jdk1.8新特性

jdk.1.8 新特性之集合

遍历集合中的对象:

     /**
     * java8 新特性
     * 遍历集合中的对象
     */
    @Test
    public  void  method01() {
        ArrayList<Person> list = new ArrayList<>();
        Person p1 = new Person("p1的名字",20);
        Person p2 = new Person("p2的名字",22);
        Person p3 = new Person("p3的名字", 30);
        list.add(p1);
        list.add(p2);
        list.add(p3);
      
        list.stream().forEach(person ->{
            System.out.println(person.getName());
        });
    }
    
    /**
     * 将 List 集合转化成 map 集合
     */
    @Test
    public void method2 () {
        ArrayList<Person> list = new ArrayList<>();
        Person p1 = new Person("p1的名字",20 , 1);
        Person p2 = new Person("p2的名字",22,2);
        Person p3 = new Person("p3的名字", 30,3);
        list.add(p1);
        list.add(p2);
        list.add(p3);
       //将id 作为 key 转成map
        Map<Integer, Person> map = list.stream().collect(Collectors.toMap(Person :: getId , a -> a ,(k1,k2)->k1));
        System.out.println(map);
       
    }
    
    
     /**
     * List 里面的对象元素, 以某个属性来分组
     */
    
    @Test
    public void method3 () {
        ArrayList<Person> list = new ArrayList<>();
        Person p1 = new Person("张三",20 , 1);
        Person p2 = new Person("李栋",22,2);
        Person p3 = new Person("李栋", 23,3); 
        Person p4 = new Person("李四", 34,3);
        list.add(p1);
        list.add(p2);
        list.add(p3);
        list.add(p4);
        
        //根据名称 name 将 list 分组
        Map<String, List<Person>> map = list.stream().collect(Collectors.groupingBy(Person::getName));
        System.out.println(map.toString());
    }
    
    
     /**
     *  从集合中过滤出符合条件的数据
     */
    @Test
    public void  method4() {
        ArrayList<Person> list = new ArrayList<>();
        Person p1 = new Person("张三",20 , 1);
        Person p2 = new Person("李栋",22,2);
        Person p3 = new Person("李栋", 23,3); 
        Person p4 = new Person("李四", 34,3);
        list.add(p1);
        list.add(p2);
        list.add(p3);
        list.add(p4);
        
        //取出名字为张三的用户信息
        List<Person> filterCtList = list.stream().filter(Person -> Person.getName().equals("张三")).collect(Collectors.toList());
        filterCtList.forEach(Person -> {
            System.out.println(Person.getName());
        });
    }
    
    
    /**
     * 将集合中的数据按照某个属性进行求和
     */
    @Test
    public void method5() {
        ArrayList<Person> list = new ArrayList<>();
        Person p1 = new Person("张三",20 , 1);
        Person p2 = new Person("李栋",22,2);
        Person p3 = new Person("李栋", 23,3); 
        Person p4 = new Person("李四", 34,3);
        list.add(p1);
        list.add(p2);
        list.add(p3);
        list.add(p4);
        
        int sum = list.stream().mapToInt(Person::getAge).sum();
        System.out.println(sum);
    }
    
     /**
     * 将list 转 map
     */
    @Test
    public void method6() {
        ArrayList<Person> list = new ArrayList<>();
        Person p1 = new Person("张三",20 , 1);
        Person p2 = new Person("李栋",22,2);
        Person p3 = new Person("李栋", 23,3); 
        Person p4 = new Person("李四", 34,4);
        list.add(p1);
        list.add(p2);
        list.add(p3);
        list.add(p4);
        
        Map<Integer, Person> map = list.stream().collect(Collectors.toMap(Person::getId, person -> person));
        System.out.println(map.toString());
    }
    

转载于:https://www.jianshu.com/p/6fa198649b6a