Develop Reference

How check if an Attribute(object) is null in Java

I need specific data for a report, then I gettin all information from a parent object
Object1
It has many attributes, object attributes
Object11, Object12, Object13, attr1, attr2...
The attributes has many attributes too
Object111, Object131, Object132,..
by now I got 5 level data attributes.
When I send information to my report it says, Error: cause:null
object1.getIdObject11().getIdObject111().getDescription;
It trows error because Object111 is null
I tried using
object1.getIdObject11().getIdObject111().getDescription==null?'':object1.getIdObject11().getIdObject111().getDescription;
but it only verify if description is null, and throws the same error
Then I tried to verify Object
if(object1.getIdObject11().getIdObject111() == null) {
var = object1.getIdObject11().getIdObject111().getDescription;
} else {
var = "";
}
But when Object11 is null, it throws same error.
I don't think its a good way doing this for each attribute (have to get like 30 attributes)
if(object1.getIdObject11()!=null) {
if(object1.getIdObject11().getIdObject111()!=null) {
if(object1.getIdObject11().getIdObject111().getIdObject1111()!=null) {
//...
}
}
}
I want to verify if is there a null object and set '' (blank) if it is, with no such a large code(because the gotten params are set inside a report, mixed with letter).
reportline1 = "Area: "+object1.getIdObject11().getIdObject111().getName;

You code breaks Demeter's law. That's why it's better to refactor the design itself.
As a workaround, you can use Optional
var = Optional.ofNullable(object1)
.map(o -> o.getIdObject11())
.map(o -> o.getIdObject111())
.map(o -> o.getDescription())
.orElse("")

The way I would probably do this to extend the functionality of the code easily in the future might take a bit of writing in the beginning but will be easily usable forever.
I would create a new method in your parent class called hasNull that returns a boolean like so:
public boolean hasNull()
{
boolean hasANull = false;
//Call another hasNull() inside of object11 which in turns calls hasNull() in object111 etc.
//If any of the calls return with a true/null value set hasANull to true
return hasANull;
}
This in turn checks to see if the current objects it contains are null. If one of the class variables is another custom class you created you can then add another hasNull into that one and keep going until you get to the lowest level where you can do a specific operation when the value is null such as set it to "".
After implementing this you will be able to just be able to use it like this any time you need it:
if (!object1.hasNull())
{
//Do whatever you want if there are no null values
}
else
{
//Do whatever you want if there is a null value
}
You can also make this a void method if you only want it to toggle the values on the lowest level, and do not need to do anything in either case.

I prefer the solution that gave dehasi.
But you can also do something like that:
getOrElse(() -> object1.getIdObject11().getIdObject111().getDescription(), "")
Where getOrElse is:
public static <T> T getOrElse(Supplier<T> getter, T elseValue) {
try {
return getter.get();
} catch (Exception e) {
// log or do something with it
}
return elseValue;
}
It may be controversial becaouse you use Exception to do this.

You can use this code to check if your object has a null attribute, the object is myclass;
for (Field f : myclass.getClass().getDeclaredFields()) {
f.setAccessible(true);
try {
if (Objects.isNull(f.get(myclass))) {
isLineContainsNull = true;
}
} catch (Exception e) {
log.error(e.getMessage());
}
}

Is it possible to return from a method using #Advice.OnMethodEnter?

Using Byte Buddy's advice API, is it possible to return from the instrumented method without actually executing it?
One use case would be to implement a cache and to return the cached value, if present, instead of computing the value again.
#Advice.OnMethodEnter
public static Object returnCachedValue(#Advice.Argument(0) String query) {
if (cache.containsKey(query)) {
// should "abort" method call
return cache.get(query);
}
}
I know that this code sample above just creates a local variable which I can get in a #Advice.OnMethodExit method. But is there a way to abort the method call on an explicit return? If yes, is this also possible for void methods?

