对HashMap进行排序

古城微笑少年丶 2022-11-28 13:34 180阅读 0赞

**目录**

初始化数据

通过TreeMap

通过ArrayList

对Key进行排序

对Value进行排序

通过Lambdas and Streams

对Key进行排序

对Value进行排序

--------------------

## 初始化数据 ##

Map<String, Employee> map = new HashMap<>();
     
    Employee employee1 = new Employee(1L, "Mher");
    map.put(employee1.getName(), employee1);
    Employee employee2 = new Employee(22L, "Annie");
    map.put(employee2.getName(), employee2);
    Employee employee3 = new Employee(8L, "John");
    map.put(employee3.getName(), employee3);
    Employee employee4 = new Employee(2L, "George");
    map.put(employee4.getName(), employee4);

另外需要注意一下Employee是实现了Comparable接口的

public class Employee implements Comparable<Employee> {
     
        private Long id;
        private String name;
     
        // constructor, getters, setters
     
        // override equals and hashCode
        @Override
        public int compareTo(Employee employee) {
            return (int)(this.id - employee.getId());
        }
    }

## 通过TreeMap ##

TreeMap<String, Employee> sorted = new TreeMap<>(map);
    // or
    TreeMap<String, Employee> sorted = new TreeMap<>();
    sorted.putAll(map);

输出结果

> Annie=Employee\{id=22, name='Annie'\}  
> George=Employee\{id=2, name='George'\}  
> John=Employee\{id=8, name='John'\}  
> Mher=Employee\{id=1, name='Mher'\}

## 通过ArrayList ##

### 对Key进行排序 ###

List<String> employeeByKey = new ArrayList<>(map.keySet());
    Collections.sort(employeeByKey);

输出：\[Annie, George, John, Mher\]

### 对Value进行排序 ###

List<Employee> employeeById = new ArrayList<>(map.values());
    Collections.sort(employeeById);

## 通过**Lambdas and Streams** ##

### 对Key进行排序 ###

map.entrySet()
      .stream()
      .sorted(Map.Entry.<String, Employee>comparingByKey())
      .forEach(System.out::println);

输出结果

> Annie=Employee\{id=22, name='Annie'\}  
> George=Employee\{id=2, name='George'\}  
> John=Employee\{id=8, name='John'\}  
> Mher=Employee\{id=1, name='Mher'\}

###  对Value进行排序 ###

map.entrySet()
      .stream()
      .sorted(Map.Entry.comparingByValue())
      .forEach(System.out::println)

输出结果

> Mher=Employee\{id=1, name='Mher'\}  
> George=Employee\{id=2, name='George'\}  
> John=Employee\{id=8, name='John'\}  
> Annie=Employee\{id=22, name='Annie'\}

可以将排序后的结果放到一个新的Map集合中

Map<String, Employee> result = map.entrySet()
      .stream()
      .sorted(Map.Entry.comparingByValue())
      .collect(Collectors.toMap(
        Map.Entry::getKey, 
        Map.Entry::getValue, 
        (oldValue, newValue) -> oldValue, LinkedHashMap::new));