StackOverflow2013

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copied!<p>A clarification about the <code>obj.getClass() != getClass()</code>.</p> <p>This statement is the result of <code>equals()</code> being inheritance unfriendly. The JLS (Java language specification) specifies that if <code>A.equals(B) == true</code> then <code>B.equals(A)</code> must also return <code>true</code>. If you omit that statement inheriting classes that override <code>equals()</code> (and change its behavior) will break this specification.</p> <p>Consider the following example of what happens when the statement is omitted:</p> <pre><code> class A { int field1; A(int field1) { this.field1 = field1; } public boolean equals(Object other) { return (other != null && other instanceof A && ((A) other).field1 == field1); } } class B extends A { int field2; B(int field1, int field2) { super(field1); this.field2 = field2; } public boolean equals(Object other) { return (other != null && other instanceof B && ((B)other).field2 == field2 && super.equals(other)); } } </code></pre> <p>Doing <code>new A(1).equals(new A(1))</code> Also, <code>new B(1,1).equals(new B(1,1))</code> result give out true, as it should.</p> <p>This looks all very good, but look what happens if we try to use both classes:</p> <pre><code>A a = new A(1); B b = new B(1,1); a.equals(b) == true; b.equals(a) == false; </code></pre> <p>Obviously, this is wrong.</p> <p>If you want to ensure the symmetric condition. a=b if b=a and the Liskov substitution principle call <code>super.equals(other)</code> not only in the case of <code>B</code> instance, but check after for <code>A</code> instance:</p> <pre><code>if (other instanceof B ) return (other != null && ((B)other).field2 == field2 && super.equals(other)); if (other instanceof A) return super.equals(other); else return false; </code></pre> <p>Which will output:</p> <pre><code>a.equals(b) == true; b.equals(a) == true; </code></pre> <p>Where, if <code>a</code> is not a reference of <code>B</code>, then it might be a be a reference of class <code>A</code> (because you extend it), in this case you call <code>super.equals()</code> <strong>too</strong>.</p>

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