Effective Java: Programming Language Guide in Java

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Then pequals(cp) returns true, while cpequals(p) returns false You might try to fix the problem by having ColorPointequals ignore color when doing mixed comparisons :
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//Broken - violates transitivity public boolean equals(Object o) { if (!(o instanceof Point)) return false; // If o is a normal Point, do a color-blind comparison if (!(o instanceof ColorPoint)) return oequals(this); // o is a ColorPoint; do a full comparison ColorPoint cp = (ColorPoint)o; return superequals(o) && cpcolor == color; }
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This approach does provide symmetry, but at the expense of transitivity:
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ColorPoint p1 = new ColorPoint(1, 2, ColorRED); Point p2 = new Point(1, 2); ColorPoint p3 = new ColorPoint(1, 2, ColorBLUE);
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At this point, p1equals(p2) and p2equals(p3) return true, while p1equals(p3) returns false, a clear violation of transitivity The first two comparisons are color-blind, while the third takes color into account So what's the solution It turns out that this is a fundamental problem of equivalence relations in object-oriented languages There is simply no way to extend an instantiable class and add an aspect while preserving the equals contract There is, however, a fine workaround Follow the advice of Item 14, Favor composition over inheritance Instead of having ColorPoint extend Point, give ColorPoint a private Point field and a public view method (Item 4) that returns the point at the same position as this color point:
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// Adds an aspect without violating the equals contract public class ColorPoint { private Point point; private Color color; public ColorPoint(int x, int y, Color color) { point = new Point(x, y); thiscolor = color; } /** * Returns the point-view of this color point */ public Point asPoint() { return point; }
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Effective Java: Programming Language Guide public boolean equals(Object o) { if (!(o instanceof ColorPoint)) return false; ColorPoint cp = (ColorPoint)o; return cppointequals(point) && cpcolorequals(color); } } // Remainder omitted
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There are some classes in the Java platform libraries that subclass an instantiable class and add an aspect For example, javasqlTimestamp subclasses javautilDate adding a nanoseconds field The equals implementation for Timestamp does violate symmetry and can cause erratic behavior if Timestamp and Date objects are used in the same collection or are otherwise intermixed The Timestamp class has a disclaimer cautioning the programmer against mixing dates and timestamps While you won't get into trouble as long as you don't mix them, there's nothing preventing you from doing so, and the resulting errors could be hard to debug The TimeStamp class is an anomaly and should not be emulated Note that you can add an aspect to a subclass of an abstract class without violating the equals contract This is important for the sort of class hierarchies that you get by following the advice in Item 20, Replace unions with class hierarchies For example, you could have an abstract Shape class with no aspects, a Circle subclass that adds a radius field, and a Rectangle subclass that adds length and width fields Problems of the sort just shown will not occur as long as it is impossible to create an instance of the superclass Consistency The fourth requirement of the equals contract says that if two objects are equal, they must remain equal for all time, unless one (or both) of them is modified This isn't so much a true requirement as a reminder that mutable objects can be equal to different objects at different times while immutable objects can't When you write a class, think hard about whether it should be immutable (Item 13) If you conclude that it should, make sure that your equals method enforces the restriction that equal objects remain equal and unequal objects remain unequal for all time Non-nullity The final requirement, which in the absence of a name I have taken the liberty of calling non-nullity, says that all objects must be unequal to null While it is hard to imagine accidentally returning true in response to the invocation oequals(null), it isn't hard to imagine accidentally throwing a NullPointerException The general contract does not allow this Many classes have equals methods that guard against it with an explicit test for null:
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public boolean equals(Object o) { if (o == null) return false; }
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This test is not necessary To test its argument for equality, the equals method must first cast the argument to an appropriate type so its accessors may be invoked or its fields accessed Before doing the cast, the method must use the instanceof operator to check that its argument is of the correct type:
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