Sort an object by different fields

Question

Hello!
Sorry for the (maybe) bad englisch but I'm a German.
If I have an object fith different fields for example field1, field2, field3.
To write a method which sorts an array of this objects by a given field?
I mean something like this
sort(String field, object[] o) {
//sort by field
instead of this
sortbyfield1(object o){
//sort by "field1"
sortbyfield2(object[] o){
//sort by "field2"
hope you understood
thanks
Stefan

Answer 1

When faced with this problem I usually use a Comparator helper class. Depending on whether the Comparator can be re-used I'll create a separate class or if not an inner or anonymous inner class. For example:
import java.util.Comparator;
import java.util.Arrays;
import java.util.List;
import java.util.Collections;
public class Main {
  // Compares Fields.f1 ascending
  public Comparator c1 = new Comparator() {
    public int compare(Object o1,
                       Object o2) {
      return ((Main.Fields)o1).f1.compareTo(((Main.Fields)o2).f1);
  public void sortem() {
    // Create our array of objects
    Object[] array = new Object[] {
      new Fields(new Integer(1),3,"3"),
      new Fields(new Integer(2),2,"1"),
      new Fields(new Integer(3),1,"2")
    // Sort on Field.f1 using reusable comparator
    Arrays.sort(array,c1);
    System.out.println(Arrays.asList(array));
    // Sort on Field.f2 using inner class
    Arrays.sort(array,new C2());
    System.out.println(Arrays.asList(array));
    // Sort on f3 using an anonymous inner class
    Arrays.sort(array, new Comparator(){
      public int compare(Object o1,
                       Object o2) {
        return((Main.Fields)o1).f3.compareTo(((Main.Fields)o2).f3);
    System.out.println(Arrays.asList(array));
  public static void main(String[] args) {
    Main m = new Main();
    m.sortem();
  // Class with many fields of different types
  class Fields {
    public Integer f1;
    public int     f2;
    public String  f3;
    public Fields(Integer f1, int f2, String f3) {
      this.f1 = f1;
      this.f2 = f2;
      this.f3 = f3;
    public String toString() {
      return ""+f1+" "+f2+" "+f3;
  // Inner class comparator on Field.f2
  class C2 implements Comparator {
    public int compare(Object o1,
                       Object o2) {
      int result = 0;
      int i1 = ((Main.Fields)o1).f2;
      int i2 = ((Main.Fields)o2).f2;
      if (i1==i1) result = 0;
      if(i1<i2) result = -1;
      else if (i1>i2) result = 1;
      return result;