对java中equals和hashCode函数的一些理解

JDK的java.lang.Object类中实现了equals函数，其定义说明如下：

引用

public boolean equals(Object obj)

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

The equals method implements an equivalence relation on non-null object references:

[list=5]


It is reflexive: for any non-null reference value x, x.equals(x) should return true.

It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.

It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.

It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.

For any non-null reference value x, x.equals(null) should return false. [/list]

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

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

其具体的实现代码如下所示：

Java代码








public boolean equals(Object obj) {
return (this == obj);
}

public boolean equals(Object obj) {
        return (this == obj);
    }

从上面的代码中可以看出，Object类的equals实现只是简单地调用了“==”运算符，也即定义中说明的，对于两个非空对象X和Y，有且仅当X和Y指向同一个对象时该函数才返回true。由于Object类是java中所有类的超类，因而其它任何类都默认继承了此函数。在大多数情况下，此默认的实现方式是合适的，因为任一个类本质上都是唯一的；但是对于那些表示“值”的类(如Integer、String等)，它们引入了自己特有的“逻辑相等”的概念，此时就必须修改equals函数的实现。

而java.lang.Integer类的实现如下：

Java代码








public boolean equals(Object obj) {
if (obj instanceof Integer) {
return value == ((Integer)obj).intValue();
}
return false;
}

hashCode()函数返回一个对象的散列值(hash code)，在java中有些集合类都是基于散列值的，如HashMap、HashSet、Hashtable等；它们都根据对象的散列值将其映射到相应的散列桶中。如Hashtable的put和get函数的实现如下所示：

Java代码








public synchronized V get(Object key) {
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
return e.value;
}
}
return null;
}

ublic synchronized V put(K key, V value) {
// Make sure the value is not null

if (value == null) {
throw new NullPointerException();
}

// Makes sure the key is not already in the hashtable.

Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
V old = e.value;
e.value = value;
return old;
}
}

modCount++;
if (count >= threshold) {
// Rehash the table if the threshold is exceeded

rehash();

tab = table;
index = (hash & 0x7FFFFFFF) % tab.length;
}

// Creates the new entry.
Entry<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
return null;
}

public synchronized V get(Object key) {
	Entry tab[] = table;
	int hash = key.hashCode();
	int index = (hash & 0x7FFFFFFF) % tab.length;
	for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
	    if ((e.hash == hash) && e.key.equals(key)) {
		return e.value;
	    }
	}
	return null;
    }

public synchronized V put(K key, V value) {
	// Make sure the value is not null
	if (value == null) {
	    throw new NullPointerException();
	}

	// Makes sure the key is not already in the hashtable.
	Entry tab[] = table;
	int hash = key.hashCode();
	int index = (hash & 0x7FFFFFFF) % tab.length;
	for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
	    if ((e.hash == hash) && e.key.equals(key)) {
		V old = e.value;
		e.value = value;
		return old;
	    }
	}

	modCount++;
	if (count >= threshold) {
	    // Rehash the table if the threshold is exceeded
	    rehash();

            tab = table;
            index = (hash & 0x7FFFFFFF) % tab.length;
	} 

	// Creates the new entry.
	Entry<K,V> e = tab[index];
	tab[index] = new Entry<K,V>(hash, key, value, e);
	count++;
	return null;
    }

因而hashCode函数极大地影响了这些集合类的正常工作。如果在改写完equals函数后，不相应改写hashCode函数，则可能会得不到想要的结果。如下例所示：

Java代码








Point p1 = new Point(1, 2);
Point p2 = new Point(1, 2);
Hashtable<Point, String> ht = new Hashtable<Point, String>();
ht.put(p1, "value");
System.out.println(p1.equals(p2));
System.out.println(ht.get(p2));

Point p1 = new Point(1, 2);
Point p2 = new Point(1, 2);
Hashtable<Point, String> ht = new Hashtable<Point, String>();
ht.put(p1, "value");
System.out.println(p1.equals(p2));
System.out.println(ht.get(p2));

由上面Point类的实现，p1和p2是相等的，第一个语句正常输出true；但是第二个语句输出的值是null，并没有得到期望中的“value”。导致这一问题的根本原因是Point类改写了equals函数，对相等的概念作了更改，但没有相应更改hashCode函数，使得两个相等的函数拥有不同的hashCode，违反了Object类关于hashCode的约定中的第2点，从而返回了错误的结果。

在Object类中定义的几个hashCode约定如下：

1. 在同一应用中，一个对象的hashCode函数在equals函数没有更改的情况下，无论调用多少次，它都必须返回同一个整数。

2. 两个对象如果调用equals函数是相等的话，那么调用hashCode函数一定会返回相同的整数。

3. 两个对象如果调用equals函数是不相等的话，那么调用hashCode函数不要求一定返回不同的整数。

我们在改写equals 和 hashCode 函数的时候，一定要遵守如上3条约定，在改写equals的同时也改写hashCode的实现，这样才能保证得到正确的结果。