do it. If callers use obviously invalid values, the exception should be raised as soon as possible so that the caller notices the problem and fixes it before it bubbles up to a catastrophic, unexplainable error in a probably completely unrelated component.

In general, it is not a good idea to check for null values because the caller (the one who invokes the setter) may really want to set the value to null.
Suppose you query for 'someField' with this:
select * from ... where someField is null
If you set it as the empty string, the query above would fail.

If you don't want a field set to null, then don't set it to null.
This can be a good idea if you have no control over the code doing the setting, but if you do, it better to fix the problem at the source rather than put in work arounds like this.

That is hard to answer. On the first look it seems to make the usage better because you don't have to check for null all the time. But you loose the quality of null that means nothing is assigned. If you do String.Empty you already have ambiguity if someone gave you a String.Empty as parameter. Maybe it doesn't matter.
I personally (if at all) wouldn't do it in the setter. Inside your class null should have a value of its own. If you are for convenience a getter
return (this.someField != null) ? this.someField: String.Empty;
will do. You would keep the null internally to deal with but the outside has a more convenient method.
Generally and personally speaking I wouldn't do it. It looks good at first and makes a lot of things harder at later time.

How to trace a NullPointerException in a chain of getters

If I get a NullPointerException in a call like this:
someObject.getSomething().getSomethingElse().
getAnotherThing().getYetAnotherObject().getValue();
I get a rather useless exception text like:
Exception in thread "main" java.lang.NullPointerException
at package.SomeClass.someMethod(SomeClass.java:12)
I find it rather hard to find out which call actually returned null, often finding myself refactoring the code to something like this:
Foo ret1 = someObject.getSomething();
Bar ret2 = ret1.getSomethingElse();
Baz ret3 = ret2.getAnotherThing();
Bam ret4 = ret3.getYetAnotherOject();
int ret5 = ret4.getValue();
and then waiting for a more descriptive NullPointerException that tells me which line to look for.
Some of you might argue that concatenating getters is bad style and should be avoided anyway, but my Question is: Can I find the bug without changing the code?
Hint: I'm using eclipse and I know what a debugger is, but I can't figuer out how to apply it to the problem.
My conclusion on the answers:
Some answers told me that I should not chain getters one after another, some answers showed my how to debug my code if I disobeyed that advice.
I've accepted an answer that taught me exactly when to chain getters:
If they cannot return null, chain them as long as you like. No need for checking != null, no need to worry about NullPointerExceptions (be warned that chaining still violates the Law of Demeter, but I can live with that)
If they may return null, don't ever, never ever chain them, and perform a check for null values on each one that may return null
This makes any good advice on actual debugging useless.

NPE is the most useless Exception in Java, period. It seems to be always lazily implemented and never tells exactly what caused it, even as simple as "class x.y.Z is null" would help a lot in debugging such cases.
Anyway, the only good way I've found to find the NPE thrower in these cases is the following kind of refactoring:
someObject.getSomething()
.getSomethingElse()
.getAnotherThing()
.getYetAnotherObject()
.getValue();
There you have it, now NPE points to correct line and thus correct method which threw the actual NPE. Not as elegant solution as I'd want it to be, but it works.

The answer depends on how you view (the contract of) your getters. If they may return null you should really check the return value each time. If the getter should not return null, the getter should contain a check and throw an exception (IllegalStateException?) instead of returning null, that you promised never to return. The stacktrace will point you to the exact getter. You could even put the unexpected state your getter found in the exception message.

In IntelliJ IDEA you can set exceptionbreakpoints. Those breakpoints fire whenever a specified exception is thrown (you can scope this to a package or a class).
That way it should be easy to find the source of your NPE.
I would assume, that you can do something similar in netbeans or eclipse.
EDIT: Here is an explanation on how to add an exceptionbreakpoint in eclipse

If you find yourself often writing:
a.getB().getC().getD().getE();
this is probably a code smell and should be avoided. You can refactor, for example, into a.getE() which calls b.getE() which calls c.getE() which calls d.getE(). (This example may not make sense for your particular use case, but it's one pattern for fixing this code smell.)
See also the Law of Demeter, which says:
Your method can call other methods in its class directly
Your method can call methods on its own fields directly (but not on the fields' fields)
When your method takes parameters, your method can call methods on those parameters directly.
When your method creates local objects, that method can call methods on the local objects.
Therefore, one should not have a chain of messages, e.g. a.getB().getC().doSomething(). Following this "law" has many more benefits apart from making NullPointerExceptions easier to debug.

I generally do not chain getters like this where there is more than one nullable getter.
If you're running inside your ide you can just set a breakpoint and use the "evaluate expression" functionality of your ide on each element successively.
But you're going to be scratching your head the moment you get this error message from your production server logs. So best keep max one nullable item per line.
Meanwhile we can dream of groovy's safe navigation operator

Early failure is also an option.
Anywhere in your code that a null value can be returned, consider introducing a check for a null return value.
public Foo getSomething()
{
Foo result;
...
if (result == null) {
throw new IllegalStateException("Something is missing");
}
return result;
}

Here's how to find the bug, using Eclipse.
First, set a breakpoint on the line:
someObject.getSomething().getSomethingElse().
getAnotherThing().getYetAnotherObject().getValue();
Run the program in debug mode, allow the debugger to switch over to its perspective when the line is hit.
Now, highlight "someObject" and press CTRL+SHIFT+I (or right click and say "inspect").
Is it null? You've found your NPE. Is it non-null?
Then highlight someObject.getSomething() (including the parenthesis) and inspect it.
Is it null? Etc. Continue down the chain to figure out where your NPE is occurring, without having to change your code.

You may want to refer to this question about avoiding != null.
Basically, if null is a valid response, you have to check for it. If not, assert it (if you can). But whatever you do, try and minimize the cases where null is a valid response for this amongst other reasons.

If you're having to get to the point where you're splitting up the line or doing elaborate debugging to spot the problem, then that's generally God's way of telling you that your code isn't checking for the null early enough.
If you have a method or constructor that takes an object parameter and the object/method in question cannot sensibly deal with that parameter being null, then just check and throw a NullPointerException there and then.
I've seen people invent "coding style" rules to try and get round this problem such as "you're not allowed more than one dot on a line". But this just encourages programming that spots the bug in the wrong place.

Chained expressions like that are a pain to debug for NullPointerExceptions (and most other problems that can occur) so I would advise you to try and avoid it. You have probably heard that enough though and like a previous poster mentioned you can add break points on the actual NullPointerException to see where it occurred.
In eclipse (and most IDEs) you can also use watch expressions to evaluate code running in the debugger. You do this bu selecting the code and use the contet menu to add a new watch.
If you are in control of the method that returns null you could also consider the Null Object pattern if null is a valid value to return.

Place each getter on its own line and debug. Step over (F6) each method to find which call returns null