Method Call Stack and Activation Records

To understand how Java performs method calls, we first need to consider a data structure (i.e., collection of related data items) known as a stack. Students can think of a stack as analogous to a pile of dishes. When a dish is placed on the pile, it is normally placed at the top (referred to as pushing the dish onto the stack). Similarly, when a dish is removed from the pile, it is always removed from the top (referred to as popping the dish off the stack). Stacks are known as last-in, first-out (LIFO) data structuresthe last item pushed (inserted) on the stack is the first item popped (removed) from the stack.

When a program calls a method, the called method must know how to return to its caller, so the return address of the calling method is pushed onto the program execution stack (sometimes referred to as the method call stack). If a series of method calls occurs, the successive return addresses are pushed onto the stack in last-in, first-out order so that each method can return to its caller.

The program execution stack also contains the memory for the local variables used in each invocation of a method during a program's execution. This data, stored as a portion of the program execution stack, is known as the activation record or stack frame of the method call. When a method call is made, the activation record for that method call is pushed onto the program execution stack. When the method returns to its caller, the activation record for this method call is popped off the stack and those local variables are no longer known to the program. If a local variable holding a reference to an object is the only variable in the program with a reference to that object, when the activation record containing that local variable is popped off the stack, the object can no longer be accessed by the program and will eventually be deleted from memory by the JVM during "garbage collection." We'll discuss garbage collection in Section 8.10.

Of course, the amount of memory in a computer is finite, so only a certain amount of memory can be used to store activation records on the program execution stack. If more method calls occur than can have their activation records stored on the program execution stack, an error known as a stack overflow occurs.