The heap and generations

Definition

Objects live on the heap. Most objects die young, so the collector splits the heap into a young area, where new objects start and are cleaned often and cheaply, and an old area for objects that survive. Cleaning the young area is called a minor collection.

Example code

Creating garbage and watching the collector run

import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;

class Demo {
    public static void main(String[] args) {
        long before = totalCollections();

        for (int i = 0; i < 3_000_000; i++) {
            byte[] temporary = new byte[1024];            // short-lived object: garbage right after this loop turn
        }

        long after = totalCollections();
        System.out.println("the collector ran while we created garbage: " + (after > before));
    }

    static long totalCollections() {
        long total = 0;
        for (GarbageCollectorMXBean gc : ManagementFactory.getGarbageCollectorMXBeans()) {
            total += gc.getCollectionCount();
        }
        return total;
    }
}

// Output:
// the collector ran while we created garbage: true