Java中NavigableMap lowerKey()方法

NavigableMap接口的lowerKey()方法用于返回严格小于给定参数键的最大键。换句话说，此方法用于查找参数后的下一个最大元素。

语法：

public K NavigableMap.lowerKey(K key)

参数：该方法带有一个参数键，它是要匹配的键。

返回值： 该方法返回严格小于键的最大键或null（如果不存在这样的键）。

异常： 该方法可能引发以下异常：

• ClassCastException ：指定键无法与Map中可用的键进行比较时。
• NullPointerException ：当Map中的指定键为空并且使用自然顺序（即比较器不允许null键）时。

以下程序演示了lowerKey()方法的使用：

示例1：

// Java program to demonstrate lowerKey() method
  
import java.util.*;
  
public class FloorKeyDemo {
    public static void main(String args[])
    {
  
        // create an empty TreeMap
        NavigableMap<Integer, String>
            navMap = new TreeMap<Integer, String>();
  
        // Insert the values
        navMap.put(6, "Six");
        navMap.put(1, "One");
        navMap.put(5, "Five");
        navMap.put(3, "Three");
        navMap.put(8, "Eight");
        navMap.put(10, "Ten");
  
        // Print the Values of TreeMap
        System.out.println("TreeMap: " + navMap.toString());
  
        // Get the greatest key mapping of the Map
  
        // As here 9 is not available it returns 8
        // because 9 is strictly less than 11, present
        System.out.print("Lower Key Entry of Element 9 is: ");
        System.out.println(navMap.lowerKey(9));
  
        // Though, 3 is available in the Map
        // it returns 1 because this method returns
        // strictly less than key according to the specified key
        System.out.print("Lower Key Entry of Element 3 is: ");
        System.out.println(navMap.lowerKey(3));
    }
}

TreeMap: {1=One, 3=Three, 5=Five, 6=Six, 8=Eight, 10=Ten}
Lower Key Entry of Element 9 is: 8
Lower Key Entry of Element 3 is: 1

示例2： 演示NullPointerException

// Java program to demonstrate lowerKey() method
  
import java.util.*;
  
public class FloorKeyDemo {
    public static void main(String args[])
    {
  
        // create an empty TreeMap
        NavigableMap<Integer, String>
            navMap = new TreeMap<Integer, String>();
  
        // Insert the values
        navMap.put(6, "Six");
        navMap.put(1, "One");
        navMap.put(5, "Five");
        navMap.put(3, "Three");
        navMap.put(8, "Eight");
        navMap.put(10, "Ten");
  
        // Print the Values of TreeMap
        System.out.println("TreeMap: " + navMap.toString());
  
        try {
            // Passing null as parameter to lowerKey()
            // This will throw exception
            System.out.println(navMap.lowerKey(null));
        }
        catch (Exception e) {
            System.out.println("Exception: " + e);
        }
   }
}

TreeMap: {1=One, 3=Three, 5=Five, 6=Six, 8=Eight, 10=Ten}
Exception: java.lang.NullPointerException