Fundamentals - SCJP Sun Certified Programmer for Java Platform SE6 - page 58

refer to it. Therefore, s1 == s2 evaluates to true . On the other hand, x1 and x2 are not
literals but actual String objects created dynamically at runtime, making them distinct
objects. Therefore, x1 and x2 point to different objects and cannot be equal. The output of
the ReferenceDemo program is
f1 != f2
today == now
s1 == s2
The important point to take from this discussion is that evaluating == and != on reference
types only compares whether or not the two references point to the same object. If you want to
compare the actual contents of two objects, the equals method is used, which we discuss next.
Equality of Objects
The exam objectives address the ability to “determine the equality of two objects or two
primitives.” As we saw in the previous section, you use the == operator to determine if
two primitives are equal. We also saw that two references are equal if and only if they
point to the same object. But what does it mean for two objects to be equal? (Don't forget:
references and objects are different entities!) As a Java programmer, you get to decide what
it means for two objects to be equal. The java.lang.Object class contains an equals
method with the following signature:
public boolean equals (Object obj)
The default implementation in Object tests for reference equality, which we can already
perform with == . The general rule of thumb is to override equals in all your classes to
defi ne what it means for two objects of your class type to be equal. Equality should be
based on the business logic of your application.
The equals Method
Because the equals method is defi ned in Object , you can invoke equals on any object,
passing in any other object. For example, the following statements are valid:
String s = “Hello”;
java.util.Date d = new java.util.Date();
boolean b = s.equals(d);
The value of b is false . Logic would tell us that a String object and a Date object should
never be equal, and that is the case. Typically two objects have to be of the same class
type for them to be equal. However, that doesn't stop you from comparing two objects of
different types, because the equals method can be invoked with any two objects.