为什么这个java equals()方法违反了对称实现规则？

Question

下面的代码是java equals方法的错误实现.它违反了对称规则.但我不知道它是如何违反这条规则的.请指出它在这种方法中违反对称规则的位置.

public class WrongEquals {

    private final String variable;

    public WrongEquals(String variable) {
            this.variable = variable;
    }

    @override public boolean equals(Object o) {
            if (this == o) {
                    return true;
            }
            if (o instanceof String) {
                    return variable.equals((String) o);
            }
            if (o instanceof WrongEquals) {
                    return variable.equals(((WrongEquals) o).variable);
            }
            return false;
    }

    @override
            public int hashCode() {
                    return (variable == null ? 0 : variable.hashCode());
            }
}

Answer 1

因为你的WrongEquals实例可以等于某些实例String,但不会String等于任何实例WrongEquals

看String的执行equals()(如JDK 7中)

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String) anObject;
        int n = value.length;
        if (n == anotherString.value.length) {
            char v1[] = value;
            char v2[] = anotherString.value;
            int i = 0;
            while (n-- != 0) {
                if (v1[i] != v2[i])
                        return false;
                i++;
            }
            return true;
        }
    }
    return false;
}

显然,此实现确保任何相等性仅存在于另一个String实例之间.