极客教程Java 根据键对HashMap进行排序
我们以HashMap的形式得到了学生的分数详情，其中学生的名字是键，分数是值。我们的任务是根据键值（即按字母顺序（lexicographical）排列的学生姓名）对该地图进行排序。
例子
输入 : Key = Jayant, Value = 80
Key = Anushka, Value = 80
Key = Amit, Value = 75
Key = Abhishek, Value = 90
Key = Danish, Value = 40
输出 : 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
建议。请先在{IDE}上尝试你的方法，然后再继续解决。
使用TreeMap (putAll方法)
我们的想法是将HashMap的所有数据放到一个TreeMap中。TreeMap遵循基于红黑树的实现。该地图根据其键的自然排序来进行排序。点击这里查看更多 …
// Java Code to sort Map by key value
import java.util.*;
class sortmapKey {
// This map stores unsorted values
static Map<String, Integer> map = new HashMap<>();
// Function to sort map by Key
public static void sortbykey()
{
// TreeMap to store values of HashMap
TreeMap<String, Integer> sorted = new TreeMap<>();
// Copy all data from hashMap into TreeMap
sorted.putAll(map);
// Display the TreeMap which is naturally sorted
for (Map.Entry<String, Integer> 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操作提供了保证的 log(n) 时间成本。
使用TreeMap (构造函数)
// Java Code to sort Map by key value
import java.util.*;
class sortmapKey {
// This map stores unsorted values
static Map<String, Integer> map = new HashMap<>();
// Function to sort map by Key
public static void sortbykey()
{
// TreeMap to store values of HashMap
TreeMap<String, Integer> sorted
= new TreeMap<>(map);
// Display the TreeMap which is naturally sorted
for (Map.Entry<String, Integer> 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ArrayList
在这种方法中，我们使用ArrayList构造函数创建一个键的列表。然后，我们使用Collections.sort()方法对该列表进行排序。
// Java Code to sort Map by key value
import java.util.*;
class sortmapKey {
// This map stores unsorted values
static Map<String, Integer> map = new HashMap<>();
// Function to sort map by Key
public static void sortbykey()
{
ArrayList<String> sortedKeys
= new ArrayList<String>(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 Lambdas
这里我们将改变我们的排序方式，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，但只是使用lambda。
下面是上述方法的实现。
// Java Code to sort Map by key value
import java.util.*;
class sortmapKey {
// This map stores unsorted key
static Map<String, Integer> map = new HashMap<>();
// function to sort hashmap by keys
public static Map<String, Integer>
sortByKey(Map<String, Integer> hm)
{
// Create a list from elements of HashMap
List<Map.Entry<String, Integer> > list
= new LinkedList<Map.Entry<String, Integer> >(
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<String, Integer> temp
= new LinkedHashMap<String, Integer>();
for (Map.Entry<String, Integer> 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<String, Integer> hm1 = sortByKey(map);
// print the sorted hashmap
for (Map.Entry<String, Integer> 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
这里我们将使用流来对地图进行排序。我们将使用 stream() 方法来获取entrySet的流，然后在sorted()方法中使用lambda表达式来对流进行排序，最后，我们将使用toMap() 方法将其转换成map。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 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<String, Integer> map = new HashMap<>();
// Function to sort map by Key
public static void sortbykey()
{
HashMap<String, Integer> 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<String, Integer> 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