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By operating on String objects, the CaseInsensitiveString.equals() method violates the second contract requirement (symmetry). Because of the asymmetry, given a String object s and a CaseInsensitiveString object cis that differ only in case, cis.equals(s)) returns true while s.equals(cis) returns false.
Note that this code also violates MET13-J. Classes that define an equals() method must also define a hashCode() method.
Compliant Solution
In this compliant solution, the CaseInsensitiveString.equals() method is simplified to operate only on instances of the CaseInsensitiveString class, consequently preserving symmetry. The class also defines a hashCode() method.
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public final class CaseInsensitiveString {
private String s;
public CaseInsensitiveString(String s) {
if (s == null) {
throw new NullPointerException();
}
this.s = s;
}
public boolean equals(Object o) {
return o instanceof CaseInsensitiveString &&
((CaseInsensitiveString)o).s.equalsIgnoreCase(s);
}
public int hashCode() {
// ...
}
public static void main(String[] args) {
CaseInsensitiveString cis = new CaseInsensitiveString("Java");
String s = "java";
System.out.println(cis.equals(s)); // Returns false now
System.out.println(s.equals(cis)); // Returns false now
}
}
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