list,set,map的相互转换补充部分（对象数组）

忘是亡心i 2023-06-24 01:26 11阅读 0赞

各个集合相互转换源码，补充部分

import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.HashMap;
    import java.util.HashSet;
    import java.util.Iterator;
    import java.util.List;
    import java.util.Map;
    import java.util.Set;
    
    public class ZhuanHuanTest {
    
        /**
         * @param args
         * @author Wang YanMing
         */
        public static void main(String[] args) {
            // TODO Auto-generated method stub
    
            // /******list 转化为set,转化为set后就变成无序了*****/
            ArrayList<Student> list2 = new ArrayList<Student>();
            list2.add(new Student("k1", 4));
            list2.add(new Student("k2", 3));
            list2.add(new Student("k3", 5));
    //        for(Student s :list2)
    //        {
    //            System.out.println(s.name);//Arraylist是有序的，
    //        }
    //
            Set set = new HashSet(list2);// 转化语句，若不采用泛型则要强制转类型
    //         Set<Student> set = new HashSet<Student>(list2);//也可以
            
            System.out.println("******list 转化为set,转化为set后就变成无序了*****");
            Iterator<Student> it = set.iterator();// 遍历
            while (it.hasNext()) {
                Student s = it.next();
                System.out.println(s.name);//
            }
    /*===========================================================================*/
            // /************set转化为list*********/
            HashSet<Student> hashset = new HashSet<Student>();
            hashset.add(new Student("k1", 4));
            hashset.add(new Student("k2", 3));
            hashset.add(new Student("k3", 5));
    //        for(Student s : hashset)
    //        {
    //            System.out.println(s.name);//此时还是set，输出无序不唯一
    //        }
            System.out.println("**********set转化为list***********");
            ArrayList<Student> list = new ArrayList<Student>(hashset);
            for(Student s :list)
            {
                System.out.println(s.name);//每次转换都不一样，所以输出不唯一
            }
    /*============================================================================*/
            //数组转化为set
            Integer[] a = { 1, 2, 3 };//这里不能是int,因为set中存的是对象
    //        Set set = new HashSet(Arrays.asList(a));
            
            Set<Integer>  set2 = new HashSet<Integer>((List)Arrays.asList(a));//注意必须强制转换
            System.out.println("******数组转化为set*******");
            //遍历1
    //        for(Integer i :set)
    //        {
    //            System.out.println(i);
    //        }
            //遍历2
            Iterator it2 = set2.iterator();
            while(it2.hasNext())
            {
                System.out.println(it2.next());
            }
    /*===========================================================================*/
            
            
            Map map = new HashMap();
            map.put("1","a");
            map.put("2","b");
            map.put("3","c");
    //        System.out.println(map);//输出键值对
    //        System.out.println(map.values());//输出值的视图
            
            System.out.println("*******map的值转化为list*********");
            List list3 = new ArrayList(map.values());//map的值转化为list
            System.out.println(list3);
            
            ArrayList list4 = new ArrayList(map.values());//map的值转化为ArrayList
            System.out.println(list4);
            Set set3 = new HashSet(map.values());
            System.out.println(set3);//map的值转化为set
    //        
    //        //同理键也可以
    //        System.out.println(map.keySet());//输出键
    //        List list3 = new ArrayList(map.keySet());
    //        System.out.println(list3);
    //        Set set2 = new HashSet(map.keySet());
    //        System.out.println(set2);
            
    /*============================================================================*/
            
    
    /*============================================================================*/
    //         String数组与list互相转化,如果是整形数组要用Integer []
             List list5 = Arrays.asList("a","b");
            
             System.out.println("String数组与list互相转化");
             System.out.println(list5);
            
             String[] arr = (String[])list5.toArray(new String[list5.size()]);
             System.out.println(Arrays.toString(arr));
    /*===========================================================================*/
    
    //         list转化为对象数组
             System.out.println("list转化为对象数组");
             ArrayList<Student> list6 = new ArrayList<Student>();
             list6.add(new Student("k1", 4));
             list6.add(new Student("k2", 3));
             list6.add(new Student("k3", 5));
                //方法一、public <T> T[] toArray(T[] a)        
             Student[] stu = list6.toArray(new Student[list6.size()]);//new Student 是指定类型
             for(Student s :stu)
             {
             System.out.println(s.name);
             }
    //        
    //         //方法二、返回object将list转化为对象数组。详细看API
    //         //public Object[] toArray()
    //         Object obj[]  = list2.toArray();
    //         for(Object o : obj)
    //         {
    //             Student stu2 = (Student)o;
    //             System.out.println(stu2.name);
    //         }
    //        
             /********************************/
             //题外话：sublist
            
    //         List list3 =list2.subList(1, 2);//左闭右开，这里就是指索引值1
    //        
    //         for(Object  o : list3)//这里不能写成Student，要显式的转换
    //         {
    //             Student s = (Student)o;
    //         System.out.println("子集合"+s.name);
    //         }
             /***********************************/
            
            // 对象数组转化为list**********************
             System.out.println(" 对象数组转化为list");
             Student[] stu2 = new Student[3];
             stu2[0] = new Student("k4", 4);
             stu2[1] = new Student("k5", 6);
             stu2[2] = new Student("k6", 5);
                        
             List<Student> list7 = Arrays.asList(stu2);//只能转化为list
             for(Student s : list7)
             {
             System.out.println(s.name);
             }
    
        }
    
    }
    
    class Student {
        String name;
        int score;
    
        Student(String name, int score) {
            this.name = name;
            this.score = score;
    
        }
    }