Java TreeMap.pollFirstEntry()、pollLastEntry()用法及代码示例

Java.util.TreeMap还包含支持在值的高端和低端进行检索和删除的函数，因此在适用性和日常使用方面具有很大的灵活性。此函数是poll()，并具有本文讨论的2个变体。

1. pollFirstEntry()：它删除并检索映射中键值最小的键值对，并且“null”为map为空。

用法：
public Map.Entry pollFirstEntry()
参数：
NA
返回值：
Retrieves and removes the least key-value if map is filled else returns null.
Exception:
NA

// Java code to demonstrate the working 
// of pollFirstEntry() 
import java.io.*; 
import java.util.*; 
public class pollFirstEntry { 
    public static void main(String[] args) 
    { 
  
        // Declaring the tree map of String and Integer 
        TreeMap<String, Integer> tmp = new TreeMap<String, Integer>(); 
  
        // Trying to retrieve and remove in empty map 
        // returns null 
        System.out.println 
        ("The smallest key value pair is:" + tmp.pollFirstEntry()); 
  
        // assigning the values in the tree map 
        // using put() 
        tmp.put("Geeks", 1); 
        tmp.put("for", 4); 
        tmp.put("geeks", 1); 
  
        // Printing the initial map 
        System.out.println 
        ("The initial Map before deletion is:" + tmp); 
  
        // Use of pollFirstEntry() 
        // Removes the first entry and returns the least key 
        // lexicographically smallest in case of String 
        // prints Geeks-1 
        System.out.println 
        ("The smallest key value pair is:" + tmp.pollFirstEntry()); 
  
        // Printing the map after deletion 
        System.out.println 
        ("The resultant Map after deletion is:" + tmp); 
    } 
}

输出：

The smallest key value pair is:null
The initial Map before deletion is:{Geeks=1, for=4, geeks=1}
The smallest key value pair is:Geeks=1
The resultant Map after deletion is:{for=4, geeks=1}

2. pollLastEntry()：删除并检索映射中具有最大键值的键值对，而“null”为map为空。

用法：
public Map.Entry pollLastEntry()
参数：
NA
返回值：
Retrieves and removes the largest key-value if map is filled else returns null.
Exception:
NA

// Java code to demonstrate the working 
// of pollLastEntry() 
import java.io.*; 
import java.util.*; 
public class pollLastEntry { 
    public static void main(String[] args) 
    { 
  
        // Declaring the tree map of String and Integer 
        TreeMap<String, Integer> tmp = new TreeMap<String, Integer>(); 
  
        // Trying to retrieve and remove in empty map 
        // returns null 
        System.out.println 
        ("The largest key value pair is:" + tmp.pollFirstEntry()); 
  
        // assigning the values in the tree map 
        // using put() 
        tmp.put("Geeks", 1); 
        tmp.put("for", 4); 
        tmp.put("geeks", 1); 
  
        // Printing the initial map 
        System.out.println 
        ("The initial Map before deletion is:" + tmp); 
  
        // Use of pollLastEntry() 
        // Removes the last(max) entry and returns the max key 
        // lexicographically greatest in case of String 
        // prints geeks-1 
        System.out.println 
        ("The largest key value pair is:" + tmp.pollLastEntry()); 
  
        // Printing the map after deletion 
        System.out.println 
        ("The resultant Map after deletion is:" + tmp); 
    } 
}

输出：

The largest key value pair is:null
The initial Map before deletion is:{Geeks=1, for=4, geeks=1}
The largest key value pair is:geeks=1
The resultant Map after deletion is:{Geeks=1, for=4}

实际应用：使用双端队列或优先队列的概念可以考虑许多应用。下面的代码显示了一个这样的示例。

// Java code to demonstrate the application 
// of pollLastEntry() and pollFirstEntry() 
import java.io.*; 
import java.util.*; 
public class pollAppli { 
    public static void main(String[] args) 
    { 
  
        // Declaring the tree map of Integer and String 
        TreeMap<Integer, String> que = new TreeMap<Integer, String>(); 
  
        // assigning the values in que 
        // using put() 
        que.put(10, "astha"); 
        que.put(4, "shambhavi"); 
        que.put(7, "manjeet"); 
        que.put(8, "nikhil"); 
  
        // Defining the priority 
        // takes highest value, if priority is high 
        // else takes lowest value 
        String prio = "high"; 
  
        // Printing the initial queue 
        System.out.println("The initial queue is:" + que); 
        if (prio == "high") { 
            System.out.println 
            ("The largest valued person is:" + que.pollLastEntry()); 
            System.out.println 
            ("The resultant queue after deletion is:" + que); 
        } 
        else { 
            System.out.println 
            ("The lowest valued person is:" + que.pollFirstEntry()); 
            System.out.println 
            ("The resultant queue after deletion is:" + que); 
        } 
    } 
}

输出：

The initial queue is:{4=shambhavi, 7=manjeet, 8=nikhil, 10=astha}
The largest valued person is:10=astha
The resultant queue after deletion is:{4=shambhavi, 7=manjeet, 8=nikhil}

相关用法

注：本文由纯净天空筛选整理自Shambhavi Singh 1大神的英文原创作品 Java.util.TreeMap.pollFirstEntry() and pollLastEntry() in Java。非经特殊声明，原始代码版权归原作者所有，本译文未经允许或授权，请勿转载或复制。