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When I have a class like this:
class Test {
private int _id = 0 ; // 4 bytes
private int _age = 0 ; // 4 bytes
}
I'm sure that each instance of it will consume more than 8 bytes in memory because of the 2 integers.
But what about methods? If I have a class with a million methods, and 2 instances of it, are the methods going to take twice as much place in memory ?
How does it work?
Thank you.
873
Methods are stored somewhere else in the memory. Notice that methods are per-class, not per-instance. So typically, the number of methods doesn't change over the run-time of a program (there are exceptions). In traditional models, the place where the methods live is called the "code segment".
The program is executed line by line, with jumps between methods. Execution causes the stored information to change. Static information (interface & class boxes) and instance information (object boxes) are stored in the heap. Method information is stored in the run-time stack.
In general a class or struct is a concept and does not occupy variable (data) space, but it does take up memory in the compiler's memory.
An object that is created using a class is said to be an instance of that class. We will sometimes say that the object belongs to the class. The variables that the object contains are called instance variables. The methods (that is, subroutines) that the object contains are called instance methods.
No. Methods occur only once in memory1. They don't vary on a per instance basis, so they don't need storage on a per-instance basis.
An object in Java basically consists of some fixed-size "housekeeping" (a pointer to the type information including the vtable), potentially GC-related bits (think mark and sweep), information about the monitor for the instance etc - and then the fields.
1 This is a bit of a simplification. There may be various representations, such as bytecode, native code etc - but that's regardless of separate instances.
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