直接看代碼,為什么我直接添加一個String對象contains()返回的是true,添加一個Dog對象的時候就返回false，還有Set不是不能添加一個重復元素，為什么Dog對象可以添加兩個，String對象則不能


class Dog{

String color;
public Dog(String s){
    color = s;
}   

}

public class SetAndHashCode {

public static void main(String[] args) {
    HashSet<Dog> dogSet = new HashSet<Dog>();
    boolean resultq;
    dogSet.add(new Dog("we have white"));
    System.out.println("We have " + dogSet.size() + " white dogs!");
    resultq = dogSet.contains(new Dog("we have white"));
    System.out.println(resultq);
       
    HashSet<String> books = new HashSet<String>();    
    //添加一個字符串對象  
    books.add(new String("Struts2權威指南"));  
    books.add(new String("Struts2權威指南"));
    boolean result = books.contains(new String("Struts2權威指南")); 
    System.out.println("We have " + books.size() + " books!");         
    System.out.println(result);          
    //下面輸出看到集合只有一個元素  
    System.out.println(books);    
}

}
執(zhí)行結果是：
**We have 2 white dogs!
false
We have 1 books!
true
[Struts2權威指南]**