Boolean references are null

Question

Can anyone explain why this code results in the below output?

@Test
public void testBooleanArray() {
    Boolean[] ab = new Boolean[]{a, b};

    a = new Boolean(true);
    b = new Boolean(false);

    for(Boolean x : ab) {
        System.out.println(x);
    }
}

Result:

null
null

Should the array ab not holds pointers to object a and object b, and therefore output:

true
false

Answer 1

a = new Boolean(true);
b = new Boolean(false);

This does not change the objects that a and b were pointing to(the elements in the array). It points them to new objects.

It is not modfying the array

To illustrate:

Boolean a = new Boolean(true);
Boolean b = new Boolean(false);
Boolean c = a;
Boolean d = b;
a = new Boolean(false);
b = new Boolean(true);

c and d will still be true/false respectively. This is the same thing that is happening with the array, except your array reference isn't named the same way.

Answer 2

You have to initialize your booleans before assigning them.

Boolean[] ab = new Boolean[]{a, b};

a = new Boolean(true);
b = new Boolean(false);

to

a = new Boolean(true);
b = new Boolean(false);

Boolean[] ab = new Boolean[]{a, b};

This is before with Objects, you copy the reference to the object, and with new statement, you create a new object, the first a,b were null when assigning.

Answer 3

Your code unrolled:

Boolean a = null;
Boolean b = null;
Boolean[] ab = new Boolean[2];
ab[0] = a;
ab[1] = b;

a = new Boolean(true); 
b = new Boolean(false); 

The moment the content of the variables named a and b was copied to the array, it was set to null. There is an important difference to copy by value and copy by reference.

As a side-note: it is recommended to use Boolean.TRUE instead or at least Boolean.valueOf(true), to avoid unnecessary object creation. There aren't that many options for a boolean value and Boolean is immutable.

Answer 4

I think it's helpful to visualize the array elements as pointers.

We first create two pointers, a and b, both pointing to null.

Boolean a = null, b = null;

Next, we create two more pointers, ab[0] and ab[1], and point them at the same place as a and b. That is, null.

Boolean[] ab = new Boolean[]{a, b};

Then, we create new Boolean true and false objects (with the new Boolean(true) and new Boolean(false) parts of the statements).

Finally, we let a and b point to them.

a = new Boolean(true);
b = new Boolean(false);

When you look at it this way, I think it's more clear why changing a and b has no effect on the array.

Answer 5

... Which is absolutely normal. You are initializing values but, sequentially speaking, a and b are still null before the process gets round to assigning the variables. It's not the variables that are being placed but their values or references as elements in an array.

@Test
public void testBooleanArray() {
    /* We have a Boolean array, Boolean being able to hold a true, false but
    also a null as an object */
    Boolean[] ab = new Boolean[]{a, b}; /* We initialize the array with the variables
    here but ... */

    a = new Boolean(true); // ... the instances are only set at this stage
    b = new Boolean(false); /* Plus, even if Boolean is immutable, the use of new
    will actually create new objects */

    for(Boolean x : ab) {
        System.out.println(x);
    }
}