My set fails

Could anyone tell me why? How do I fix it?

package setTest;
import java.util.*;

public class SetTest implements Comparable<SetTest> {
    public String toString(){
        return this.s1 + " " + this.x1;
    }
    public static void display(){
        for(SetTest s : set){
            System.out.println(s);
        }
    }
    public static void populate(){
        for (int i = 1; i < 3; i++){
            set.add(new SetTest(1, "One"));
            set.add(new SetTest(2, "One"));
            set.add(new SetTest(1, "Two"));
        }
    }
    public String getX1(){
        return x1;
    }
    public int getS1(){
        return s1;
    }

    public boolean equals(Object o) {
        if (!(o instanceof SetTest))
            return false;
        SetTest s = (SetTest)o;
        if(s.getS1() == this.getS1() && s.getX1() == this.getX1())
            return true;
        else
            return false;
    }
    public int compareTo(SetTest s){
        if(this == s)
            return 1;
        else
            return -1;
    }

    public SetTest(int s, String x){
        s1 = s;
        x1 = x;
    }
    public static void main(String[] args){
        populate();
        System.out.println("The size of set is: " + set.size());
        display();
    }
    private int s1;
    private String x1;
    static protected Set<SetTest> set = new TreeSet<SetTest>();
}