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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.

  • Four objects are created: two equal objects b1 and b2 of type BParticle and two unequal objects c1 and c2 of type CParticle .
  • As expected, 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:

  • Since the parameter can now be any object, a check is first made that the parameter is not null .
  • Similarly, a check is made that the parameter is of the same type as this object.
  • Now that we know that the parameter is actually of this type, it can be cast from Object to the type.
  • Only then is class-specific code performed—usually a field-by-field comparison.

Trace the execution of the program:

  • Four objects are created: one object b1 of type BParticle and three objects c1 , c2 and c3 of type CParticle .
  • Clearly, comparing c1 to null or b1 returns false .
  • Field-by-field comparisons are used if the parameter is of type CParticle : c1.equals(c2) returns false and c1.equals(c3) returns true .

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Source:  OpenStax, Learning objects for java (with jeliot). OpenStax CNX. Dec 28, 2009 Download for free at http://cnx.org/content/col10915/1.2
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