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Stack and heap

The two memory areas of an object program: the stack, with one frame per call, and the heap, where objects created by new live. References from one to the other, lifetime and overflow.

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  • Call stack: what is it?

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    The memory area where the frames of the running methods are stacked, the most recently called one on top.

    When main calls f which calls g, the stack holds main, f, g from bottom to top. When g returns, its frame is removed and f resumes.

  • What does this program print?

    ```java
    static void deplacer(Point p, int dx) {
    p.x += dx;
    }

    static void remettre(Point p) {
    p = new Point(0);
    }

    public static void main(String[] args) {
    Point pt = new Point(1);
    deplacer(pt, 2);
    remettre(pt);
    System.out.println(pt.x);
    }
    `

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    3

    deplacer receives a copy of the reference: p.x += dx modifies the heap object, which pt sees too, so x is 3. In remettre, p = new Point(0) only changes the frame's local variable, which disappears on return: pt still refers to the old object.

  • Each method call pushes a … that holds the parameters and local variables; it is popped when the method ….

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    Each method call pushes a frame that holds the parameters and local variables; it is popped when the method returns.

    Pushing and popping follow the order of calls: the last one called is the first to finish. This is what makes recursion possible.

  • Stack frame: what is it?

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    The area of the stack belonging to one method call, which holds its parameters and local variables.

    Each call has its own frame, even for a method calling itself: the local variables of two recursive calls are distinct. The frame disappears on return, and its variables with it.

  • A program stops with java.lang.StackOverflowError, the trace showing the same factorial method hundreds of times. What is the cause?

    • Recursive calls that never end, for lack of a base case
    • The heap is full: too many objects were created
    • A local variable went past the maximum value of an int
    • The garbage collector did not free the stack frames
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    Recursive calls that never end, for lack of a base case

    The repeated trace shows frames piling up. Frames do not depend on the garbage collector (they are removed on return), and a full heap gives another error, OutOfMemoryError.

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