Stack and Heap Example: One Method, Five Things, Two Regions

Four lines of a method, and where each variable and object actually sits.

Inside a method, local variables sit in the method's stack frame, the space set aside for that one call, and every object they refer to sits on the heap. Four lines, and each one puts something in a different place:

java

void process() {
    int count = 3;
    String label = "total";
    List<String> rows = new ArrayList<>();
    rows.add(label);
}

Where each thing in the method lives
Thing	Lives	Gone when
`count`, holding 3	The stack frame	`process()` returns
`label`, holding an address	The stack frame	`process()` returns
The string `"total"`	The heap, in the pooled area	Effectively never, while the class is loaded
`rows`, holding an address	The stack frame	`process()` returns
The `ArrayList` object	The heap	Nothing can reach it any more

The three variables vanish together when the frame is discarded. The list does not vanish at that moment: it becomes unreachable, which is a different thing, and the collector removes it later. The pooled string stays reachable, still referred to, from the pool.

Notice that nothing in the method decided when the list would be freed. The frame going away only removed the last thing pointing at it.

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