Memory Leaks

A memory leak in Java occurs when heap objects are no longer needed by the program, but remain reachable from GC Roots. Because they are reachable, the GC cannot reclaim them, leading to an eventual OutOfMemoryError.

Common Memory Leak Sources

• Static Collections: Static fields persist for the lifetime of the application. Placing objects into a static collection (like HashMap) without cleanup leaks them.
• Unclosed Resources: Forgetting to close database connections, sockets, or file streams leaks OS descriptors and memory.
• Unregistered Listeners: Registering callbacks or event listeners without removing them preserves the subscriber instance reference.
• Array Slices / Custom Buffers: Failing to null out array elements in custom stacks or lists when popping items.

Code Walkthrough

The following class simulates a leak using custom arrays and demonstrates the fix.

public class LeakyStack {
    private Object[] elements = new Object[100];
    private int size = 0;
public void push(Object o) {
        elements[size++] = o;
    }
public Object pop() {
        if (size == 0) throw new IllegalStateException();
        // Return object but keep reference in array -> LEAK!
        // return elements[--size];
// FIX: Reclaim index allocation
        Object result = elements[--size];
        elements[size] = null; // Dereference pointer
        return result;
    }
}

Interview-Relevant Information