Java中的 TreeMap firstEntry() 和 firstKey() 方法及示例

Java.util.TreeMap 中的 first() 有两种变体，本文将讨论这两种变体。

方法一： firstEntry()

它返回与此映射中的最小键关联的键值映射，如果映射为空，则返回 null。

句法：

public Map.Entry firstEntry()

返回类型：具有最少键的条目，如果映射为空，则返回 null。

例子：

Java

// Java Program to Illustrate Working of firstKey() Method
// of TreeMap class
  
// Importing required classes
import java.io.*;
import java.util.*;
  
// Main class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an empty TreeMap by
        // declaring object of integer, strings pairs
        TreeMap treemap
            = new TreeMap();
  
        // Populating values in the TreeMap
        // using put() method
        treemap.put(2, "two");
        treemap.put(7, "seven");
        treemap.put(3, "three");
        treemap.put(1, "one");
        treemap.put(6, "six");
        treemap.put(9, "nine");
  
        // Printing the lowest entry in TreeMap by
        // using firstEntry() method
        System.out.println("Lowest entry is: "
                           + treemap.firstEntry());
    }
}

输出：

Lowest entry is: 1=one

方法二：firstKey()

它返回地图中当前的第一个（最低）键。

句法：

public K firstKey()

返回类型：当前地图中的第一个（最低）键。

抛出异常： NoSuchElementException 如果此地图为空，则抛出。

例子：

Java

// Java Program to Demonstrate firstKey() Method
// of TreeMap class
  
// Importing required classes
import java.io.*;
import java.util.*;
  
// Main class
class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an empty TreeMap by
        // declaring object of integer, strings pairs
        TreeMap treemap
            = new TreeMap();
  
        // Populating values in the TreeMap
        // using put() method
        treemap.put(2, "two");
        treemap.put(1, "one");
        treemap.put(3, "three");
        treemap.put(6, "six");
        treemap.put(5, "five");
        treemap.put(9, "nine");
  
        // Printing the lowest entry in TreeMap by
        // using firstKey() method
        System.out.println("Lowest key is: "
                           + treemap.firstKey());
    }
}

输出：

Lowest key is: 1

实现：这些函数可用于获取给定列表中排名最高的人，或者可用于分配获胜者，其中完成任务的时间最短的人获胜。后一种将在下面讨论。

示例：实际应用

Java

// Java Program to Demonstrate Application Usage
// of firstKey() and firstEntry() Methods
// of TreeMap class
  
// Importing required classes
import java.io.*;
import java.util.*;
  
// Main class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an empty TreeMap
        // of Integer and String times of participants
        // In seconds
        TreeMap time
            = new TreeMap();
  
        // Populating the time taken to complete task
        // using put() method
        time.put(2.32f, "Astha");
        time.put(7.43f, "Manjeet");
        time.put(1.3f, "Shambhavi");
        time.put(5.63f, "Nikhil");
        time.put(6.26f, "Vaishnavi");
  
        // Printing person with least time
        // using of firstEntry() method
        System.out.println("Winner with lowest time is : "
                           + time.firstEntry());
    }
}

输出：

Winner with lowest time is : 1.3=Shambhavi