极客教程Java 根据数值对HashMap进行排序
给出一个学生在100个科目中的分数，其中科目的名称是关键，分数是值。我们的任务是根据数值对HashMap进行排序，即根据分数进行排序。
示例：
输入 : Key = Math, Value = 98
Key = Data Structure, Value = 85
Key = Database, Value = 91
Key = Java, Value = 95
Key = Operating System, Value = 79
Key = Networking, Value = 80
输出 : Key = Operating System, Value = 79
Key = Networking, Value = 80
Key = Data Structure, Value = 85
Key = Database, Value = 91
Key = Java, Value = 95
Key = Math, Value = 98
解决方案： 我们的想法是将条目集存储在一个列表中，并根据数值对列表进行排序。然后从列表中获取值和键，并将其放入一个新的哈希玛。这样，一个新的HashMap就根据值进行了排序。
以下是上述想法的实现：
// Java program to sort hashmap by values
import java.util.*;
import java.lang.*;
public class GFG {
// function to sort hashmap by values
public static HashMap<String, Integer> sortByValue(HashMap<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
Collections.sort(list, new Comparator<Map.Entry<String, Integer> >() {
public int compare(Map.Entry<String, Integer> o1,
Map.Entry<String, Integer> o2)
{
return (o1.getValue()).compareTo(o2.getValue());
}
});
// 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)
{
HashMap<String, Integer> hm = new HashMap<String, Integer>();
// enter data into hashmap
hm.put("Math", 98);
hm.put("Data Structure", 85);
hm.put("Database", 91);
hm.put("Java", 95);
hm.put("Operating System", 79);
hm.put("Networking", 80);
Map<String, Integer> hm1 = sortByValue(hm);
// print the sorted hashmap
for (Map.Entry<String, Integer> en : hm1.entrySet()) {
System.out.println("Key = " + en.getKey() +
", Value = " + en.getValue());
}
}
}
输出
Key = Operating System, Value = 79
Key = Networking, Value = 80
Key = Data Structure, Value = 85
Key = Database, Value = 91
Key = Java, Value = 95
Key = Math, Value = 98
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 program to sort hashmap by values
import java.lang.*;
import java.util.*;
public class GFG {
// function to sort hashmap by values
public static HashMap<String, Integer>
sortByValue(HashMap<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.getValue().compareTo(i2.getValue()));
// 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)
{
HashMap<String, Integer> hm
= new HashMap<String, Integer>();
// enter data into hashmap
hm.put("Math", 98);
hm.put("Data Structure", 85);
hm.put("Database", 91);
hm.put("Java", 95);
hm.put("Operating System", 79);
hm.put("Networking", 80);
Map<String, Integer> hm1 = sortByValue(hm);
// print the sorted hashmap
for (Map.Entry<String, Integer> en :
hm1.entrySet()) {
System.out.println("Key = " + en.getKey()
+ ", Value = "
+ en.getValue());
}
}
}
输出
Key = Operating System, Value = 79
Key = Networking, Value = 80
Key = Data Structure, Value = 85
Key = Database, Value = 91
Key = Java, Value = 95
Key = Math, Value = 98
在Java 8中使用流
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 program to sort hashmap by values
import static java.util.stream.Collectors.*;
import java.lang.*;
import java.util.*;
import java.util.stream.*;
import java.util.stream.Collectors;
public class gfg3 {
// function to sort hashmap by values
public static HashMap<String, Integer>
sortByValue(HashMap<String, Integer> hm)
{
HashMap<String, Integer> temp
= hm.entrySet()
.stream()
.sorted((i1, i2)
-> i1.getValue().compareTo(
i2.getValue()))
.collect(Collectors.toMap(
Map.Entry::getKey,
Map.Entry::getValue,
(e1, e2) -> e1, LinkedHashMap::new));
return temp;
}
// Driver Code
public static void main(String[] args)
{
HashMap<String, Integer> hm
= new HashMap<String, Integer>();
// enter data into hashmap
hm.put("Math", 98);
hm.put("Data Structure", 85);
hm.put("Database", 91);
hm.put("Java", 95);
hm.put("Operating System", 79);
hm.put("Networking", 80);
Map<String, Integer> hm1 = sortByValue(hm);
// print the sorted hashmap
for (Map.Entry<String, Integer> en :
hm1.entrySet()) {
System.out.println("Key = " + en.getKey()
+ ", Value = "
+ en.getValue());
}
}
}
输出
Key = Operating System, Value = 79
Key = Networking, Value = 80
Key = Data Structure, Value = 85
Key = Database, Value = 91
Key = Java, Value = 95
Key = Math, Value = 98