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Exercise Move the declaration of
equals to class
BParticle , changing the code as needed. What now is the value of
c1.equals(b1) ? Explain.
Concept Assigning a variable containing a reference to another
of the same type merely copies thereference so that two fields refer to the same object. The method
clone is used to copy the content of an object into a new one.
clone is defined in class
Object and can be
overridden in any class definition.
Program: Inheritance08.java
// Learning Object Inheritance08
// cloneclass Particle implements Cloneable {
int position;
Particle(int p) { position = p;
}
void newPosition(int delta) { position = position + delta;
}
protected Object clone() { try {
Particle p = (Particle) super.clone(); p.newPosition(10);
return p; }
catch (CloneNotSupportedException e) { e.printStackTrace();
throw new Error(); }
}}
class Inheritance08 {
public static void main(String[] args) {
Particle p1 = new Particle(20); Particle p2 = p1;
p1.newPosition(10); System.out.println(p1.position);
System.out.println(p2.position);
Particle p3 = (Particle) p1.clone(); System.out.println(p1.position);
System.out.println(p3.position);
p3.newPosition(10); System.out.println(p1.position);
System.out.println(p3.position); }
}
clone is overridden in class
Particle .
The class must implement the interface
Cloneable , the
method of the superclass should be called, and we have to take intoaccount that the method might raise an exception. The method returns
the object returned by superclass method after calling
newPosition .
Particle is allocated and its reference
assigned to the field
p1 .p2 .
Check that they have the same value.newPosition is called on
p1 , but the
value of
p2.position is also changed, showing that the two fields
point to the same object.Particle is obtained by calling
p1.clone() and its reference assigned to the field
p3 . Since
clone returns a value of type
Object , it must be cast to type
Particle before the assignment.
Check that the objects referenced by
p1 and
p3 have different values.p3.newPosition changes only the field in the object
referenced by
p3 and not the separate object referenced by
p1 .
Exercise The method
clone can perform arbitrary
computation. Modify the program so that new objects are initialized withthe absolute value of the field of the object that is being cloned.
Concept Overloading is the use of the same method name with a different parameter signature. Overriding is the use in a subclass of the same method name with the same parameter signature as a method of the superclass.
Program: Inheritance09.java
// Learning Object Inheritance09
// overloading vs. overridingclass 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; }
void newPosition(double delta) {
if (position < delta)
spin = spin + delta; }
}
class Inheritance09 { public static void main(/*String[] args*/) { Particle p = new Particle(10);
AParticle a1 = new AParticle(20, -1.0); AParticle a2 = new AParticle(20, -1.0);
p.newPosition(10);
int pPosition = p.position; a1.newPosition(10);
int a1Position = a1.position; a2.newPosition(10.0);
int a2Position = a2.position; }
}
The method
newPosition(int delta) is declared in
Particle and
overridden in
AParticle . It is also
overloaded by a method
with the same name takes a parameter of type
double .
p ,
a1 and
a2 ,
newPosition is called on each one.p.newPosition calls the method declared in class
Particle .a1.newPosition calls the method declared
in class
AParticle that overrides the method in
Particle .a2.newPosition calls the overloaded method
because the actual parameter is of type
double .
Exercise At the end of the program add an assignment
p =
a1 . Add the method invocations
p.newPosition(10) and
p.newPosition(10.0) in the main method. Explain what happens.
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