Question

I get an error in my sorting method.

Comparison method violates its general contract

This is my sorting object with sort method

public abstract class ComparablePerson extends IDValueItem implements
        Comparable<ComparablePerson> {

    private int score;
    private String itemID,itemName;

    //setters and getters

    public int compareTo(ComparablePerson another) {
    if (score == another.getScore())
        return this.getItemName().compareToIgnoreCase(another.getItemName());
    else if ((score) > another.getScore())
        return 1;
    else
        return -1;
}

@Override
public boolean equals(Object o) {
    final ComparablePerson other = (ComparablePerson) o; 

    if (score == other.getScore() && this.getItemName().equalsIgnoreCase(other.getItemName())) 
        return true; 
    else 
        return false; 
}

I just call
Collections.sort(ComparablePersonCollection);

What can be the cause of this?

Answer 1

The compareTo and equals method implementations seem to be inconsistent, the error is telling you that for the same two objects equals gives true while compareTo does not produce zero, which is incorrect. I suggest you invoke compareTo from equals to ensure consistency or otherwise define a custom Comparator<T>.

Simply do:

public abstract class ComparablePerson extends IDValueItem implements Comparable<ComparablePerson> {

    private int score;
    private String itemID,itemName;

    //setters and getters

    public int compareTo(ComparablePerson another) {
    if (score == another.getScore())
        return this.getItemName().compareToIgnoreCase(another.getItemName());
    else if ((score) > another.getScore())
        return 1;
    else
        return -1;
    }

    @Override
    public boolean equals(Object o) {
        return compareTo(o) == 0; 
    }   
}