Which mechanism is responsible for initial initialiazation of fields in Java

Question

As far as we all know Java object's fields is initialized in this way:

• boolean variables get false
• other primitive types (such as int, byte, long,) get 0
• objects get null

My question is which mechanism (method) is doing that job and when is it doing that?

Answer 1

These initializations are done when the new instance of the class is created by JVM . See Creation of new class instances section . It states:

Whenever a new class instance is created, memory space is allocated for it with room for all the instance variables declared in the class type and all the instance variables declared in each superclass of the class type, including all the instance variables that may be hidden (§8.3).

If there is not sufficient space available to allocate memory for the object, then creation of the class instance completes abruptly with an OutOfMemoryError. Otherwise, all the instance variables in the new object, including those declared in superclasses, are initialized to their default values (§4.12.5).

The default value is as follows for each type of instance variable:

• 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, '\' .
• For type boolean , the default value is false .
• For all reference types (§4.3), the default value is null .