What is the default initialization of an array in Java?

Question

So I'm declaring and initializing an int array:

static final int UN = 0;
int[] arr = new int[size];
for (int i = 0; i < size; i++) {
    arr[i] = UN;
}

Say I do this instead...

int[] arr = new int[5];
System.out.println(arr[0]);

... 0 will print to standard out. Also, if I do this:

static final int UN = 0;
int[] arr = new int[5];
System.out.println(arr[0]==UN);

... true will print to standard out. So how is Java initializing my array by default? Is it safe to assume that the default initialization is setting the array indices to 0 which would mean I don't have to loop through the array and initialize it?

Thanks.

Answer 1

Everything in Java not explicitly set to something, is initialized to a zero value.

• For references (anything that holds an object) that is null.
• For int/short/byte that is a 0.
• For float/double that is a 0.0
• For booleans that is a false.

When you create an array of something, all entries are also zeroed. So your array contains five zeros right after it is created by new.

Answer 2

From the Java Language Specification:

* Each class variable, instance variable, or array component is initialized with a default value when it is created (§15.9, §15.10):
      o For type byte, the default value is zero, that is, the value of (byte)0.
      o For type short, the default value is zero, that is, the value of (short)0.
      o For type int, the default value is zero, that is, 0.
      o For type long, the default value is zero, that is, 0L.
      o For type float, the default value is positive zero, that is, 0.0f.
      o For type double, the default value is positive zero, that is, 0.0d.
      o For type char, the default value is the null character, that is, '\u0000'.
      o For type boolean, the default value is false.
      o For all reference types (§4.3), the default value is null.