“Comparison method violates its general contract!”

Question

Can someone explain me in simple terms, why does this code throw an exception, \"Comparison method violates its general contract!\", and how do I fix it?

pri         


        

Answer 1

The violation of the contract often means that the comparator is not providing the correct or consistent value when comparing objects. For example, you might want to perform a string compare and force empty strings to sort to the end with:

if ( one.length() == 0 ) {
    return 1;                   // empty string sorts last
}
if ( two.length() == 0 ) {
    return -1;                  // empty string sorts last                  
}
return one.compareToIgnoreCase( two );

But this overlooks the case where BOTH one and two are empty - and in that case, the wrong value is returned (1 instead of 0 to show a match), and the comparator reports that as a violation. It should have been written as:

if ( one.length() == 0 ) {
    if ( two.length() == 0 ) {
        return 0;               // BOth empty - so indicate
    }
    return 1;                   // empty string sorts last
}
if ( two.length() == 0 ) {
    return -1;                  // empty string sorts last                  
}
return one.compareToIgnoreCase( two );