如何在Java中向 TreeSet 添加自定义类对象？

TreeSet 是Java中 SortedSet 接口的一个实现，它使用红黑树进行存储。默认情况下，它保持升序。它只包含独特的元素。它不允许空元素。访问和检索时间非常快。要将用户定义的对象添加到 TreeSet 中，我们需要实现一个 Comparable 接口。我们要添加到 TreeSet 中的元素必须是可比较的类型。如果我们不实现 Comparable 接口，那么它将抛出ClassCastException。

示例 1

Java

// Importing util package
import java.util.*;
  
// Custome class Car implements Comparable interface
class Car implements Comparable {
  
    // attributes
    int Modelno;
    String name, city;
    int stock;
  
    // Car constructor
    public Car(int Modelno, String name, String city,
               int stock)
    {
        this.Modelno = Modelno;
        this.name = name;
        this.city = city;
        this.stock = stock;
    }
  
    // Override the compareTo() method
    public int compareTo(Car c)
    {
        if (stock > c.stock) {
            return 1;
        }
        else if (stock < c.stock) {
            return -1;
        }
        else {
            return 0;
        }
    }
}
  
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
        // Define an objects of TreeSet class
        TreeSet set = new TreeSet();
  
        // Creating Car objects
        Car c1 = new Car(132, "BMW", "Rajkot", 35);
        Car c2 = new Car(269, "Audi", "Surat", 20);
        Car c3 = new Car(560, "Kia", "Vadodara", 15);
        Car c4 = new Car(109, "Creta", "Ahmedabad", 26);
  
        // Adding Car objects to TreeSet
        set.add(c1);
        set.add(c2);
        set.add(c3);
        set.add(c4);
  
        // Traversing TreeSet
        for (Car c : set) {
            System.out.println(c.stock + " " + c.name + " "
                               + c.city + " " + c.Modelno);
        }
    }
}

输出

15 Kia Vadodara 560
20 Audi Surat 269
26 Creta Ahmedabad 109
35 BMW Rajkot 132

我们需要覆盖compareTo()方法，以便它按特定顺序对我们的集合进行排序。

示例 2：让我们采用相同的示例，但现在我们根据汽车名称覆盖 compareTo() 方法。

Java

// Importing util package
import java.util.*;
  
// Custome class Car implements Comparable interface
class Car implements Comparable {
  
    // attributes
    int Modelno;
    String name, city;
    int stock;
  
    // Car constructor
    public Car(int Modelno, String name, String city,
               int stock)
    {
        this.Modelno = Modelno;
        this.name = name;
        this.city = city;
        this.stock = stock;
    }
  
    // Override the compareTo() method
    public int compareTo(Car c)
    {
        return name.compareTo(c.name);
    }
}
  
public class GFG {
    // Main driver method
    public static void main(String[] args)
    {
  
        // Define an objects of TreeSet class
        TreeSet set = new TreeSet();
        
        // Creating Car objects
        Car c1 = new Car(132, "BMW", "Rajkot", 35);
        Car c2 = new Car(269, "Audi", "Surat", 20);
        Car c3 = new Car(560, "Kia", "Vadodara", 15);
        Car c4 = new Car(109, "Creta", "Ahmedabad", 26);
  
        // Adding Car objects to TreeSet
        set.add(c1);
        set.add(c2);
        set.add(c3);
        set.add(c4);
  
        // Traversing TreeSet
        for (Car c : set) {
            System.out.println(c.name + " " + c.stock + " "
                               + c.city + " " + c.Modelno);
        }
    }
}

输出

Audi 20 Surat 269
BMW 35 Rajkot 132
Creta 26 Ahmedabad 109
Kia 15 Vadodara 560