Java中的 TreeMap ceilingKey() 和示例

TreeMap 类的ceilingKey()函数返回大于或等于给定键的最小键，如果不存在这样的键，则返回 null。

句法：

public K ceilingKey(K key)

参数：此方法接受一个强制参数键，它是要搜索的键。

返回值：此方法返回大于或等于给定键值的最小键。
如果没有这样的键，则返回 null。

异常：此方法引发以下异常：

• ClassCastException – 如果指定的键无法与给定的键值进行比较，则抛出该异常。
• NullPointerException – 如果给定键为空且映射使用自然排序或比较器不允许空值，则抛出此异常。

以下是说明 ceilingKey() 方法的示例：

程序 1：演示对带有比较器的 TreeMap 使用 ceilingKey() 方法

import java.util.*;
  
public class Main {
    public static void main(String[] args)
    {
  
        // creating tree map
        NavigableMap
            treemap = new TreeMap((a, b)
                                              -> ((a > b)
                                                      ? 1
                                                      : ((a == b)
                                                             ? 0
                                                             : -1)));
  
        // populating tree map
        treemap.put(1, " A ");
        treemap.put(2, " B ");
        treemap.put(3, " C ");
        treemap.put(4, " D ");
        treemap.put(6, " E ");
        try {
            System.out.println("Ceiling key entry for 5: "
                               + treemap.ceilingKey(5));
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

输出：

Ceiling key entry for 5: 6

程序 2：演示在没有任何比较器的 TreeMap 中使用 ceilingKey() 方法

import java.util.*;
  
public class Main {
    public static void main(String[] args)
    {
  
        // creating tree map
        NavigableMap
            treemap = new TreeMap();
  
        // populating tree map
        treemap.put(1, " A ");
        treemap.put(2, " B ");
        treemap.put(3, " C ");
        treemap.put(4, " D ");
        treemap.put(6, " E ");
        treemap.put(7, " F ");
  
        // Since 6 is the least value greater than 5,
        // it is returned as the key.
        System.out.println("Ceiling key entry for 5: "
                           + treemap.ceilingKey(5));
    }
}

输出：

Ceiling key entry for 5: 6

程序 3：演示在返回 null 时使用 ceilingKey() 方法

import java.util.*;
  
public class Main {
    public static void main(String[] args)
    {
  
        // creating tree map
        NavigableMap
            treemap = new TreeMap();
  
        // populating tree map
        treemap.put(1, " A ");
        treemap.put(2, " B ");
        treemap.put(3, " C ");
        treemap.put(4, " E ");
        treemap.put(5, " D ");
  
        // Since 10 is not present in the Map
        // and neither any Key is present greater than 10
        // Hence this will return null
        System.out.println("Ceiling key entry for 10: "
                           + treemap.ceilingKey(10));
    }
}

输出：

Ceiling key entry for 10: null

程序 4：显示 NullPointerException

import java.util.*;
  
public class Main {
    public static void main(String[] args)
    {
  
        // creating tree map
        TreeMap
            treemap = new TreeMap();
  
        // populating tree map
        treemap.put(2, " two ");
        treemap.put(1, " one ");
        treemap.put(3, " three ");
        treemap.put(6, " six ");
        treemap.put(5, " five ");
  
        try {
            // returns a NullPointerException
            // as key value can't be null
            // because of natural ordering
            System.out.println("Ceiling key entry for null value : "
                               + treemap.ceilingKey(null));
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

输出：

Exception: java.lang.NullPointerException

程序5：演示ClassCastException

import java.util.*;
  
public class Main {
    public static void main(String[] args)
    {
  
        // creating tree map
        NavigableMap
            treemap = new TreeMap();
  
        // populating tree map
        treemap.put(1, " A ");
        treemap.put(2, " B ");
        treemap.put(3, " C ");
        treemap.put(4, " E ");
        treemap.put(5, " D ");
  
        try {
            // returns ClassCastException
            // as we cannot compare a String object with an Integer object
            System.out.println("Ceiling key entry for \"asd\": "
                               + treemap.ceilingKey(new String("asd")));
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
    }
}

输出：

Exception: java.lang.ClassCastException: 
           java.lang.Integer cannot be cast to java.lang.String