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Exercise It is possible to declare a nonabsract method in an abstract class. Give an example for this program, and explain why it is areasonable thing to do.
Concept There are two concepts of equality in Java: the
operator
== compares primitives types and references, while the
method
equals compares objects. The default implementation of
equals is like
== , but it can be overridden in any class.
Program: Inheritance07A.java
// Learning Object Inheritance07A
// equality (== vs. equals)class Particle {
int position;
Particle(int p) { position = p;
}
void newPosition(int delta) { position = position + delta;
}}
class AParticle extends Particle {
double spin;
AParticle(int p, double s) { super(p);
spin = s; }
void newPosition(int delta) {
if (spin < delta)
position = position + delta; }
}
class Inheritance07A { public static void main(/*String[] args*/) { AParticle a1 = new AParticle(20, 2.0);
AParticle a2 = a1; AParticle a3 = new AParticle(20, 2.0);
boolean eqop12 = a1 == a2; boolean eqop13 = a1 == a3;
boolean eqmethod = a1.equals(a3); }
}
a1 of type
AParticle is created.a1 is assigned to
a2 using
== .a3 of type
AParticle is created with the
same values for its fields as the object referenced by
a1 .a1==a2 returns
true because they both reference the same object.a1==a3 returns
false because they reference different objects.a1.equals(a3) returns
false .
Although their fields are equal, the default implementation of
equals is
the same as
== !
Exercise Add the follow method to
AParticle and run the
program again. What happens now?
public boolean equals(AParticle a) {
return this.position == a.position && this.spin == a.spin;
}
Program: Inheritance07B.java
// Learning Object Inheritance07B
// equality (overloading equals)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; }
public boolean equals(BParticle b) {
return this.position == b.position && this.charge == b.charge;
}}
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(CParticle c) { return this.position == c.position && this.charge == c.charge && this.strange == c.strange;
}}
class Inheritance07B {
public static void main(/*String[] args*/) {
BParticle b1 = new BParticle(20, 2); BParticle b2 = new BParticle(20, 2);
CParticle c1 = new CParticle(20, 2, false); CParticle c2 = new CParticle(20, 2, true);
boolean eqb1b2 = b1.equals(b2); boolean eqc1c2 = c1.equals(c2);
boolean eqb1c1 = b1.equals(c1); boolean eqc1b1 = c1.equals(b1);
}}
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