Introduction to Objects - Advanced Topics in Java: Core Concepts in Data Structures - page 48

2.8.3 Comparing Object Variables
Consider the following that creates two identical, but separate, objects and stores their addresses in a and b :

Part a = new Part("Air Filter", 8.75);
Part b = new Part("Air Filter", 8.75);

Assume these statements create the situation shown in Figure 2-9 .

name:Air Filter
price: 8.75

name:Air Filter
price:8.75

2000
a

4000

b

Figure 2-9. After creation of two identical objects

Since the objects are identical, it may come as a surprise that the following condition is false :

a == b

However, if you remember that a and b contain addresses and not objects, then we are comparing the address
in a ( 2000 ) with the address in b ( 4000 ). Since these are different, the comparison is false .
Two object variables would compare equal only when they contain the same address (in which case they point to
the same object). This could happen, for instance, when we assign one object variable to another.
We would, of course, need to know whether two objects are the same. That is, if a and b point to two objects, are
the contents of these objects the same? To do this, we must write our own method that compares the fields, one by one.
Using the class Part as an example, we write a method equals , which returns true if one object is identical to
another and false otherwise. The method can be used as follows to compare Part objects pointed to by a and b :

if (a.equals(b)) ...

The method simply checks whether the name fields and the price fields of the two objects are the same. Since
the name fields are String objects, we call the equals method of the String class to compare them. 1

public boolean equals(Part p) {
return name.equals(p.name) && (price == p.price);
}

In the method, the variables name and price (without being qualified) refer to the fields of the object via which
the method is invoked. Suppose we had used the following expression:

a.equals(b)

1 There is no conflict in using the same name equals to compare Part s and String s. If equals is invoked via a Part object, then
the equals method from the Part class is used. If equals is invoked via a String object, then the equals method from the String
class is used.