Java中List集合按需求转换为Map集合

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最新推荐文章于 2024-03-31 18:03:42 发布

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分类专栏： Java集合类文章标签： Java8新特性

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本文介绍了如何使用Java 8的Stream API将List<Faculty>转换为Map<Integer, List<String>>，以实现按专业ID分组并收集每个专业对应的班级名称。展示了两种方法，一种是使用Stream collect，另一种是使用增强for循环。

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一、List集合按照专业id对应的所包含班级名称name进行转换

class Faculty{
    //专业id
    private Integer id;
    //班级名称
    private String name;
    public Faculty(Integer id,String name){
        this.id = id;
        this.name= name;
    }
    public void setId(Integer id){
        this.id = id;
    }
    public Integer getId(){
        return id;
    }
    public void setName(String name){
        this.name= name;
    }
    public String getName(){
        return name;
    }
}


public class Main{
    public static void main(String[] args){
        //声明一个List集合
        List<Faculty> list = new ArrayList();  
        list.add(new Faculty(1001, "计科01"));  
        list.add(new Faculty(1002, "计科02"));  
        list.add(new Faculty(1003, "计科03"));
        System.out.println(list);
        //将list转换map
        Map<Integer, String> map = list.stream().collect(Collectors.toMap(Faculty::getId, Faculty::getName));                                                                                 
        System.out.println(map);
    }
}

二、将list转换map key为专业id， value是相同专业id的所包含的不同班级名称

1、使用Java8的新特性steam

class Faculty{
    //专业id
    private Integer id;
    //班级名称
    private String name;
    public Faculty(Integer id,String name){
        this.id = id;
        this.name= name;
    }
    public void setId(Integer id){
        this.id = id;
    }
    public Integer getId(){
        return id;
    }
    public void setName(String name){
        this.name= name;
    }
    public String getName(){
        return name;
    }
}


public class Main{
    public static void main(String[] args){
        //声明一个List集合
        List<Faculty> list = new ArrayList();  
        list.add(new Faculty(1001, "A"));  
        list.add(new Faculty(1002, "B"));  
        list.add(new Faculty(1002, "C"));
        list.add(new Faculty(1003, "D"));
        list.add(new Faculty(1003, "E"));
        list.add(new Faculty(1003, "F"));
        System.out.println(list);


        //将list转换map  key为专业id， value是 相同专业id的所包含的不同班级名称
        Map<Integer, List<String>> map = list.stream().collect(Collectors.toMap(Faculty::getId,
	    		p ->  {
	    		 	List<String> getNameList = new ArrayList<>();
	    		 		getNameList.add(p.getName());
	    		 		return getNameList;
	    		 	},
    		     	(List<String> value1, List<String> value2) -> {
    		     		value1.addAll(value2);
    		     		return value1;
    		     	}
	    		 ));
       
         System.out.println(map);

    }
}

2、使用增强for循环

class Faculty{
    //专业id
    private Integer id;
    //班级名称
    private String name;
    public Faculty(Integer id,String name){
        this.id = id;
        this.name= name;
    }
    public void setId(Integer id){
        this.id = id;
    }
    public Integer getId(){
        return id;
    }
    public void setName(String name){
        this.name= name;
    }
    public String getName(){
        return name;
    }
}


public class Main{
    public static void main(String[] args){
        //声明一个List集合
        List<Faculty> list = new ArrayList();  
        list.add(new Faculty(1001, "A"));  
        list.add(new Faculty(1002, "B"));  
        list.add(new Faculty(1002, "C"));
        list.add(new Faculty(1003, "D"));
        list.add(new Faculty(1003, "E"));
        list.add(new Faculty(1003, "F"));
        System.out.println(list);

		//将list转换map  key为专业id， value是 相同专业id的所包含的不同班级名称
		Map<Integer, List<String>> map = new HashMap<>();
		for(Faculty faculty : list){
			List<String> nameList = map.getOrDefault(faculty.getId(),new ArrayList<>());
			nameList.add(faculty.getName());
			map.put(faculty.getId(),nameList);
		}
        System.out.println(map);

    }
}