Java 中的树形图 lowerKey()，示例
原文:https://www . geesforgeks . org/tree map-lowerkey-in-Java-with-examples/
先决条件:Java 中的树形图
lowerKey() 方法用于返回严格小于给定键的最大键，作为参数传递。更简单地说，这个方法用于在作为参数传递的元素之后查找下一个最大的元素。
语法:
public K TreeMap.lowerKey(K key)
参数:该方法取一个强制参数键，即这是要匹配的键。
返回值:这个方法返回最大的键严格小于到键，或者空值如果没有这样的键。
异常:该方法抛出以下异常:
- ClassCastException :当指定的键无法与地图中可用的键进行比较时。
- NullPointRexception:当映射中指定的键为空并且使用自然的 排序时，这意味着比较器不允许空键。
下面是一些例子来说明 lowerKey()方法的使用:
例 1:
// Java program to demonstrate lowerKey() method
import java.util.TreeMap;
public class FloorKeyDemo {
public static void main(String args[])
{
// create an empty TreeMap
TreeMap<Integer, String>
numMap = new TreeMap<Integer, String>();
// Insert the values
numMap.put(6, "Six");
numMap.put(1, "One");
numMap.put(5, "Five");
numMap.put(3, "Three");
numMap.put(8, "Eight");
numMap.put(10, "Ten");
// Print the Values of TreeMap
System.out.println("TreeMap: " + numMap.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(numMap.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(numMap.lowerKey(3));
}
}
Output:
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: 演示空指针异常
// Java program to demonstrate lowerKey() method
import java.util.TreeMap;
public class FloorKeyDemo {
public static void main(String args[])
{
// create an empty TreeMap
TreeMap<Integer, String>
numMap = new TreeMap<Integer, String>();
// Insert the values
numMap.put(6, "Six");
numMap.put(1, "One");
numMap.put(5, "Five");
numMap.put(3, "Three");
numMap.put(8, "Eight");
numMap.put(10, "Ten");
// Print the Values of TreeMap
System.out.println("TreeMap: " + numMap.toString());
try {
// Passing null as parameter to lowerKey()
// This will throw exception
System.out.println(numMap.lowerKey(null));
}
catch (Exception e) {
System.out.println("Exception: " + e);
}
}
}
Output:
TreeMap: {1=One, 3=Three, 5=Five, 6=Six, 8=Eight, 10=Ten}
Exception: java.lang.NullPointerException
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