使用带有用户定义对象的比较器对 TreeMap 中的键进行排序的Java程序

Java中的TreeMap用于实现 Map 接口和 NavigableMap 以及 AbstractMap 类。映射根据其键的自然顺序排序，或按 比较器 在地图创建时提供，具体取决于使用的构造函数。要在Java中使用带有用户定义对象的比较器对 TreeMap 中的键进行排序，我们必须创建一个实现 Comparator 接口的类来覆盖 compare 方法。

// AccordingMarks class that implements the 
// comparator interface to override compare method

class AccordingMarks implements Comparator {
    public int compare(Student s1, Student s2) {
        return s1.getMarks().compareTo(s2.getMarks());
    }
}

在下面的代码中，我们传递一个自定义对象作为 TreeMap 中的键，即 Student 用户定义类。在这种情况下，我们需要在构造函数中传递比较器根据标记，同时创建根据学生的分数对 TreeMap 进行排序的 TreeMap 对象。

示例 1：按标记的升序对键进行排序

Java

// Java program to demonstrate how to sort TreeMap of custom
// class objects
import java.util.*;
 
// Custom class
class Student {
 
    private String name;
    private int marks;
 
    public Student(String name, Integer marks)
    {
        this.name = name;
        this.marks = marks;
    }
 
    public String getName() { return this.name; }
 
    public Integer getMarks() { return this.marks; }
    // override toString method
    public String toString()
    {
        return this.name + ": " + marks;
    }
}
 
// Comparator that sort elements according to marks in
// Ascending order
class AccordingMarks implements Comparator {
    public int compare(Student s1, Student s2)
    {
        return s1.getMarks().compareTo(s2.getMarks());
    }
}
 
// Driver Code
public class GFG {
    public static void main(String[] args)
    {
 
        // New TreeMap of custom class Student
        TreeMap map
            = new TreeMap<>(new AccordingMarks());
 
        // Add elements to TreeMap
        map.put(new Student("Akshay", 400), 1);
        map.put(new Student("Bina", 500), 2);
        map.put(new Student("Chintu", 300), 3);
 
        System.out.println(
            "TreeMap keys sorting in ascending order of the marks:");
 
        // Print map using Entry
        for (Map.Entry entry :
             map.entrySet()) {
            System.out.println("Key : (" + entry.getKey()
                               + "), Value : "
                               + entry.getValue());
        }
    }
}

输出：

TreeMap keys sorting in ascending order of the marks:
Key : (Chintu: 300), Value : 3
Key : (Akshay: 400), Value : 1
Key : (Bina: 500), Value : 2

示例 2：将键按标记的降序排序

Java

// Java program to demonstrate how to sort TreeMap of custom
// class objects
import java.util.*;
 
// Custom class
class Student {
 
    private String name;
    private int marks;
 
    public Student(String name, Integer marks)
    {
        this.name = name;
        this.marks = marks;
    }
 
    public String getName() { return this.name; }
 
    public Integer getMarks() { return this.marks; }
    // override toString method
    public String toString()
    {
        return this.name + ": " + marks;
    }
}
 
// Comparator that sort elements according to marks in
// Descending order
class AccordingMarks implements Comparator {
    public int compare(Student s1, Student s2)
    {
        return s2.getMarks().compareTo(s1.getMarks());
    }
}
 
// Driver Code
public class GFG {
    public static void main(String[] args)
    {
 
        // New TreeMap of custom class Student
        TreeMap map
            = new TreeMap<>(new AccordingMarks());
 
        // Add elements to TreeMap
        map.put(new Student("Akshay", 400), 1);
        map.put(new Student("Bina", 500), 2);
        map.put(new Student("Chintu", 300), 3);
 
        System.out.println(
            "TreeMap Keys sorted in descending order of the marks: ");
 
        // Print map using Entry
        for (Map.Entry entry :
             map.entrySet()) {
            System.out.println("Key : (" + entry.getKey()
                               + "), Value : "
                               + entry.getValue());
        }
    }
}

输出：

TreeMap Keys sorted in descending order of the marks: 
Key : (Bina: 500), Value : 2
Key : (Akshay: 400), Value : 1
Key : (Chintu: 300), Value : 3