经常看到面试题有这个问题,今天查了下资料mark一下。
我觉得英文的注释写的特别好,总结一下:
1)如果两个对象equals方法返回true,那么hashCode方法返回的值必然相同
2)如果两个对象equals方法返回false,但是不要求hashCode方法返回的值不同。
其实HashCode方法主要用于Set容器来提高性能,极大的减少调用equals方法的次数。
Set容器中有A对象,然后放入B对象。
如果B和A的hashCode不相同,可以确保A,B不相同,直接放入容器中。
如果B和A的hashCode相同,那么再判断equals方法是否相同。
/** * Returns a hash code value for the object. This method is * supported for the benefit of hashtables such as those provided by * <code>java.util.Hashtable</code>. * <p> * The general contract of <code>hashCode</code> is: * <ul> * <li>Whenever it is invoked on the same object more than once during * an execution of a Java application, the <tt>hashCode</tt> method * must consistently return the same integer, provided no information * used in <tt>equals</tt> 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 <tt>equals(Object)</tt> * method, then calling the <code>hashCode</code> method on each of * the two objects must produce the same integer result. * <li>It is <em>not</em> required that if two objects are unequal * according to the {@link java.lang.Object#equals(java.lang.Object)} * method, then calling the <tt>hashCode</tt> 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 hashtables. * </ul> * <p> * As much as is reasonably practical, the hashCode method defined by * class <tt>Object</tt> 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<font size="-2"><sup>TM</sup></font> programming language.) * * @return a hash code value for this object. * @see java.lang.Object#equals(java.lang.Object) * @see java.util.Hashtable */ public native int hashCode();