Java Reference
In-Depth Information
You cannot override equals() with arbitrary code. Doing so will probably prove
fied in the Java documentation for this method, and which I present next.
The equals() methodimplementsanequivalencerelationonnonnullobjectrefer-
It is reflexive : For any nonnull reference value x , x .equals( x ) returns true.
It is symmetric :Foranynonnullreferencevalues x and y , x .equals( y ) re-
turns true if and only if y .equals( x ) returns true.
It is transitive :Foranynonnullreferencevalues x , y ,and z ,if x .equals( y )
returnstrueand y .equals( z ) returnstrue,then x .equals( z ) returnstrue.
It is consistent :Foranynonnullreferencevalues x and y ,multipleinvocations
of x .equals( y ) consistentlyreturntrueorconsistentlyreturnfalse,provided
no information used in equals() comparisons on the objects is modified.
• For any nonnull reference value x , x .equals(null) returns false.
satisfy.Forproof,takealookattheimplementationofthe equals() methodin List-
ing 2-27 ' s Point class.
Listing 2-27. Logically comparing Point objects
class Point
private int x, y;
Point(int x, int y)
this.x = x;
this.y = y;
int getX() { return x; }
int getY() { return y; }
public boolean equals(Object o)
if (!(o instanceof Point))
return false;
Point p = (Point) o;
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