No, this is not possible, a return value can only be set from exit advice. But it can be emulated by skipping the original method in case that a value already exists and by setting this value from the exit advice in case that the enter advice defines a value:
class MyAdvice {
#Advice.OnMethodEnter(skipOn = Advice.OnNonDefaultValue.class)
public static Object returnCachedValue(#Advice.Argument(0) String query) {
if (cache.containsKey(query)) {
return cache.get(query);
} else {
return null;
}
}
#Advice.OnMethodExit
public static void processCachedValue(
#Advice.Return(readOnly = false, typing = DYNAMIC) Object returned,
#Advice.Enter Object enter) {
if (enter != null) {
returned = enter;
} else {
cache.put(query, returned);
}
}
}
Of course, this does not work if the cached value is null. To avoid this, you could wrap the value in some instance to make sure that the enter value is never null. Doing so would also allow to use the above pattern to void methods.
This might look inconvenient to program but the idea of advice is that Byte Buddy can use the advice class as a template and inline the byte code without much work to avoid a runtime overhead.

Java convention in practice - return mutliple values from method

I have two questions about Java Convention. I try to make use od Robert C. Martin's "Clean Code".
Following case:
public void startProgressIfAllowed() {
try {
tryStartProgressIfAllowed();
} catch (Exception exception) {
// log error
}
}
private void tryStartProgressIfAllowed() {
if (isStartProgressAllowed()) {
stopProgressOnCurrentlyStartedTask();
startProgressOnThisTask();
}
}
private boolean isStartProgressAllowed() {
// Calls JOptionPane.showConfirmDialog with JOptionPane.YES_NO_OPTION.
// Created dialog contains checkbox indicating that saving currently started task is required.
// returns boolean depending on JOptionPane.YES_NO_OPTION clicked button
}
private void stopProgressOnCurrentlyStartedTask() {
// Saves currently started task depending on checkbox selecion property and stops currently started.
// What is the correct way to get checkbox selecion property?
}
Proposed solution:
public void tryStartProgressIfAllowed() {
if (tryToStopProgressOnStartedTaskIfNecessary()) {
startProgressOnThisTask();
}
}
private boolean tryToStopProgressOnStartedTaskIfNecessary() {
// Calls JOptionPane.showConfirmDialog with JOptionPane.YES_NO_OPTION.
// Created dialog contains checkbox indicating that saving currently started task is required.
// Depending on checkbox selecion property saves task.
// returns boolean depending on JOptionPane.YES_NO_OPTION clicked button
}
But this approach doesn't meet the "Command Query Separation" principle, because tryToStopProgressOnStartedTaskIfNecessary(...) method performs some logic and returns success/failure value.
I think this approach also doesn't meet the "One level of abstraction per function" principle, because I suppose "check" and "save" operations are on different levels of abstraction.
Is the method name correct to avoid disinformation? Maybe better name would be tryToStopProgressAndSaveStartedTaskIfNecessary(...)?
Is there any better solution for above problem?

What about the following:
public void tryStartProgressOnThisTaskIfAllowed() {
tryStopTaskInProgressIfAllowed()
if (!isTaskInProgress()) {
tryStartProgressOnThisTask();
}
}
private void tryStopTaskInProgressIfAllowed() {
if (!isTaskInProgress()) {
return;
}
TaskInProgressResult result = whatToDoWithTaskInProgress();
if (result == Result.KEEP) {
return;
} else if (result == Result.DROP)
tryDropTaskInProgress();
} else if (result == Result.SAVE) {
trySaveTaskInProgress();
}
}
About your points:
You now have two separate methods for C and Q
I think the two things whatToDoWithTaskInProgress and tryDropTaskInProgress are the same level of abstraction. If you'd inline the code of one or the other you were absolutely right of course.
I changed some of the method names according to my taste :) The only thing I still don't like is the part "OnThisTask" because this task is somewhat meaningless. Maybe it's only because the rest of the code is unknown maybe OnNextTask or OnNewTask are better.
The problem we were having is that we were thinking in UI terms YES/NO + checkbox value. But it is much better to think in business terms here. I identified three different outcomes that are of interest: KEEP, SAVE, DROP How the answer is obtained should not matter to the calling method.

