Java-Map

Collections

Posted by Haven Tong on 2020-03-05

Java-Map

受到上篇C++ STL中map的排序方式的启发,这次又来探究一下Java中对于map的不同排序方式

依然分为两种排序

  • Sort By Key
  • Sort By Value

打印map

打印map的元素使用以下的show()方法:

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public static void show(Map<String, Integer> map) {
for (Map.Entry<String, Integer> entry: map.entrySet()) {
System.out.println(entry.getKey() + " " + entry.getValue());
}
System.out.println("=======================");
System.out.println();
}

Sort By Value

升序

Java中对于Map的value进行排序依然需要借助其他容器,这里使用ArrayList

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   /**
* 获取按照value升序排序后的map
* @param map 需要排序的map
* @return 排好序的map
*/
public static Map<String, Integer> sortByValueAsc(Map<String, Integer> map) {
List<Map.Entry<String, Integer>> list = new ArrayList<>(map.entrySet());
// 升序
Collections.sort(list, (e1, e2) -> e1.getValue().compareTo(e2.getValue()));
Map<String, Integer> sortedMap = new LinkedHashMap<>();
for (Map.Entry<String, Integer> entry: list) {
sortedMap.put(entry.getKey(), entry.getValue());
}
return sortedMap;
}

public static void main(String[] args) {
Map<String, Integer> map = new HashMap<>();
map.put("boss", 2);
map.put("abuse", 1);
map.put("unit", 3);
map.put("duty", 4);

// 根据Value升序排序
System.out.println("🚀 SORT BY VALUE ASC:");
Map<String, Integer> sortedMapAsc = sortByValueAsc(map);
show(sortedMapAsc);
}

这里将Map中的entry都放到了一个List中,然后使用自定义Comparator的方式对entry进行排序;

同时也使用了一种新的MapLinkedHashMap

LinkedHashMap是有序的,它会按照元素的插入顺序对元素进行排序

结果如下:

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🚀 SORT BY VALUE ASC:
abuse 1
boss 2
unit 3
duty 4
=======================

降序

同理,只是更换一下Comparator

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/**
* 获取按照value降序排序后的map
* @param map 需要排序的map
* @return 排好序的map
*/
public static Map<String, Integer> sortByValueDesc(Map<String, Integer> map) {
List<Map.Entry<String, Integer>> list = new ArrayList<>(map.entrySet());
// 降序
Collections.sort(list, (e1, e2) -> e2.getValue().compareTo(e1.getValue()));
Map<String, Integer> sortedMap = new LinkedHashMap<>();
for (Map.Entry<String, Integer> entry: list) {
sortedMap.put(entry.getKey(), entry.getValue());
}
return sortedMap;
}

public static void main(String[] args) {
Map<String, Integer> map = new HashMap<>();
map.put("boss", 2);
map.put("abuse", 1);
map.put("unit", 3);
map.put("duty", 4);

// 根据Value降序排序
System.out.println("🚀 SORT BY VALUE DESC");
Map<String, Integer> sortedMapDesc = sortByValueDesc(map);
show(sortedMapDesc);
}

结果如下:

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🚀 SORT BY VALUE DESC
duty 4
unit 3
boss 2
abuse 1
=======================

Sort By Key

升序

对于key的排序也可以按照对value的排序一样,借助ArrayListLinkedHashMap,但这里采用另一种方式,借助TreeMap进行排序

TreeMap是另一种Map,会对Map中的元素按照key大小进行排序

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  /**
* 通过TreeMap的方式对key按照升序排序
* @param map 要排序的map
* @return 按照key升序排好序的map
*/
public static Map<String, Integer> sortByKeyByTreeMapAsc(Map<String, Integer> map) {
/*
The naturalOrder() method of Comparator Interface in Java
returns a comparator that use to compare Comparable objects in natural order.
*/
Map<String, Integer> sortedMap = new TreeMap<>(Comparator.naturalOrder());
sortedMap.putAll(map);
return sortedMap;
}

public static void main(String[] args) {
Map<String, Integer> map = new HashMap<>();
map.put("boss", 2);
map.put("abuse", 1);
map.put("unit", 3);
map.put("duty", 4);


// 根据Key升序排序,使用TreeMap方式
System.out.println("🚀 SORT BY KEY ASC");
Map<String, Integer> sortedByKeyAsc = sortByKeyByTreeMapAsc(map);
show(sortedByKeyAsc);
}

Comparable对象的升序排序可以使用Comparator.naturalOrder()返回的Comparator来指定

执行的结果如下:

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🚀 SORT BY KEY ASC
abuse 1
boss 2
duty 4
unit 3
=======================

