关于hashCode与equals

标签：equals   hashcode   native   object   string   

public native int hashCode();

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

两个准则

因为有的时候equals方法比较复杂,我们如果通过hashcode方法已经说明两个对象不相等了就可以不用调用equals了

hashCode是个本地方法,返回的结果是对对象存储位置的一系列复杂运算;

代码说明

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;
    }

      /**
     * Returns a hash code for this string. The hash code for a
     * <code>String</code> object is computed as
     * <blockquote><pre>
     * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
     * </pre></blockquote>
     * using <code>int</code> arithmetic, where <code>s[i]</code> is the
     * <i>i</i>th character of the string, <code>n</code> is the length of
     * the string, and <code>^</code> indicates exponentiation.
     * (The hash value of the empty string is zero.)
     *
     * @return  a hash code value for this object.
     */
    public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;

            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

        String s1=new String("zhaoxudong");
        String s2=new String("zhaoxudong");
        String s3="zhaoxudong";
        String s4="zhaoxudong";
        System.out.println(s1==s2);      //两个new出来的对象 s1 s2里面放的是两个不同对象的地址 自然不相等
        System.out.println(s1==s3);         //false 一个指向堆内存 一个指向常量池
        System.out.println(s3==s4);      //true 都在常量池中
        System.out.println("##### ");
        System.out.println(s1.equals(s2));//true
        System.out.println(s1.hashCode());//s1.hashcode()等于s2.hashcode()等于s3.hashcode()
        System.out.println(s2.hashCode());
        System.out.println(s3.hashCode());
    
        Set hashset=new HashSet();
        hashset.add(s1);
        hashset.add(s2);
        Iterator it=hashset.iterator();
        while(it.hasNext())
            System.out.println(it.next());  //只打印出一个

import java.util.HashSet;
import java.util.Set;
import java.util.Iterator;

public class hashCode
{
    public static void main(String[] args)
    {

        HashSet hs=new HashSet();
        hs.add(new Student(1,"zhangsan"));
        hs.add(new Student(2,"lisi"));
        hs.add(new Student(3,"wangwu"));
        hs.add(new Student(1,"zhangsan"));
 
        Iterator it=hs.iterator();
        while(it.hasNext())
            System.out.println(it.next());
      }
}
public class Student
   {
     int num;
     String name;
     Student(int num,String name)
                {
                this.num=num;
                 this.name=name;
                 }
              public String toString()
                {
                    return num+":"+name;
                 }
   }   

public int hashCode()
{
            return num*name.hashCode();
}
public boolean equals(Object o)
{
            Student s=(Student)o;
            return num==s.num && name.equals(s.name);
} 

改完之后,就只有一个zhangsan了。

参考资料

http://www.iteye.com/topic/257191

关于hashCode与equals

标签：equals   hashcode   native   object   string   

原文地址：http://blog.csdn.net/dlf123321/article/details/40274355