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This noncompliant code example violates the second requirement in the contract (symmetry). This requirement means that if one object is equal to another then the other must also be equal to this one. Consider defines a CaseInsensitiveString class that defines a String and overrides the equals() method. The CaseInsensitiveString class knows about ordinary strings but the String class has no idea about knowledge of case-insensitive strings. As a resultConsequently, sCaseInsensitiveString.equalsIgnoreCaseequals(((CaseInsensitiveString)o).s) returns true but s.equalsIgnoreCase((String)o) always returns false) method should not attempt to interoperate with objects of the String class.

public final class CaseInsensitiveString {
  private String s;

  public CaseInsensitiveString(String s) {
    if (s == null) {
      throw new NullPointerException();
    } 
    this.s = s;
  }

  // This method violates asymmetrysymmetry
  public boolean equals(Object o) {
    if (o instanceof CaseInsensitiveString) {
      return s.equalsIgnoreCase(((CaseInsensitiveString)o).s);
    }

    if (o instanceof String) {
      return s.equalsIgnoreCase((String)o);
    }
    return false;
  }

  public static void main(String[] args) {
    CaseInsensitiveString cis = new CaseInsensitiveString("Java");
    String s = "java";
    System.out.println(cis.equals(s)); // Returns true
    System.out.println(s.equals(cis)); // Returns false
  }
}

Compliant Solution

As a result, a String object s and CaseInsensitiveString object cis that only differ in case, cis.equals(s)) returns true while s.equals(cis) returns false. This violates the second contract requirement (symmetry).

Compliant Solution

Do not try to inter-operate with String from within the equals() method. The new equals() method is highlighted in this compliant solution.

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