降序

同理,只是需要更改Comparator

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/**
* 通过TreeMap的方式对key按照将序排序
* @param map 要排序的map
* @return 按照key降序排好序的map
*/
public static Map<String, Integer> sortByKeyByTreeMapDesc(Map<String, Integer> map) {
/*
The reverseOrder() Returns a comparator that imposes the reverse of the natural ordering.
*/
Map<String, Integer> sortedMap = new TreeMap<>(Comparator.reverseOrder());
sortedMap.putAll(map);
return sortedMap;
}

public static void main(String[] args) {
Map<String, Integer> map = new HashMap<>();
map.put("boss", 2);
map.put("abuse", 1);
map.put("unit", 3);
map.put("duty", 4);

// 根据Key降序排序,使用TreeMap方式
System.out.println("🚀 SORT BY KEY DESC");
Map<String, Integer> sortedByKeyDesc = sortByKeyByTreeMapDesc(map);
show(sortedByKeyDesc);

}

Comparable对象的降序排序可以使用Comparator.reverseOrder()返回的Comparator来指定

结果如下:

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🚀 SORT BY KEY DESC
unit 3
duty 4
boss 2
abuse 1
=======================

完整代码

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import java.util.*;

/**
* @author HavenTong
* @date 2020/3/5 5:34 下午
*/
public class MapUtil {

public static void show(Map<String, Integer> map) {
for (Map.Entry<String, Integer> entry: map.entrySet()) {
System.out.println(entry.getKey() + " " + entry.getValue());
}
System.out.println("=======================");
System.out.println();
}

/**
* 获取按照value升序排序后的map
* @param map 需要排序的map
* @return 排好序的map
*/
public static Map<String, Integer> sortByValueAsc(Map<String, Integer> map) {
List<Map.Entry<String, Integer>> list = new ArrayList<>(map.entrySet());
// 升序
Collections.sort(list, (e1, e2) -> e1.getValue().compareTo(e2.getValue()));
Map<String, Integer> sortedMap = new LinkedHashMap<>();
for (Map.Entry<String, Integer> entry: list) {
sortedMap.put(entry.getKey(), entry.getValue());
}
return sortedMap;
}

/**
* 获取按照value降序排序后的map
* @param map 需要排序的map
* @return 排好序的map
*/
public static Map<String, Integer> sortByValueDesc(Map<String, Integer> map) {
List<Map.Entry<String, Integer>> list = new ArrayList<>(map.entrySet());
// 降序
Collections.sort(list, (e1, e2) -> e2.getValue().compareTo(e1.getValue()));
Map<String, Integer> sortedMap = new LinkedHashMap<>();
for (Map.Entry<String, Integer> entry: list) {
sortedMap.put(entry.getKey(), entry.getValue());
}
return sortedMap;
}


/**
* 通过TreeMap的方式对key按照升序排序
* @param map 要排序的map
* @return 按照key升序排好序的map
*/
public static Map<String, Integer> sortByKeyByTreeMapAsc(Map<String, Integer> map) {
/*
The naturalOrder() method of Comparator Interface in Java
returns a comparator that use to compare Comparable objects in natural order.
*/
Map<String, Integer> sortedMap = new TreeMap<>(Comparator.naturalOrder());
sortedMap.putAll(map);
return sortedMap;
}

/**
* 通过TreeMap的方式对key按照将序排序
* @param map 要排序的map
* @return 按照key降序排好序的map
*/
public static Map<String, Integer> sortByKeyByTreeMapDesc(Map<String, Integer> map) {
/*
The reverseOrder() Returns a comparator that imposes the reverse of the natural ordering.
*/
Map<String, Integer> sortedMap = new TreeMap<>(Comparator.reverseOrder());
sortedMap.putAll(map);
return sortedMap;
}



public static void main(String[] args) {
Map<String, Integer> map = new HashMap<>();
map.put("boss", 2);
map.put("abuse", 1);
map.put("unit", 3);
map.put("duty", 4);

// 根据Value升序排序
System.out.println("🚀 SORT BY VALUE ASC:");
Map<String, Integer> sortedMapAsc = sortByValueAsc(map);
show(sortedMapAsc);

// 根据Value降序排序
System.out.println("🚀 SORT BY VALUE DESC");
Map<String, Integer> sortedMapDesc = sortByValueDesc(map);
show(sortedMapDesc);

// 根据Key升序排序,使用TreeMap方式
System.out.println("🚀 SORT BY KEY ASC");
Map<String, Integer> sortedByKeyAsc = sortByKeyByTreeMapAsc(map);
show(sortedByKeyAsc);

// 根据Key降序排序,使用TreeMap方式
System.out.println("🚀 SORT BY KEY DESC");
Map<String, Integer> sortedByKeyDesc = sortByKeyByTreeMapDesc(map);
show(sortedByKeyDesc);

}
}