LambdaGCTest.java

package com.mastfrog.lambda.gc;

import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import static org.junit.jupiter.api.Assertions.assertTrue;
import org.junit.jupiter.api.Test;

public class LambdaGCTest {

    // This test will fail - member references are the same as
    // creating a new WeakReference<Runnable>(new Whatever()) - it is
    // instantly garbage collected

    @Test
    public void testSomeMethod() {
        Touchable touchable = new Touchable();
        Listener listener = new Listener(touchable);
        for (int i = 0; i < 100; i++) {
            touchable.touch();
            listener.assertIncremented();
            System.gc();
        }
    }

    static class Touchable {

        private final List<WeakReference<Runnable>> listeners
                = new ArrayList<>(10);

        void listen(Runnable listener) {
            listeners.add(new WeakReference<>(listener));
        }

        void touch() {
            for (Iterator<WeakReference<Runnable>> it = listeners.iterator(); it.hasNext();) {
                WeakReference<Runnable> ref = it.next();
                Runnable toRun = ref.get();
                if (toRun == null) {
                    it.remove();
                } else {
                    toRun.run();
                }
            }
        }
    }

    static class Listener {
        private int lastTouchCount;
        private int touchCount;

        Listener(Touchable touchable) {
            touchable.listen(this::increment);
        }

        void increment() {
            touchCount++;
        }

        void assertIncremented() {
            assertTrue(touchCount > lastTouchCount, "Was not incremented");
            lastTouchCount = touchCount;
        }
    }

}