Try to make JVM only do full GC

OnlyFullGc.java

class OnlyFullGc {
    private static final int _1M = 1024 * 1024;

    //java -verbose:gc -XX:+PrintGCDetails -XX:PretenureSizeThreshold=3145728 -Xms20M -Xmx20M -Xmn10M OnlyFullGc
    //1
    //2
    //[GC [Tenured: 9216K->4237K(10240K), 0.0110910 secs] 9543K->4237K(19456K), [Perm : 91K->91K(12288K)], 0.0113940 secs] [Times: user=0.01 sys=0.00, real=0.01 secs] 
    //3
    //Heap
    //def new generation   total 9216K, used 491K [0x8e510000, 0x8ef10000, 0x8ef10000)
    //eden space 8192K,   6% used [0x8e510000, 0x8e58af00, 0x8ed10000)
    //from space 1024K,   0% used [0x8ed10000, 0x8ed10000, 0x8ee10000)
    //to   space 1024K,   0% used [0x8ee10000, 0x8ee10000, 0x8ef10000)
    //tenured generation   total 10240K, used 9357K [0x8ef10000, 0x8f910000, 0x8f910000)
    //the space 10240K,  91% used [0x8ef10000, 0x8f833468, 0x8f833600, 0x8f910000)
    //compacting perm gen  total 12288K, used 91K [0x8f910000, 0x90510000, 0x93910000)
    //the space 12288K,   0% used [0x8f910000, 0x8f926eb8, 0x8f927000, 0x90510000)
    //ro space 10240K,  60% used [0x93910000, 0x93f1dba0, 0x93f1dc00, 0x94310000)
    //rw space 12288K,  61% used [0x94310000, 0x94a72ed0, 0x94a73000, 0x94f10000)

    public static void main(String[] args) {
        byte[] a0, a1;
        a0 = new byte[5 * _1M];
        System.out.println(1);
        a1 = new byte[4 * _1M];
        System.out.println(2);
        a0 = null;
        a0 = new byte[5 * _1M];
        System.out.println(3);
    }
}

second version works