JAVA.IDEF.CTONOEQ : compareTo without equals (Java)

Summary

compareTo() is defined but equals() is inherited from Object.

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 without equals (Java)"

Example

Consider now the following program:

public abstract class CompareTo implements Comparable<CompareTo> { // "compareTo without equals (Java)" warning issued here
  private static int nextId;
  private final int id = nextId++;

  public final int compareTo(CompareTo other) {
    return id - other.id;
  }
}

class A extends CompareTo {
  private final String name;

  public A(String name) {
    this.name = name;
  }
}

A warning is issued because there might be non-equal instances of class A whose comparison yields 0. In this example, the programmer could modify class CompareTo as follows.

public abstract class CompareTo implements Comparable<CompareTo> {
  private static int nextId;
  private final int id = nextId++;

  @Override
  public final int compareTo(CompareTo other) {
    return id - other.id;
  }

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

The following implementation of compareTo() is not symmetrical. In addition, it might return 0 when the inherited equals() method returns false.

public final class CompareToVsEquals3 implements Comparable<CompareToVsEquals3> {
  private int f;
        
  public CompareToVsEquals3(int f) {
    this.f = f;
  }

  @Override
  public int compareTo(CompareToVsEquals3 o) { // Asymmetric compareTo (Java) warning issued here
                                               // compareTo without equals (Java) warning issued here
    if (f > 0)
      return -1;
    else
      return 0;
  }

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

The following code corrects these problems.

public final class CompareToVsEquals3 implements Comparable<CompareToVsEquals3> {
  private int f;
        
  public CompareToVsEquals3(int f) {
    this.f = f;
  }

  @Override
  public int compareTo(CompareToVsEquals3 o) {
    return f - o.f;
  }

  @Override
  public boolean equals(Object other) {
    return other instanceof CompareToVsEquals3 && ((CompareToVsEquals3) other).f == f;
  }

  @Override
  public int hashCode() {
    return f;
  }

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

Resolution

Use the non-raw interface java.lang.Comparable<C>; make equals() consistent with compareTo(). In most cases, this just amounts to providing the missing definition of equals().

Relevant Configuration File Parameters

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

• REPORT_SIMILAR_WARNINGS
• WARNING_FILTER