CODEWITHSUNDEEP
javabooleanautoboxing
StuPointerException
StuPointerException

Reputation: 7267

Boolean references are null

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

Upvotes: 13

Views: 1282

Answers (5)

Flight Odyssey
Flight Odyssey

Reputation: 2287

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;

Pointers a and b point to 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};

All four pointers point to null

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

Two new Booleans have been created

Finally, we let a and b point to them.

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

a and b point to the new Booleans

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

Upvotes: 4

TwoThe
TwoThe

Reputation: 14269

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.

Upvotes: 6

James P.
James P.

Reputation: 19607

... 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);
    }
}

Upvotes: 3

Cruncher
Cruncher

Reputation: 7812

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.

Upvotes: 21

RamonBoza
RamonBoza

Reputation: 9038

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.

Upvotes: 8

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