Deque getFirst() method in Java

The getFirst() method of Deque Interface returns the first element or the head of the Deque. It does not deletes the element. It throws an exception when the Deque is empty.

Syntax:

E getFirst()

Parameters: This method does not accepts any parameter.

Returns: Thie method returns the first element or the head of the Deque but does not delete it.

Exception: The function throws NoSuchElementException when the Deque is empty and the function is called.



Below programs illustrate getFirst() method of Deque:

Program 1: With the help of LinkedList.

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// Java Program Demonstrate getFirst()
// method of Deque
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Deque
        Deque<Integer> DQ
            = new LinkedList<Integer>();
  
        // Add numbers to end of Deque
        DQ.add(7855642);
        DQ.add(35658786);
        DQ.add(5278367);
        DQ.add(74381793);
  
        // print Deque
        System.out.println("Deque: " + DQ);
  
        // print head
        System.out.println("Deque's head: " + DQ.getFirst());
    }
}

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Output:

Deque: [7855642, 35658786, 5278367, 74381793]
Deque's head: 7855642

Program 2: With the help of ArrayDeque.

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// Java Program Demonstrate getFirst()
// method of Deque
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Deque
        Deque<Integer> DQ
            = new ArrayDeque<Integer>();
  
        // Add numbers to end of Deque
        DQ.add(7855642);
        DQ.add(35658786);
        DQ.add(5278367);
        DQ.add(74381793);
  
        // print Deque
        System.out.println("Deque: " + DQ);
  
        // print head
        System.out.println("Deque's head: " + DQ.getFirst());
    }
}

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Output:

Deque: [7855642, 35658786, 5278367, 74381793]
Deque's head: 7855642

Program 3: With the help of LinkedBlockingDeque.

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// Java Program Demonstrate getFirst()
// method of Deque
import java.util.*;
import java.util.concurrent.LinkedBlockingDeque;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Deque
        Deque<Integer> DQ
            = new LinkedBlockingDeque<Integer>();
  
        // Add numbers to end of Deque
        DQ.add(7855642);
        DQ.add(35658786);
        DQ.add(5278367);
        DQ.add(74381793);
  
        // print Deque
        System.out.println("Deque: " + DQ);
  
        // print head
        System.out.println("Deque's head: " + DQ.getFirst());
    }
}

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Output:

Deque: [7855642, 35658786, 5278367, 74381793]
Deque's head: 7855642

Program 4: With the help of ConcurrentLinkedDeque.

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// Java Program Demonstrate getFirst()
// method of Deque
import java.util.*;
import java.util.concurrent.ConcurrentLinkedDeque;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Deque
        Deque<Integer> DQ
            = new ConcurrentLinkedDeque<Integer>();
  
        // Add numbers to end of Deque
        DQ.add(7855642);
        DQ.add(35658786);
        DQ.add(5278367);
        DQ.add(74381793);
  
        // print Deque
        System.out.println("Deque: " + DQ);
  
        // print head
        System.out.println("Deque's head: " + DQ.getFirst());
    }
}

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Output:

Deque: [7855642, 35658786, 5278367, 74381793]
Deque's head: 7855642

Program 2:

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// Java Program Demonstrate getFirst()
// method of Deque when it is empty
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
        throws IllegalStateException
    {
  
        // create object of Deque
        Deque<Integer> DQ
            = new LinkedList<Integer>();
  
        // Add numbers to end of Deque
        DQ.add(7855642);
        DQ.add(35658786);
        DQ.add(5278367);
        DQ.add(74381793);
  
        // print Deque
        System.out.println("Deque: " + DQ);
  
        // print head
        System.out.println("Deque's head: " + DQ.getFirst());
  
        DQ.clear();
  
        // Deque is empty now hence exception
        System.out.println("Deque's head: " + DQ.getFirst());
    }
}

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Output:

Exception in thread "main" java.util.NoSuchElementException
    at java.util.LinkedList.getFirst(LinkedList.java:244)
    at GFG.main(GFG.java:29)

Reference: https://docs.oracle.com/javase/8/docs/api/java/util/Deque.html#getFirst–




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