在Java中使用 TreeMap 时如何修复Java .lang.ClassCastException ？

Java.lang.ClassCastException是Java中未经检查的异常之一。当我们尝试将一种类类型的对象转换为另一种类类型的对象时，它可能会发生在我们的程序中。

当我们使用自定义类对象作为 TreeMap 中的键并且既没有实现可比较接口也没有实现比较器接口时，就会出现Java .lang.ClassCastException。

因此，在Java中使用 TreeMap 时，有两种方法可以修复Java .lang.ClassCastException ：

• 使用可比
• 使用比较器

方法 1：使用Comparable

我们可以通过用作 TreeMap 键的对象实现 Comparable 接口来修复Java.lang.ClassCastException。

伪代码：

// Custom class Student implements comparable interface

class Student implements Comparable {

  String name;
  Integer marks;

  public Student(String name, Integer marks) {
    this.name = name;
    this.marks = marks;
  }

  // Override toString method
  public String toString() {
    return this.name + " : " + this.marks;
  }

  public int getMarks() {
    return this.marks;
  }

  // Override compareTo method that sort treemap in the ascending order of the marks
  public int compareTo(Student stu) {
    return this.getMarks() - stu.getMarks();
  }
}

执行：

Java

// Java program to demonstrate how to fix
// java.lang.ClassCastException while using the TreeMap
 
import java.util.*;
 
// Custom class Student implements comparable interface
class Student implements Comparable {
 
    String name;
    Integer marks;
 
    public Student(String name, Integer marks)
    {
        this.name = name;
        this.marks = marks;
    }
 
    // Override toString method
    public String toString()
    {
        return this.name + " : " + this.marks;
    }
 
    public int getMarks() { return this.marks; }
 
    // Override compareTo method that sort treemap in the
    // ascending order of the marks
    public int compareTo(Student stu)
    {
        return this.getMarks() - stu.getMarks();
    }
}
 
public class GFG {
    public static void main(String[] args)
    {
 
        // New TreeMap
        TreeMap map = new TreeMap<>();
 
        map.put(new Student("Akshay", 500), 1);
        map.put(new Student("Bhanu", 600), 2);
        map.put(new Student("Chetan", 300), 3);
 
        System.out.println("The Treemap : " + map);
    }
}

输出

The Treemap : {Chetan : 300=3, Akshay : 500=1, Bhanu : 600=2}

方法 2：使用比较器

我们可以通过在创建 TreeMap 时为构造函数提供自定义比较器来修复Java.lang.ClassCastException。

伪代码：

// Custom comparator

class MyComparator implements Comparator {
  // Compare method that sort TreeMap in the ascending order of the marks
  public int compare(Student stu1, Student stu2) {
    return stu1.getMarks() - stu2.getMarks();
  }
}

执行：

Java

// Java program to demonstrate how to fix
// java.lang.ClassCastException while using the TreeMap
 
import java.util.*;
 
// Custom class Student implements comparable interface
class Student {
 
    String name;
    Integer marks;
 
    public Student(String name, Integer marks)
    {
        this.name = name;
        this.marks = marks;
    }
 
    // Override toString method
    public String toString()
    {
        return this.name + " : " + this.marks;
    }
 
    public int getMarks() { return this.marks; }
}
 
// Custom comparator
class MyComparator implements Comparator {
   
    // Compare method that sort TreeMap in the ascending
    // order of the marks
    public int compare(Student stu1, Student stu2)
    {
        return stu1.getMarks() - stu2.getMarks();
    }
}
 
public class GFG {
    public static void main(String[] args)
    {
 
        // New TreeMap
        TreeMap map
            = new TreeMap<>(new MyComparator());
 
        map.put(new Student("Akshay", 500), 1);
        map.put(new Student("Bhanu", 600), 2);
        map.put(new Student("Chetan", 300), 3);
 
        System.out.println("The Treemap : " + map);
    }
}

输出

The Treemap : {Chetan : 300=3, Akshay : 500=1, Bhanu : 600=2}