Java Reference
In-Depth Information
Sometimes you want to treat two objects as equal if they have the same state based on some or all of their
instance variables. If you want to compare two objects of your class for equality based on criteria other than their
references (identities), your class needs to reimplement the
equals()
method of the
Object
class. The default
implementation of the
equals()
method in the
Object
class compares the references of the object being passed as the
parameter and the object on which the method is called. If the two references are equal, it returns
true
. Otherwise,
it returns
false
. In other words, the
equals()
method in the
Object
class performs identity based comparison for
equality. The implementation of the method is as follows. Recall that the keyword
this
inside an instance method of a
class refers to the reference of the object on which the method is called.
public boolean equals(Object obj) {
return (this == obj);
}
Consider the following snippet of code. It compares some
Point
objects using the equality operator (
==
), which
always compares the references of its two operands. It also uses the
equals()
method of the
Object
class to compare
the same two references. The output shows that the result is the same. Note that your
Point
class does not contain an
equals()
method. When you call the
equals()
method on a
Point
object, the
equals()
method's implementation of
the
Object
class is used.
Point pt1 = new Point(10, 10);
Point pt2 = new Point(10, 10);
Point pt3 = new Point(12, 19);
Point pt4 = pt1;
System.out.println("pt1 == pt1: " + (pt1 == pt1));
System.out.println("pt1.equals(pt1): " + pt1.equals(pt1));
System.out.println("pt1 == pt2: " + (pt1 == pt2));
System.out.println("pt1.equals(pt2): " + pt1.equals(pt2));
System.out.println("pt1 == pt3: " + (pt1 == pt3));
System.out.println("pt1.equals(pt3): " + pt1.equals(pt3));
System.out.println("pt1 == pt4: " + (pt1 == pt4));
System.out.println("pt1.equals(pt4): " + pt1.equals(pt4));
pt1 == pt1: true
pt1.equals(pt1): true
pt1 == pt2: false
pt1.equals(pt2): false
pt1 == pt3: false
pt1.equals(pt3): false
pt1 == pt4: true
pt1.equals(pt4): true
In practice, two points are considered the same if they have the same (x, y) coordinates. If you want to implement
this rule of equality for your
Point
class, you must reimplement the
equals()
method as shown in Listing 7-2.
