Java How to access data from local inner class

Question

Below code is some example analogous to what I am trying to achieve. I have to store the data from internal class but the compiler complains following:

  returnedValue=(MyXYZClass)value.toString();

Local variable selectedBusinessArea defined in an enclosing scope must be final or effectively final

 private String methodInMainClass(Object someRef){
      String returnedValue="";

      new SomeInnerClass(Object someRef){
          @Override
          public String getData(Object value){
              if(value instanceof MyXYZClass){
                  returnedValue=(MyXYZClass)value.toString(); // Not Happening. I cannot assign it to the outer variable.
              }
              return super.getData(value);
          }
      }

How can I store the value from the overriden method in internal class?

this might be helpful to you stackoverflow.com/questions/18825718/… — Avinash T.
– Avinash T., Commented Sep 25, 2018 at 11:27

LuCio · Accepted Answer · 2018-09-25 20:35:49Z

2

The local variable in your code sample is returnedValue. It has to be final or effectively final but it's neither declared final nor effectively final since it gets a value assigned.

How to solve this:

All you need is final or effectively final variable which can store the state of returnedValue. There are many ways to do it.

One way would be to define a class like BoxedValue.

public class BoxedValue<T> {

    private T value;

    public void set(T value) {
        this.value = value;
    }

    public T get() {
        return value;
    }
}

Using it in the code:

private String methodInMainClass(Object someRef){
    BoxedValue<String> returnedValue = new BoxedValue<>("");

    new SomeInnerClass(Object someRef){
        @Override
        public String getData(Object value){
            if(value instanceof MyXYZClass){
                returnedValue.set((MyXYZClass)value.toString()); // Not Happening. I cannot assign it to the outer variable.
            }
            return super.getData(value);
        }
    }

    return returnedValue.get();
}

The variable returnedValue is now effectively final.

If you don't want to define an additional class, you can use any other to store the state, i.e. Optional<String> returnedValue = Optional.of("").

edited Sep 25, 2018 at 20:35

answered Sep 25, 2018 at 12:06

LuCio

5,2232 gold badges21 silver badges36 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user1717259 Over a year ago

+1 but the AtomicReference class already does this; docs.oracle.com/javase/8/docs/api/java/util/concurrent/atomic/…

LuCio Over a year ago

@user1717259 Yes ... but as you know it actually does more. It has also a more sophisticated interface. I just wanted to keep it simple.

ToYonos · Accepted Answer · 2018-09-25 12:01:20Z

1

You can use an array in order to do the trick

private String methodInMainClass(Object someRef){
      String[] returnedValue= {""};

      new SomeInnerClass(someRef){
          @Override
          public String getData(Object value){
              if(value instanceof MyXYZClass){
                  returnedValue[0] = ((MyXYZClass)value).toString(); // Not Happening. I cannot assign it to the outer variable.
              }
              return super.getData(value);
          }
    }
    // [...]

answered Sep 25, 2018 at 12:01

ToYonos

16.9k3 gold badges62 silver badges74 bronze badges

1 Comment

ToYonos Over a year ago

Because returnedValue reference is never modified, only the first and only element of the array

Bassem Reda Zohdy · Accepted Answer · 2018-09-25 11:47:58Z

1

it is not about inner class this also happened in lambda, the you can not change value of var from out of scope of lambda or inner class, otherwise you may make access issue and for multithread race condition, as workaround you may use one of Atomic family classes for that like AtomicReference<T> or AtomicInteger for ex. :

    AtomicInteger count = new AtomicInteger(0);
    Runnable r = () -> {
        count.getAndIncrement();
    };
    r.run();
    System.out.println(count);

Note here that I used only main thread not created new one to not face race condition.

answered Sep 25, 2018 at 11:47

Bassem Reda Zohdy

13k3 gold badges37 silver badges40 bronze badges

2 Comments

lesnar Over a year ago

thanks for the answer but we are using unfortunately java 7 . +1 for the answer

Bassem Reda Zohdy Over a year ago

@Roxy Atomic family in java from Java 1.5 you still can use it.

Collectives™ on Stack Overflow

Java How to access data from local inner class

3 Answers 3

2 Comments

1 Comment

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related