This seems something to ask on CodeReview, see the drop down at the top left of the page.
An example of how such stateliness is realized in Java SE: the regex Matcher class.
String s = ...
Pattern pattern = Pattern.compile("...");
Matcher m = pattern.matcher(s);
StringBuffer sb = new StringBuffer();
while (m.find()) {
m.appendReplacement(sb, ... m.group(1) ...);
}
m.appendTail(sb);
with m.matches() and m.lookingAt as alternative circuits too.
In short state is held in a processing class on the actual data (String here).

set methods in Java

Could anubody explain how to use set methods? Problem:
class Sonum {
private int prior;
public Sonum(int prior) {
this.prior = prior;
}
public int getPrior() {
return prior;
}
public void setPrior(int prior) {
this.prior = prior;
}
class Tel {
// Please explain how can I set the value for prior? (for example 2)
}

Well, first you need an instance of Sonum on which you want to set the prior value. For example:
class Test {
public void foo() {
Sonum sonum = new Sonum(5);
// Use it with a prior of 5
// ...
sonum.setPrior(10);
// Now use it with a prior of 10
}
}

Sonum mySonum = new Sonum(1); //prior is currently 1
mySonum.setPrior(2); //now prior is 2

Take a deep breath. The Java Tutorial. Read it. You will understand.

Refer
Creating Objects & Using Objects
http://download.oracle.com/javase/tutorial/java/javaOO/objectcreation.html

"Setter methods" aren't magic. They're just regular methods. You need an instance of that class, and then you can call the methods on it. Just like any other Java object.

set method deal with a private value that we would like to prevent the direct way to him using our client, therefor there are get \ set method.
The biggest advantage of get \ set methods is the control ability !
We can for example control a minimum age when we want to set an age, and many other simple examples.
Example:
setAge (int age)
{
if ( age < 0 )
{
System.out.println ( "Wrong age !!" );
}
}
Now I think you can easily understand this HW :)

Can I un-assign (clear) all fields of an instance?

Is there a simple way to clear all fields of an instance from a an instance? I mean, I would like to remove all values assigned to the fields of an instance.
ADDED
From the main thread I start a window and another thread which controls state of the window (the last thread, for example, display certain panels for a certain period of time). I have a class which contains state of the window (on which stage the user is, which buttons he already clicked).
In the end, user may want to start the whole process from the beginning (it is a game). So, I decided. So, if everything is executed from the beginning, I would like to have all parameter to be clean (fresh, unassigned).
ADDED
The main thread, creates the new object which is executed in a new thread (and the old thread is finished). So, I cannot create a new object from the old thread. I just have a loop in the second thread.

I don't get it. How can you programmatically decide how to clear various fields?
For normal attributes it can be easy (var = null) but what about composite things or collection? Should it be collection = null, or collection.removeAll()?
This question is looking for synctactic sugar that wouldn't make so much sense..
The best way is to write out your own reset() method to customize the behaviour for every single object.. maybe you can patternize it using an
interface Resettable
{
void reset()
}
but nothing more than that..

Is there a simple way to clear all fields of an instance from a an instance? I mean, I would like to remove all values assigned to the fields of an instance.
Yes, just assign a default value to each one of them. It would take you about 20-30 mins. and will run well forever*( YMMV)
Create a method: reset and invoke it
class YourClass {
int a;
int b;
boolean c;
double d;
String f;
// and so on...
public void method1(){}
public void method2(){}
public void method3(){}
// etc.
// Magic method, reset all the attributes of your instance...
public void reset(){
a = 0;
b = 0;
c = false;
d = 0.0;
f = "";
}
}
And then just invoke it in your code:
....
YourClass object = new YourClass();
Thread thread = YourSpecificNewThread( object );
thread.start();
... // Later on you decide you have to reset the object just call your method:
object.reset(); // like new
I don't really see where's the problem with this approach.

You may use reflection:
Try something like this:
Field[] fields = object.getClass().getDeclaredFields();
for (Field f : fields) {
f.setAccessible(true);
f.set(object, null);
}
It's not a beautifull solution, but may work for you.

There is no other way than setting null to all of them.
As an aside, i find that a particular weird idea. You would have better re-creating a new instance, instead of trying to reset your old one.

If you want to clear a filter (Serializable) that your application "can handle his null" fields, you can use BeanUtils (Apache Commons):
Field[] fields = filter.getClass().getDeclaredFields();
for (Field f : fields) {
if (f.getName().endsWith("serialVersionUID")) {
continue;
}
try {
BeanUtils.setProperty(filter, f.getName(), null);
} catch (IllegalAccessException | InvocationTargetException e) {
FacesUtils.handleError(LOG, "Erro limpar filtro...", e);
}
}
I hope it can help you.