Java集合类之Set

1 介绍HashSet和TreeSet

1.1 接口继承图

• Set接口
  
  • SortedSet接口
    
    • TreeSet接口
  • HashSet实现类
  • LinkedHashSet实现类

1.2 共同点

元素对象唯一性（不重复），都不是线程安全的（Note that this implementation is not synchronized.）

1.3 差异点

• 实现方式：
  
  • HashSet: Hash表实现
  • TreeSet: 红黑树实现
• 存入对象：
  
  • HashSet: 可存入null
  • TreeSet: 不能添加null，添加对象必须是可排序的
• 其他差异：
  
  • HashSet: 添加对象要重写hashCode和equals方法
  • TreeSet: 元素必须实现Comparable接口，该接口中只用一个方法，就是compare()方法

2 比较HashSet和TreeSet

2.1 代码片段

package com.loongshaw;

import java.util.HashSet;
import java.util.Set;
import java.util.TreeSet;

public class SetComparation {

    public static void hashSetStringTest(){

         Set<String> set = new HashSet<String>();

         set.add("a");
         set.add("b");
         set.add("c");
         set.add("d");
         set.add("d");

         System.out.println("-------hashset 输出无序结果---------");  

         for(String value : set){  
                System.out.print(value+" ");  
         }  

         System.out.println();  
    }

    public static void hashSetStudentTest(){

         Set<Student> set = new HashSet<Student>();

         set.add(new Student(7));
         set.add(new Student(6));
         set.add(new Student(1));
         set.add(new Student(2));
         set.add(new Student(3));
         set.add(new Student(4));
         set.add(new Student(5));

         System.out.println("-------hashset 输出Student结果---------");  

         for(Student obj : set){  
                System.out.print(obj+" ");  
         }  

         System.out.println();  
    }

    public static void hashSetPersonTest(){

         Set<Person> set = new HashSet<Person>();

         set.add(new Person("a"));
         set.add(new Person("b"));
         set.add(new Person("c"));
         set.add(new Person("d"));
         set.add(new Person("e"));
         set.add(new Person("f"));
         set.add(new Person("g"));

         System.out.println("-------hashset 输出Person结果---------");  

         for(Person obj : set){  
                System.out.print(obj+" ");  
         }  

         System.out.println();  
    }


    public static void hashSetObjectTest(){

         Set<Integer> set = new HashSet<Integer>();

         set.add(6);
         set.add(3);
         set.add(2);
         set.add(1);
         set.add(4);  
         set.add(5);

         System.out.println("-------hashset 输出有序结果---------");  

         for(Integer value : set){  
                System.out.print(value+" ");  
         } 

         System.out.println();  
    }

    public static void treeSetStringTest(){

         Set<String> set = new TreeSet<String>();

         set.add("a");
         set.add("b");
         set.add("c");
         set.add("d");
         set.add("e");  
         set.add("A");

         System.out.println("-------treeset 输出有序结果---------");  

         for(String value : set){  
                System.out.print(value+" ");  
         } 

         System.out.println();  
    }

    public static void treeSetObjectTest(){

         Set<Integer> set = new TreeSet<Integer>();

         set.add(6);
         set.add(3);
         set.add(2);
         set.add(1);
         set.add(4);  
         set.add(5);

         System.out.println("-------treeset 输出有序结果---------");  

         for(Integer value : set){  
                System.out.print(value+" ");  
         } 

         System.out.println();  
    }

    public static void treeSetStudentTest(){

         @SuppressWarnings("unchecked")
        Set<Student> set = new TreeSet<Student>(new MyComparator());

         set.add(new Student("lili",18));
         set.add(new Student("jim",17));
         set.add(new Student("green",19));
         set.add(new Student("brown",20));
         set.add(new Student("jack",16));  
         set.add(new Student("ocean",15));

         System.out.println("-------treeset 输出有序结果---------");  

         for(Student value : set){  
                System.out.print(value.getName()+"-"+value.getAge() +" ");  
         } 

         System.out.println();  
    }


    public static void main(String[] args){

        hashSetStringTest();

        hashSetObjectTest();

        hashSetStudentTest();

        hashSetPersonTest();

        treeSetStringTest();

        treeSetObjectTest();

        treeSetStudentTest();
    }
}

2.2 Person对象类

package com.loongshaw;

public class Person {

    String name;

    public Person(String name){
        this.name = name;
    }

    @Override
    public String toString() {
        return this.name;
    }
    @Override
    public int hashCode() {
        return this.name.hashCode();
    }
    @Override
    public boolean equals(Object obj) {
        if (obj instanceof Person){
            Person  p = (Person) obj;
            if (p.name == this.name)
                return true;
        }
        return false;
    }

}

2.3 Student对象类

package com.loongshaw;

public class Student {

    int id;
    String name;
    int age;

    public Student(int id){
        this.id = id;
    }

    public Student(String name,int age){
        this.name = name;
        this.age = age;
    }

    public void setAge(int age){        
        this.age = age;
    }

    public int getAge(){
        return age;
    }

    public void setName(String name){
        this.name = name;
    }

    public String getName(){
        return name;
    }

    @Override
    public String toString() {
        return this.id+"";
    }
    @Override
    public int hashCode() {
        return this.id;
    }
    @Override
    public boolean equals(Object obj) {
        if (obj instanceof Student){
            Student  stu = (Student) obj;
            if (stu.id == this.id)
                return true;
        }
        return false;
    }

}

2.4 TreeSet实现Comparator接口，改写排序规则

package com.loongshaw;

import java.util.Comparator;

@SuppressWarnings("rawtypes")
public class MyComparator implements Comparator{  

        public int compare(Object o1, Object o2){  
            if(!((o1 instanceof Student)&&(o2 instanceof Student))){  
                throw new RuntimeException("Sorry");  
            }  
            Student s1 =(Student)o1;  
            Student s2 =(Student)o2;  

            int num = s1.getAge() -s2.getAge();  
            //int num =s1.getName().compareTo(s2.getName()); 
            //if(num ==0) 
            // return s1.getAge() -s2.getAge(); 
            return num;  
        }  

}

2.5 输出结果

-------hashset 输出无序结果---------
d b c a 
-------hashset 输出有序结果---------
1 2 3 4 5 6 
-------hashset 输出Student结果---------
1 2 3 4 5 6 7 
-------hashset 输出Person结果---------
f g d e b c a 
-------treeset 输出有序结果---------
A a b c d e 
-------treeset 输出有序结果---------
1 2 3 4 5 6 
-------treeset 输出有序结果---------
ocean-15 jack-16 jim-17 lili-18 green-19 brown-20