根据Java中的键对 HashMap 进行排序

我们以 HashMap 的形式给出学生得分的详细信息，其中学生的名字是 Key，得分是 Value。我们的任务是根据键值对地图进行排序，即按字母（字典）顺序排列的学生姓名。
例子：

Input : Key = Jayant, Value = 80
        Key = Anushka, Value = 80
        Key = Amit, Value = 75
        Key = Abhishek, Value = 90
        Key = Danish, Value = 40
Output : Sorted Map according to Names:
         Key = Abhishek, Value = 90
         Key = Amit, Value = 75
         Key = Anushka, Value = 80
         Key = Danish, Value = 40
         Key = Jayant, Value = 80

使用 TreeMap（putAll 方法）

想法是将HashMap的所有数据放入TreeMap中。 TreeMap 遵循基于红黑树的实现。地图根据其键的自然顺序进行排序。点击这里查看更多。

Java

// Java Code to sort Map by key value
import java.util.*;
class sortmapKey {
 
    // This map stores unsorted values
    static Map map = new HashMap<>();
 
    // Function to sort map by Key
    public static void sortbykey()
    {
        // TreeMap to store values of HashMap
        TreeMap sorted = new TreeMap<>();
 
        // Copy all data from hashMap into TreeMap
        sorted.putAll(map);
 
        // Display the TreeMap which is naturally sorted
        for (Map.Entry entry : sorted.entrySet())
            System.out.println("Key = " + entry.getKey() +
                         ", Value = " + entry.getValue());       
    }
     
    // Driver Code
    public static void main(String args[])
    {
        // putting values in the Map
        map.put("Jayant", 80);
        map.put("Abhishek", 90);
        map.put("Anushka", 80);
        map.put("Amit", 75);
        map.put("Danish", 40);
 
        // Calling the function to sortbyKey
        sortbykey();
    }
}

输出

Key = Abhishek, Value = 90
Key = Amit, Value = 75
Key = Anushka, Value = 80
Key = Danish, Value = 40
Key = Jayant, Value = 80

注意： TreeMap 为 containsKey、get、put 和 remove 操作提供有保证的log(n)时间成本。

使用 TreeMap（构造函数）

Java

// Java Code to sort Map by key value
import java.util.*;
class sortmapKey {
 
    // This map stores unsorted values
    static Map map = new HashMap<>();
 
    // Function to sort map by Key
    public static void sortbykey()
    {
        // TreeMap to store values of HashMap
        TreeMap sorted
            = new TreeMap<>(map);
 
        // Display the TreeMap which is naturally sorted
        for (Map.Entry entry :
             sorted.entrySet())
            System.out.println("Key = " + entry.getKey()
                               + ", Value = "
                               + entry.getValue());
    }
 
    // Driver Code
    public static void main(String args[])
    {
        // putting values in the Map
        map.put("Jayant", 80);
        map.put("Abhishek", 90);
        map.put("Anushka", 80);
        map.put("Amit", 75);
        map.put("Danish", 40);
 
        // Calling the function to sortbyKey
        sortbykey();
    }
}

输出

Key = Abhishek, Value = 90
Key = Amit, Value = 75
Key = Anushka, Value = 80
Key = Danish, Value = 40
Key = Jayant, Value = 80

使用数组列表

在这种方法中，我们使用 ArrayList 构造函数创建一个键列表。然后我们使用 Collections.sort() 方法对列表进行排序。

Java

// Java Code to sort Map by key value
import java.util.*;
class sortmapKey {
 
    // This map stores unsorted values
    static Map map = new HashMap<>();
 
    // Function to sort map by Key
    public static void sortbykey()
    {
        ArrayList sortedKeys
            = new ArrayList(map.keySet());
 
        Collections.sort(sortedKeys);
 
        // Display the TreeMap which is naturally sorted
        for (String x : sortedKeys)
            System.out.println("Key = " + x
                               + ", Value = " + map.get(x));
    }
 
