Memory leaks

Definition

Java frees unreachable objects, so a "leak" means objects you no longer need that are still reachable, for example items added to a static list that is never cleared. Keep caches bounded and remove listeners and entries when you are done.

Example code

An ever-growing static list, and a bounded cache instead

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

class Demo {
    static final List<byte[]> leaky = new ArrayList<>();     // static: reachable for the whole run, never cleared

    static class BoundedCache<K, V> extends LinkedHashMap<K, V> {
        private final int max;
        BoundedCache(int max) { super(16, 0.75f, true); this.max = max; }
        @Override protected boolean removeEldestEntry(Map.Entry<K, V> eldest) { return size() > max; }
    }

    public static void main(String[] args) {
        for (int i = 0; i < 1_000; i++) leaky.add(new byte[1024]);   // every item stays reachable: this grows forever
        System.out.println("leaky list size: " + leaky.size());

        BoundedCache<Integer, byte[]> cache = new BoundedCache<>(100);
        for (int i = 0; i < 1_000; i++) cache.put(i, new byte[1024]);
        System.out.println("bounded cache size: " + cache.size());   // old entries were removed
    }
}

// Output:
// leaky list size: 1000
// bounded cache size: 100