String concatenation vs String Builder in java

public class StringTest
{

    private int iterations;

    public static void main(String[] args) {
        StringTest test = new StringTest(20000);
        for(int i = 0; i < 10; i++) {
            test.concat();
            test.append();
            test.alloc();
            test.setval();

        }
    }

    public StringTest(int iterations)
    {
        this.iterations = iterations;
    }

    private void concat() {
        String foo = "";
        long start = System.currentTimeMillis();
        for(int i = 0; i < iterations; i++)
       {
            foo += "foo";
        }
        System.out.println("String concat took "+(System.currentTimeMillis()-start)+" ms");
    }

    private void append() {
        StringBuilder sb = new StringBuilder();
        long start = System.currentTimeMillis();
        for(int i = 0; i < iterations; i++) {
            sb.append("foo");
        }
        System.out.println("String append took "+(System.currentTimeMillis()-start)+" ms");
    }

    private void alloc() {
        byte[] z;

        long start = System.currentTimeMillis();
        for(int i = 0; i < iterations; i++) {
            z=new byte[3*iterations];

        }
        System.out.println("String alloc took "+(System.currentTimeMillis()-start)+" ms");
    }

    private void setval()
    {
        byte[] z=new byte[60000];

        long start = System.currentTimeMillis();
        for(int i = 0; i < iterations; i++) {
            for(int j=0;j<i*3;j++)
                z[j]=0;
        }
        System.out.println("String setval took "+(System.currentTimeMillis()-start)+" ms");
    }
}