Sort a vector of objects by one of its attributes

Question

Hi all!
I have created this class:
public class Client {
private int numero;
private double tempsEntrada;
private double tempsServei;
private double tempsIniciSessio;
private char tipus;
* Creates a new instance of Client
public Client(int num, double temp,double servei, char tip) {
numero = num;
arrivalTime = temp;
serviceTime = servei;
tipus = tip;
public char getTipus()
return tipus;
public int getNumero()
return numero;
public double getTempsEntrada()
return arrivalTime;
public double getTempsServei()
return serviceTime;
Now I've done a java vector of this object, and I want to sort it by arrivalTime.
The class comparer looks like this
public class Comparer implements Comparator {
public int compare(Object obj1, Object obj2)
double temps1 = ((Client)obj1).getTempsEntrada();
double temps2 = ((Client)obj2).getTempsEntrada();
return (int)(temps1 - temps2);
now when I do Collections.sort(vector);
this is the error reported:
Exception in thread "main" java.lang.ClassCastException: cafevirtual.Client cannot be cast to java.lang.Comparable
at java.util.Arrays.mergeSort(Arrays.java:1144)
at java.util.Arrays.mergeSort(Arrays.java:1155)
at java.util.Arrays.mergeSort(Arrays.java:1155)
at java.util.Arrays.sort(Arrays.java:1079)
at java.util.Collections.sort(Collections.java:117)
at cafevirtual.Main.main(Main.java:76)
Java Result: 1
what's wrong?
thanks!

Answer 1

Your Client class does not implement Comparable. You have to specify the Comparator to use: Collections.sort(vector, new Comparer());