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

Strings1 = new String(“kvill”),String s2 = new String(“kvill”);s1.equals(s2)为ture，

public native int hashCode(); 

public class TestEquals { 
  public static void main(String args[]) { 
    Student s1 = new Student("张一", 6); 
    Student s2 = new Student("张一", 6); 
if (s1.equals(s2)) { 
      System.out.println("相同 s1的代码:" + s1.hashCode() + " s2的代码:" 
          + s2.hashCode()); 
    } else { 
      System.out.println("不相同"); 
    } 
  } 
} 
class Student { 
  private int age; 
  private String name; 
   
  public Student() { 
  } 
public Student(String name, int age) { 
    this.age = age; 
    this.name = name; 
  } 
public int getAge() { 
    return age; 
  } 
public void setAge(int age) { 
    this.age = age; 
  } 
public String getName() { 
    return name; 
  } 
public void setName(String name) { 
    this.name = name; 
  } 
public int hashCode() { 
    return (this.name.hashCode() + this.age) * 31; 
  } 
public boolean equals(Object obj) { 
    boolean result = false; 
    if (obj == null) { 
      result = false; 
    } 
    if (this == obj) { 
      result = true; 
    } 
if (obj instanceof Student) { 
      Student stu = (Student) obj; 
      if (stu.getName().equals(this.name) && stu.getAge() == (this.age)) { 
        result = true; 
      } 
} else { 
      result = false; 
    } 
    return result; 
  } 
} 

public static void main(String args[]){ 
String s1=new String("zhaoxudong"); //此语句创建了两个对象，一个是字符串对象“zhaoxudong”（存放于栈中的字面量），另一个是new后在堆中产生的对象。详细见下面的四.4 
String s2=new String("zhaoxudong"); 
//上述两条语句一共是产生了三个对象，因为栈中只有产生了一个对象。 
System.out.println(s1==s2);//false 
System.out.println(s1.equals(s2));//true 
System.out.println(s1.hashCode());//s1.hashcode()等于s2.hashcode() ，指向同一内存的引用 
System.out.println(s2.hashCode()); //equals和hashCode方法只用于两个对象的比较和容器中，与对象的创建没有关系 
Set hashset=new HashSet(); 
hashset.add(s1); 
hashset.add(s2); /*在添加s1,s2时， hashset认为s1和s2是相等的，所以让s2覆盖了s1;*/ 
Iterator it=hashset.iterator(); 
      while(it.hasNext()){ 
       System.out.println(it.next()); 
      } //最后在while循环的时候只打印出了一个”zhaoxudong”。

>public class HashSetTest { 
  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()); 
         } } } 
class Student { 
   int num; 
   String name; 
   Student(int num,String name) { 
        this.num=num; 
         this.name=name; } 
   public String toString() { return num+":"+name; } 
      } 

          1:zhangsan
          1:zhangsan
          3:wangwu
          2:lisi 

class Student{ 
 
int num; 
 
String name; 
 
Student(int num,String name){ 
 
      this.num=num; 
 
      this.name=name; } 
 
public int hashCode(){ //重写hashCode的方法 
 
      return num*name.hashCode(); } 
 
public boolean equals(Object o) { 
 
      Student s=(Student)o; 
 
      return num==s.num && name.equals(s.name); //&&的优先级比==低，所以前面不必加括号 
 
} 
 
public String toString(){return num+":"+name; } 
 
}