    // Driver Code
    public static void main(String args[])
    {
        // putting values in the Map
        map.put("Jayant", 80);
        map.put("Abhishek", 90);
        map.put("Anushka", 80);
        map.put("Amit", 75);
        map.put("Danish", 40);
 
        // Calling the function to sortbyKey
        sortbykey();
    }
}

输出

Key = Abhishek, Value = 90
Key = Amit, Value = 75
Key = Anushka, Value = 80
Key = Danish, Value = 40
Key = Jayant, Value = 80

使用Java 8 Lambda

在这里，我们将更改排序方式，并使用 lambda 表达式进行排序。逻辑是一样的，甚至我们也传递了比较器对象，但只使用了 lambda。

下面是上述方法的实现：

Java

// Java Code to sort Map by key value
import java.util.*;
class sortmapKey {
 
    // This map stores unsorted key
    static Map map = new HashMap<>();
 
    // function to sort hashmap by keys
    public static Map
    sortByKey(Map hm)
    {
        // Create a list from elements of HashMap
        List > list
            = new LinkedList >(
                hm.entrySet());
 
        // Sort the list using lambda expression
        Collections.sort(
            list,
            (i1, i2) -> i1.getKey().compareTo(i2.getKey()));
 
        // put data from sorted list to hashmap
        HashMap temp
            = new LinkedHashMap();
        for (Map.Entry aa : list) {
            temp.put(aa.getKey(), aa.getValue());
        }
        return temp;
    }
 
    // Driver Code
    public static void main(String args[])
    {
        // putting values in the Map
        map.put("Jayant", 80);
        map.put("Abhishek", 90);
        map.put("Anushka", 80);
        map.put("Amit", 75);
        map.put("Danish", 40);
 
        // Calling the function to sortbyKey
        Map hm1 = sortByKey(map);
 
        // print the sorted hashmap
        for (Map.Entry en :
             hm1.entrySet()) {
            System.out.println("Key = " + en.getKey()
                               + ", Value = "
                               + en.getValue());
        }
    }
}

输出

Key = Abhishek, Value = 90
Key = Amit, Value = 75
Key = Anushka, Value = 80
Key = Danish, Value = 40
Key = Jayant, Value = 80

使用Java 8 流

在这里，我们将使用流对地图进行排序。我们将使用stream()方法获取 entrySet 的流，然后使用 sorted() 方法中的 lambda 表达式对流进行排序，最后，我们将使用 toMap() 将其转换为映射 方法。在 toMap() 方法中，我们使用LinkedHashMap::new方法引用来保留地图的排序顺序。

Java

// Java Code to sort Map by key value
import static java.util.stream.Collectors.*;
 
import java.lang.*;
import java.util.*;
import java.util.stream.*;
import java.util.stream.Collectors;
class sortmapKey {
 
    // This map stores unsorted values
    static Map map = new HashMap<>();
 
    // Function to sort map by Key
    public static void sortbykey()
    {
        HashMap temp
            = map.entrySet()
                  .stream()
                  .sorted((i1, i2)
                              -> i1.getKey().compareTo(
                                  i2.getKey()))
                  .collect(Collectors.toMap(
                      Map.Entry::getKey,
                      Map.Entry::getValue,
                      (e1, e2) -> e1, LinkedHashMap::new));
 
        // Display the HashMap which is naturally sorted
        for (Map.Entry entry :
             temp.entrySet()) {
            System.out.println("Key = " + entry.getKey()
                               + ", Value = "
                               + entry.getValue());
        }
    }
 
    // Driver Code
    public static void main(String args[])
    {
        // putting values in the Map
        map.put("Jayant", 80);
        map.put("Abhishek", 90);
        map.put("Anushka", 80);
        map.put("Amit", 75);
        map.put("Danish", 40);
 
        // Calling the function to sortbyKey
        sortbykey();
    }
}

输出

Key = Abhishek, Value = 90
Key = Amit, Value = 75
Key = Anushka, Value = 80
Key = Danish, Value = 40
Key = Jayant, Value = 80