Java ArrayDeque peekFirst()用法及代码示例

Java中的java.util.ArrayDeque.peekFirst()方法用于检索或获取双端队列的第一个元素。检索到的元素不会从Queue中删除或删除，该方法仅返回它。如果双端队列中没有元素或该元素为空，则返回Null。

用法:

Array_Deque.peekFirst()

参数：该方法不带任何参数。

返回值：该方法返回双端队列的第一个元素。

下面的程序演示了Java.util.ArrayDeque.peekFirst()方法：
示例1:

// Java code to illustrate peekFirst() 
import java.util.*; 
  
public class ArrayDequeDemo { 
    public static void main(String args[]) 
    { 
        // Creating an empty ArrayDeque 
        Deque<String> de_que = new ArrayDeque<String>(); 
  
        // Use add() method to add elements into the Deque 
        de_que.add("Welcome"); 
        de_que.add("To"); 
        de_que.add("Geeks"); 
        de_que.add("4"); 
        de_que.add("Geeks"); 
  
        // Displaying the ArrayDeque 
        System.out.println("Initial ArrayDeque: " + de_que); 
  
        // Displaying the first element 
        System.out.println("The first element is: " +  
                                           de_que.peekFirst()); 
  
        // Displaying the ArrayDeque after operation 
        System.out.println("Final ArrayDeque: " + de_que); 
    } 
}

输出：

Initial ArrayDeque: [Welcome, To, Geeks, 4, Geeks]
The first element is: Welcome
Final ArrayDeque: [Welcome, To, Geeks, 4, Geeks]

示例2:

// Java code to illustrate peekFirst() 
import java.util.*; 
  
public class ArrayDequeDemo { 
    public static void main(String args[]) 
    { 
        // Creating an empty ArrayDeque 
        Deque<Integer> de_que = new ArrayDeque<Integer>(); 
  
        // Use add() method to add elements into the Deque 
        de_que.add(10); 
        de_que.add(15); 
        de_que.add(30); 
        de_que.add(20); 
        de_que.add(5); 
  
        // Displaying the ArrayDeque 
        System.out.println("Initial ArrayDeque: " + de_que); 
  
        // Displaying the first element 
        System.out.println("The first element is: " +  
                                           de_que.peekFirst()); 
  
        // Displaying the ArrayDeque after operation 
        System.out.println("Final ArrayDeque: " + de_que); 
    } 
}

输出：

Initial ArrayDeque: [10, 15, 30, 20, 5]
The first element is: 10
Final ArrayDeque: [10, 15, 30, 20, 5]

示例3:对于空的双端队列：

// Java code to illustrate peekFirst() 
import java.util.*; 
  
public class ArrayDequeDemo { 
    public static void main(String args[]) 
    { 
        // Creating an empty ArrayDeque 
        ArrayDeque<Integer> de_que = new ArrayDeque<Integer>(); 
  
        // Displaying the ArrayDeque 
        System.out.println("ArrayDeque: " + de_que); 
  
        // Displaying the first element 
        System.out.println("The first element is: " +  
                                     de_que.peekFirst()); 
    } 
}

输出：

ArrayDeque: []
The first element is: null

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注：本文由纯净天空筛选整理自Chinmoy Lenka大神的英文原创作品 ArrayDeque peekFirst() Method 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。