Question

In practice can I assume that all int arrays in Java will start out filled with zeros? for all machines in which the JVM runs?

Is this true for all types? char? boolean? enums?

Where is this officially documented?

The textbooks I have say that int arrays are set to zero but they also recommend that one should write a for-loop to set all values to zero just “to be clearer”.

Answer 1

Java Language Specification is the right place to look for such information:

Array components are unnamed variables that are created and initialized to default values (§4.12.5) whenever a new object that is an array is created

Default values themselves are given in section 4.12.5.

• For type byte, the default value is zero, that is, the value of (byte)0.
• For type short, the default value is zero, that is, the value of (short)0.
• For type int, the default value is zero, that is, 0.
• For type long, the default value is zero, that is, 0L.
• For type float, the default value is positive zero, that is, 0.0f.
• For type double, the default value is positive zero, that is, 0.0d.
• For type char, the default value is the null character, that is, ‘\u0000’.
• For type boolean, the default value is false.
• For all reference types, the default value is null.