java中的equals()方法

Java的基类（Object类）提供了一些方法，其中equals()方法用于判断两个对象是否相等。

equals()方法不是final方法，可以被重写（Overwrite），因此各个引用类型的equals()方法的实现不尽相同。

Object类中的equals()方法

Object类中的equals()方法实现如下：

/**
 * Indicates whether some other object is "equal to" this one.
 * 

 * The {@code equals} method implements an equivalence relation
 * on non-null object references:
 * 

 * 
It is reflexive: for any non-null reference value
 *     {@code x}, {@code x.equals(x)} should return
 *     {@code true}.
 * 
It is symmetric: for any non-null reference values
 *     {@code x} and {@code y}, {@code x.equals(y)}
 *     should return {@code true} if and only if
 *     {@code y.equals(x)} returns {@code true}.
 * 
It is transitive: for any non-null reference values
 *     {@code x}, {@code y}, and {@code z}, if
 *     {@code x.equals(y)} returns {@code true} and
 *     {@code y.equals(z)} returns {@code true}, then
 *     {@code x.equals(z)} should return {@code true}.
 * 
It is consistent: for any non-null reference values
 *     {@code x} and {@code y}, multiple invocations of
 *     {@code x.equals(y)} consistently return {@code true}
 *     or consistently return {@code false}, provided no
 *     information used in {@code equals} comparisons on the
 *     objects is modified.
 * 
For any non-null reference value {@code x},
 *     {@code x.equals(null)} should return {@code false}.
 * 
 * 

 * The {@code equals} method for class {@code Object} implements
 * the most discriminating possible equivalence relation on objects;
 * that is, for any non-null reference values {@code x} and
 * {@code y}, this method returns {@code true} if and only
 * if {@code x} and {@code y} refer to the same object
 * ({@code x == y} has the value {@code true}).
 * 

 * Note that it is generally necessary to override the {@code hashCode}
 * method whenever this method is overridden, so as to maintain the
 * general contract for the {@code hashCode} method, which states
 * that equal objects must have equal hash codes.
 *
 * @param   obj   the reference object with which to compare.
 * @return  {@code true} if this object is the same as the obj
 *          argument; {@code false} otherwise.
 * @see     #hashCode()
 * @see     java.util.HashMap
 */
public boolean equals(Object obj) {
    return (this == obj);
}

从实现上可以看出，Object类的equals()方法实现采用的是区分度最高的算法，即只要两个对象不是同一个对象（同一个内存地址），那么equals()就一定返回false。

Object obj = new Object();
Object obj1 = new Object();
System.out.println(obj.equals(obj1)); // false

虽然我们在定义类（Java中所有类都默认继承自Object类）时，可以重写equals()方法，但是JDK中（在方法的注释上）说明了实现equals()方法应该遵守的一些约定。

1.自反性（Reflexive）：x.equals(x)必须返回true。

2.对称性（Symmetric）：x.equals(y)与y.equals(x)的返回值必须相等。

3.传递性（Transitive）：如果x.equals(y)为true，y.equals(z)也为true，那么x.equals(z)必须为true。

4.一致性（Consistence）：如果对象x和y在equals()中使用的信息都没有改变，那么x.equals(y)值始终不变。

5.非null（Non-null）：如果x不是null，y为null，则x.equals(y)必须为false。

String类中的equals()方法

String类中的equals()方法实现如下：

/**
 * Compares this string to the specified object.  The result is {@code
 * true} if and only if the argument is not {@code null} and is a {@code
 * String} object that represents the same sequence of characters as this
 * object.
 *
 * @param  anObject
 *         The object to compare this {@code String} against
 *
 * @return  {@code true} if the given object represents a {@code String}
 *          equivalent to this string, {@code false} otherwise
 *
 * @see  #compareTo(String)
 * @see  #equalsIgnoreCase(String)
 */
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;
}

可以看出，String类中的equals()方法内部采用的是每个字符逐一做比较的算法，意思就是，只要两个字符串的值相等而不需要内存地址也相等，equals()方法的结果就是true。

String str = "yanggb";
String str1 = new String("yanggb");
System.out.println(str == str1); // false
System.out.println(str.equals(str1)); // true

String类是比较特殊的，因为JVM中有一个字符串常量池的概念，相同的字面量声明实际上都是指向的同一个内存地址（都在字符串常量池中），只有new出来的String对象，才是在堆中开辟空间，并指向不同的内存地址。

 

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