Class overrides `compareTo()` but not `equals()`JAVA-W1056

This class implements Comparable<T> and overrides compareTo(), but it does not override equals() so that the implementations of compareTo and equals are in sync.

This will cause issues when performing comparison/equality checks, and may cause inconsistent behaviour when collections of this class are sorted.

Make sure to add a corresponding equals() implementation which agrees with compareTo().

This class defines a compareTo(...) method but inherits its equals() method from java.lang.Object. Generally, the value of compareTo() should return zero if and only if equals() returns true.

From the JavaDoc for Comparable<T>.compareTo():

It is strongly recommended, but not strictly required that (x.compareTo(y)==0) == (x.equals(y)). Generally speaking, any class that implements the Comparable interface and violates this condition should clearly indicate this fact. The recommended language is:

Note: this class has a natural ordering that is inconsistent with equals.

Bad Practice

class OnlyCompareTo implements Comparable<OnlyCompareTo> {
    int field1 = 0;

    @Override
    int compareTo(OnlyCompareTo other) {
        if (other == null) return 1;
        return Integer.compare(field1, other.field1);
    }
}

Recommended

Consider implementing an equals method that matches the behavior of the defined compareTo method.

class CompareToAndEquals implements Comparable<CompareToAndEquals> {
    int field1 = 0;

    @Override
    int compareTo(CompareToAndEquals other) {
        if (other == null) return 1;
        return Integer.compare(field1, other.field1);
    }

    @Override
    boolean equals(Object other) {
        return this == other || (other != null && other.field1 == this.field1);
    }
}