how is Array object different from other objects

Question

I am confused by the way arrays work. If I pass an array reference to some method, then that method is able to change the index values previously stored in array. However, if do the same with, say, a wrapper class object then that method is not able to change the value. Here is the code:

public class TestArray {
    public static void main(String[] args) {
        ChangeValues ch=new ChangeValues();
        int intArr[]={1,2,3};
        Integer iWrapper=new Integer(123);
        ch.changeArray(intArr);
        for(int i:intArr){
            System.out.print(i);// o/p: 789
        }
        System.out.println("\n-----------------");
        ch.changeWrapper(iWrapper);
        System.out.println(iWrapper);// o/p: 123
    }
}

class ChangeValues{
    void changeArray(int i[]){
        i[0]=7;
        i[1]=8;
        i[2]=9;
    }
    void changeWrapper(Integer i){
        i=789;
    }
}

output:

789
-----------------
123

Why is it that an array is able to change and not wrapper object. can any one clear away my doubts? thanks.

Answer 1

All arguments to Java methods are passed by value. When the argument is of a reference type (as opposed to a primitive type) then the reference is passed by value. Essentially, that means the method receives a copy of the caller's value.

Any reference to a mutable object may be used to modify that object, but that's different from modifying the variable holding the reference. The two change methods you provided are unlike in that way: changeArray() modifies the array object to which its argument refers, but changeWrapper() only assigns a new value to the local variable (initially) containing its argument.

Answer 2

Because the wrapper objects are immutable. You have to create a new instance, and since you can't modify a caller's reference you can't modify the wrapper instance. You could (if using Java 8) use an Optional, or create your own POJO and pass it (as long as you modify the POJO's reference, then a caller can access it from the POJO). Something like,

class POJO<T> {
    T v;

    public POJO(T v) {
        this.v = v;
    }

    public T getValue() {
        return v;
    }

    public void setValue(T v) {
        this.v = v;
    }

    @Override
    public String toString() {
        return String.valueOf(v);
    }
}

public static void changeIt(POJO<Integer> a) {
    a.setValue(123);
}

public static void main(String args[]) throws Exception {
    POJO<Integer> p = new POJO<>(1);
    System.out.println(p);
    changeIt(p);
    System.out.println(p);
}

Output is

1
123

Answer 3

Instead of using the Integer wrapper, use something like this (note that auto-boxing will be broken, so it will have to be done manually):

public class MyInteger {
    private int intValue;
    public MyInteger(int val) {
        intValue = val;
    }
    public void setValue(int val) {
        intValue = val;
    }
    pubic int intValue() {
        return intValue;
    }
}

Then your changeWrapper method can be:

void changeWrapper(MyInteger i){
    i.setValue(789);
}

And

    ch.changeWrapper(iWrapper);
    System.out.println(iWrapper.intValue());

Will print "789".

Alternatively, you could make intValue public:

public int intValue;

And then your changeWrapper method could become:

void changeWrapper(MyInteger i){
    i.intValue = 789;
}

This is effectively what an array is -- an object with a whole bunch of public fields.

Answer 4

The changeArray method gets a copy of the reference to the array object. Now, both its i and intArr from main refer to the same object. The array access assignment ensures that the element in the referred array is updated. There is only one array, and it gets modified. No reference variables are updated to refer to other objects.

The changeWrapper method gets a copy of the reference to the Integer object. Now, both its i and iWrapper from main refer to the same object. But this is not array access. Saying i = here changes the reference to another reference, the auto-boxed Integer 789. The original reference iWrapper in main is not updated, so it still refers to the original Integer 123, explaining the output.

Answer 5

Because when you pass a primitive as a parameter (Second case) you are taking a COPY of the primitive to be used for the method. However, when you use a non primitive or reference data type as a parameter (Your array), the argument is SHARED to be used for your method so whatever you do with this object will affect the original object.

You can find a better explanation of this here in the section "Passing parameters": http://pages.cs.wisc.edu/~bahls/cs302/PrimitiveVsReference.html

PD: Sorry for my English, it's so bad.