Java中的ArrayDeque peekFirst()方法

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队列中删除或移除，方法只是返回它。如果deque中不存在任何元素或它是空的，则返回Null。

语法：

Array_Deque.peekFirst()

参数： 该方法不接受任何参数。

返回值： 该方法返回Deque的第一个元素。

下面的程序说明了Java.util.ArrayDeque.peekFirst()方法：

程序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);
    }
}

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

程序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);
    }
}

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

程序3： 对于一个空的deque：

// 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());
    }
}

ArrayDeque: []
The first element is: null