CODEWITHSUNDEEP
javacollectionscomparable
Sanjay Chandlekar
Sanjay Chandlekar

Reputation: 170

JAVA_Runtime_Error :- Comparison method violates its general contract

I am getting this error while I'm trying to sort the arraylist of nodes.I've tried most of the solutions, but none of them worked in my case.

java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.ComparableTimSort.mergeHi(ComparableTimSort.java:866)
at java.util.ComparableTimSort.mergeAt(ComparableTimSort.java:483)
at java.util.ComparableTimSort.mergeCollapse(ComparableTimSort.java:406)
at java.util.ComparableTimSort.sort(ComparableTimSort.java:213)
at java.util.Arrays.sort(Arrays.java:1312)
at java.util.Arrays.sort(Arrays.java:1506)
at java.util.ArrayList.sort(ArrayList.java:1454)
at j...

Code for this is

static class Node implements Comparable<Node>
{
    int key;
    int value;
    int double_value;

    public Node(int key , int value , int double_value)
    {
        this.key = key;
        this.value = value;
        this.double_value = double_value;
    }

    public int compareTo(Node node)
    {
        if(double_value < node.double_value)
            return 1;
        else if(double_value > node.double_value)
            return -1;

        return -1;
    }
}

It works for small inputs, but when the the number of inputs are large it gives this error. I've also read about the Transitivity rule in comparison method, but I can't figure out how it is applied in this case.

Thanks in advance.

Upvotes: 0

Views: 1137

Answers (1)

Stephen P
Stephen P

Reputation: 14810

Since you are actually comparing int values (despite it being named double_value) you can have a very simple compareTo method

public int compareTo(Node node)
{
    return this.double_value - node.double_value;
}

You don't have to return -1, 0, or +1 — any negative or positive value will do.
The documentation for compareTo() says:

Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

So

Node node1 = new Node(5, 5, 5);
Node node2 = new Node(7, 7, 7);
System.out.println( node1.compareTo(node2) );

will produce -2 because 5 is less than 7.

When you override compareTo you should also override .equals(Node other), and when you override equals you should also override hashCode().

Update for better safety per Chris Parker's comment, this goes back to using -1, 0, 1 for the results:

public int compareTo(Node other)
{
    final long m = (long) this.double_value = (long) other.double_value;
    return m < 0 ? -1 :
           m > 0 ?  1 :
           0;
}

Upvotes: 3

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