JAVA.IDEF.CTOEQ : compareTo/equals mismatch (Java)

Summary

compareTo() is defined is defined inconsistently from equals().

By convention the compareTo should return 0 if two instances are equal, and it should return 1 or -1 only if the instances are different in order to preserve the total ordering.

A class C that implements the raw interface java.lang.Comparable must implement the compareTo(Object) method. If C is made to implement the non-raw interface java.lang.Comparable<C>, then it must implement the compareTo(C) method instead.

Moreover, it is good practice to make compareTo() consistent with equals(): if the comparison of two objects yields 0, then they should be equal. The validity of this implication is in general undecidable. There are, however, frequent situations when, typically, this implication does not hold. An example is when equals() is inherited from java.lang.Object.

This checker verifies that compareTo(Object) is defined for classes implementing the raw java.lang.Comparable interface, instead of the (possibly more logical) method compareTo(C). Moreover, it verifies the consistency of compareTo() wrt equals().

Inconsistent definitions of compareTo()/equals() induce unexpected behaviors when objects are put inside most SortedSet classes of the standard Java library.

Properties

WARNING_FILTER += discard class="compareTo/equals mismatch (Java)"

Example

// CompareToVsEquals9.java
public final class CompareToVsEquals9 implements Comparable<CompareToVsEquals9> {
  private int f;
        
  public CompareToVsEquals9(int f) {
    this.f = f;
  }

  @Override
  public int compareTo(CompareToVsEquals9 o) {
    if (this == o || this.equals(o)) // "compareTo/equals mismatch (Java)" warning issued here
      return 1;
    else
      return -1;
  }

  @Override
  public boolean equals(Object other) {
    return other instanceof CompareToVsEquals9 && ((CompareToVsEquals9) other).f == f;
  }

  @Override
  public String toString() {
    return String.valueOf(f);
  }
}    

// Mango.java
public class Mango implements Comparable<Mango> {
  public int compareTo(Mango o) { // "compareTo/equals mismatch (Java)" warning issued here
      if (this == o || this.equals(o))
          return 1;
      else
          return -1;
  }
        
  //...
}

Resolution

RespectJava convention, the compareTo should return 0 if two instances are equal, and it should return 1 or -1 only if the instances are different in order to preserve the total ordering.

Relevant Configuration File Parameters

The following configuration file parameters affect checks for this warning class.

• REPORT_SIMILAR_WARNINGS
• WARNING_FILTER