Java Reference
In-Depth Information
defined in the Object class, it can be used to compare any two objects. However, the default
implementation in the Object class is equivalent to the “==” operator: it just checks if the
two objects are identical. Consider the following rewrite.
Superhero s1 = new Superhero( "Superman" ,5,5) ;
Superhero s2 = new Superhero( "Superman" ,5,5) ;
if (s1.equals(s2)) {
If the Superhero does not override the equals method, then the condition in the if state-
ment will be false again because the two objects are distinct. In order for the condition to
be true, we need to override the equals method in the Superman class by defining what
does it mean for two Supermans to be equal. Let us start with the FictionalCharacter
public class FictionalCharacter
private String name;
public boolean equals(Object other) {
if (other . getClass ()!= getClass ()) {
return false ;
return name. equals (((FictionalCharacter)other) .name) ;
The equals method compares the input object to the this object. If they have different
runtime types, then the objects cannot be the same. If they have the same runtime type, then
the input object must be of type FictionalCharacter . It therefore must have the attribute
name and we can simply compare the names of the two objects. Remember that strings
should always be compared using the equals method. The reason is that the creators of the
String class created a meaningful equals method for objects of type String . Remember as
well that the input to the equals method in the class Object is of type Object . Therefore,
when we override the equals method, we must perform the following tasks.
1. Determine the type of the input object.
2. Cast the input object to the appropriate type.
3. Compare the variables of the two objects.
Extra care should be taken when creating classes that have both the equals and
compareTo methods. Although not required, it is good programming practice that
o1.equals(o2) is true exactly when o1.compareTo(o2)==0 .
Next, let us focus on the Superhero class and its equal methods.
public class Superhero extends FictionalCharacter
private int goodPower ;
private int respect ;
public boolean equals(Object other) {
if (! super .equals(other)) {
return false ;
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