HashCode和Equals的区别

曾经终败给现在 2022-03-24 20:40 402阅读 0赞

我们首先来看一下HashCode的源码：

/**
 * Returns a hash code value for the object. This method is
 * supported for the benefit of hash tables such as those provided by
 * {@link java.util.HashMap}.
 * 
 * The general contract of {@code hashCode} is:
 * <ul>
 * <li>Whenever it is invoked on the same object more than once during
 * an execution of a Java application, the {@code hashCode} method
 * must consistently return the same integer, provided no information
 * used in {@code equals} comparisons on the object is modified.
 * This integer need not remain consistent from one execution of an
 * application to another execution of the same application.
 * <li>If two objects are equal according to the {@code equals(Object)}
 * method, then calling the {@code hashCode} method on each of
 * the two objects must produce the same integer result.
 * <li>It is not required that if two objects are unequal
 * according to the {@link java.lang.Object#equals(java.lang.Object)}
 * method, then calling the {@code hashCode} method on each of the
 * two objects must produce distinct integer results. However, the
 * programmer should be aware that producing distinct integer results
 * for unequal objects may improve the performance of hash tables.
 * </ul>
 * 
 * As much as is reasonably practical, the hashCode method defined by
 * class {@code Object} does return distinct integers for distinct
 * objects. (This is typically implemented by converting the internal
 * address of the object into an integer, but this implementation
 * technique is not required by the
 * Java™ programming language.)
 *
 * @return a hash code value for this object.
 * @see java.lang.Object#equals(java.lang.Object)
 * @see java.lang.System#identityHashCode
 */
 public native int hashCode();

刨到这种地步，也差不多了，我们看一下上面的注释是啥意思？

返回对象的Hash Code值，支持此方法的好处是可以使用\{@link java.util.HashMap\}.提供的哈希表

每当在Java应用程序执行期间在同一个对象上多次调用该方法时，  
\{@code hashCode\}方法必须一致地返回相同的整数，  
前提是不修改对象上的\{@code =\}比较中使用的信息。  
从应用程序的一次执行到同一应用程序的另一次执行，该整数不必保持一致。

String string = new String();
    string="bbbb";
    System.out.println(string.hashCode());
    string="dddd";
    System.out.println(string.hashCode());

![在这里插入图片描述][2019011922505266.png]

如果根据\{@code equals(Object)\}方法两个对象相等，  
那么在两个对象上调用\{@code hashCode\}方法必须产生相同的整数结果。

如果根据\{@link java.lang.Object\#equals(java.lang.Object)\}方法两个对象不相等，  
那么在两个对象上调用\{@code hashCode\}方法必须产生不同的整数结果，  
这是不必要的。但是，程序员应该意识到，为不相等的对象生成不同的整数结果可能会提高哈希表的性能。

在相当实际的情况下，类\{@code Object\}定义的hashCode方法确实为不同的对象返回不同的整数。  
(这通常是通过将对象的内部地址转换为整数来实现的，  
但是Java&trade并不需要这种实现技术;编程语言)。

/**
 * Indicates whether some other object is "equal to" this one.
 * 
 * The {@code equals} method implements an equivalence relation
 * on non-null object references:
 * <ul>
 * <li>It is reflexive: for any non-null reference value
 * {@code x}, {@code x.equals(x)} should return
 * {@code true}.
 * <li>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}.
 * <li>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}.
 * <li>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.
 * <li>For any non-null reference value {@code x},
 * {@code x.equals(null)} should return {@code false}.
 * </ul>
 * 
 * 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);
 }

1、{@code =}方法在非空对象引用上实现等价关系:
    2、它是自反的:对于任何非空引用值{@code x}，
      {@code x.equals(x)}应该返回{@code true}。
    3、它是对称的:对于任何非空的引用值{@code x}和{@code y}，
      当且仅当{@code y = (x)}返回{@code true}时，
      {@code x.equals(y)}应该返回{@code true}。
      
    
    
    4、 它是传递的:对于任何非空的参考值{@code x}， {@code y}， {@code z}，如果{@code x.equals(y)}返回{@code true}，
    {@code y.equals(z)}返回{@code true}，那么{@code x.equals(z)}应该返回{@code true}。
    
    5、它是一致的:对于任何非空引用值{@code x}和{@code y}，对{@code x.equals(y)}的多次调用
    一致返回{@code true}或一致返回{@code false}，但在对象被修改。
    6、对于任何非空引用值{@code x}， {@code x.equals(null)}应该返回{@code false}。
    
    7、{@code eauals}方法类{@codeObject}实现最歧视可能等价关系对象,也就是说,对于任何非空引用值{@code x}和
    {@code y},该方法返回{@code true}当且仅当{@code x}和{@code y}引用同一个对象({@code x = = y}
    {@code true})的值。
    
    8、注意，通常需要在重写该方法时重写{@code hashCode}方法，以便维护{@code hashCode}方法的通用契约，
    该契约声明相等的对象必须具有相等的散列代码。

[2019011922505266.png]: /images/20220325/ea94b28e7bb6440c9a32d5d8667db4e5.png