stream api 中 list to map and to list

秒速五厘米 2022-04-10 08:29 172阅读 0赞

学习stream api 记录一下笔记

public class StreamToMap {
    	List<Student> list = Arrays.asList(new Student(1, 18, "阿才"),
                new Student(2, 18, "小强" ),
                new Student(3, 18, "啊星"));
    		@Test
    		public void listToMap(){
    			  / /jdk1.8 lambda 表达式
    			 Map<Integer, Student> map = list.stream().collect(Collectors.toMap(Student::getId, student->student));
    			  // 遍历打印结果
    				map.forEach((key, value) -> {
    				System.out.println("key: " + key + "    value: " + value);
    			});
    		}
    		
    		@Test
    		public void listToMapByName){
    			// value 为对象中的属性
    			Map<Integer, String> map = list.stream().collect(Collectors.toMap(Student::getId, Student::getName));
    			map.forEach((key, value) -> {
    			System.out.println("key: " + key + "    value: " + value);
    		});
           }
    		
    		@Test
    		public void listToListByName(){
    			// value 为对象中的属性
    			List< String> list1 = list.stream().map(e -> e.getName()).collect(Collectors.toList());
    			list1.forEach((value) -> {
    			  System.out.println( "value: " + value);
    		});
           }
    		
    		@Test
    		public void listToList(){  
    			List<String> wordList = new ArrayList<>();
    			wordList.add("a");
    			wordList.add("b");
    			wordList.add("c");
    			// value 为对象中的属性
    			List<String> output = wordList.stream().
    					map(String::toUpperCase).
    					collect(Collectors.toList());		
    			output.forEach((value)-> System.out.println(value));
           }
    		
    		@Test
    		public void filtList() {
    			Integer[] sixNums = {1, 2, 3, 4, 5, 6};
    			Integer[] evens =
    			Stream.of(sixNums).filter(n -> n%2 == 0).toArray(Integer[]::new);  //过滤操作
    			System.out.println(evens[0]);
    		}
    
    }	
    		
    		
    class Student{	
    	Integer id;
    	Integer age;
    	String name;	
    	public Student(Integer id, Integer age, String name) {
    		super();
    		this.id = id;
    		this.age = age;
    		this.name = name;
    	}
    		public Integer getId() {
    			return id;
    		}
    		public void setId(Integer id) {
    			this.id = id;
    		}
    		public Integer getAge() {
    			return age;
    		}
    		public void setAge(Integer age) {
    			this.age = age;
    		}
    		public String getName() {
    			return name;
    		}
    		public void setName(String name) {
    			this.name = name;
    		}
    		
    	
    }