Fortolkning af brug af memory: Runtime.getRuntime().freeMemory()

Hvordan bruger og fortolker man brug af memory? Jeg forsøger med:

import java.util.*;
public class Memory {
    public static void main(String[] args) {
        System.out.println(Runtime.getRuntime().totalMemory()+" "+Runtime.getRuntime().freeMemory());
        ArrayList x=new ArrayList();
        for (int i=0;i<1000000;i++) x.add(new Integer(i));
        System.out.println(Runtime.getRuntime().totalMemory()+" "+Runtime.getRuntime().freeMemory());
        ArrayList y=new ArrayList();
        for (int i=0;i<1000000;i++) y.add(new Integer(i));
        System.out.println(Runtime.getRuntime().totalMemory()+" "+Runtime.getRuntime().freeMemory());
        ArrayList z=new ArrayList();
        for (int i=0;i<1000000;i++) z.add(new Integer(i));
        System.out.println(Runtime.getRuntime().totalMemory()+" "+Runtime.getRuntime().freeMemory());
        ArrayList v=new ArrayList();
        for (int i=0;i<1000000;i++) v.add(new Integer(i));
        System.out.println(Runtime.getRuntime().totalMemory()+" "+Runtime.getRuntime().freeMemory());
    }
   
}

- og får flg. output:
2031616 1876408
24272896 1473320
62799872 18498232
66650112 6488824
java.lang.OutOfMemoryError