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Let us try to override the method
equals in classes
BParticle and
CParticle ; the method returns true if the all fields of the
two objects are equal.
b1 and
b2 of type
BParticle and two unequal objects
c1 and
c2 of type
CParticle .b1.equals(b2) returns
true and
c1.equals(c2) returns
false .b1.equals(c1) returns
true : since
CParticle is a subclass of
BParticle , the variable
c1 is acceptable
as a parameter to the method
equals declared in
BParticle .
c1
is equal to
b1 , because we are only comparing
the first two fields inherited from
BParticle and these are equal.
Exercise Explain what happens if you try to evaluate
c1.equals(b1) .
Program: Inheritance07C.java
// Learning Object Inheritance07C
// equality (robust overriding)class Particle {
int position;
Particle(int p) { position = p;
}
void newPosition(int delta) {
position = position + delta; }
}
class BParticle extends Particle { int charge;
BParticle(int p, int c) {
super(p); charge = c;
}}
class CParticle extends BParticle {
boolean strange;
CParticle(int p, int c, boolean s) { super(p, c);
strange = s; }
void newPosition(int delta) {
if (strange) position = position * charge;
}
public boolean equals(Object obj) { if (obj == null) return false;
if (!(obj instanceof CParticle)) return false; CParticle c = (CParticle) obj;
return this.position == c.position && this.charge == c.charge && this.strange == c.strange;
}}
class Inheritance07C {
public static void main(/*String[] args*/) {
BParticle b1 = new BParticle(20, 2); CParticle c1 = new CParticle(20, 2, false);
CParticle c2 = new CParticle(20, 2, true); CParticle c3 = new CParticle(20, 2, false);
boolean eqc1null = c1.equals(null); boolean eqc1b1 = c1.equals(b1);
boolean eqc1c2 = c1.equals(c2); boolean eqc1c3 = c1.equals(c3);
}}
It would be unusual for two objects to be considered equal if they
are of different types, even if one type is a subclass of another.In fact,
public boolean equals(CParticle c) does not override the method
equals in
BParticle ,
because an overriding method must have the
same signature as the overridden
method.
The method
equals is declared in the root class
Object as:
public boolean equals(Object obj) and this is the method that must be overridden.
This program shows the correct technique:
null .Object to the type.Trace the execution of the program:
b1 of type
BParticle and three objects
c1 ,
c2 and
c3 of type
CParticle .c1 to
null or
b1 returns
false .CParticle :
c1.equals(c2) returns
false and
c1.equals(c3) returns
true .
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