ArrayDeque peekFirst() Method in Java - GeeksforGeeks

10 Dec, 2018
The java.util.ArrayDeque.peekFirst() method in Java is used to retrieve or fetch the first element of the deque. The element retrieved does not get deleted or removed from the Queue instead the method just returns it. If no element is present in the deque or it is empty, then Null is returned.
Syntax:
Array_Deque.peekFirst()
Parameters: The method does not take any parameter.
Return Value: The method returns the first element of the Deque.
Below programs illustrate the Java.util.ArrayDeque.peekFirst() method:Program 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]

Program 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]

Program 3: For an empty 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

Java - util package
Java-ArrayDeque
Java-Collections
Java-Functions
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Java-Collections
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