Stack push() Method in Java

The push() method in Java is used to add an element to the top of a Stack. It is commonly used to insert elements while maintaining the LIFO (Last-In, First-Out) order.

• It accepts one element as a parameter and returns the pushed element.
• It allows null elements to be added to the Stack.

import java.util.Stack;

public class GFG{
    
    public static void main(String[] args){
        
        Stack<Integer> stack = new Stack<>();

        stack.push(10);
        stack.push(20);
        stack.push(30);

        System.out.println(stack);
    }
}

[10, 20, 30]

Explanation: The push() method adds each element to the top of the stack. Here, 30 is the top element because it was added last.

Syntax

public E push(E item);

Here, E represents the type of element stored in the Stack, and item is the element to be added.

• Parameter: item -> The element to be added to the top of the stack.
• Return Value: Returns the element that was pushed onto the stack.

Note: Stack.push() allows null elements. In contrast, ArrayDeque.push() throws a NullPointerException when null is passed.

Examples on push() Method

The following examples demonstrate how the push() method can be used to add different types of elements to a Stack.

Example: Adding String Elements to a Stack

The following program demonstrates how to add string elements to a Stack using the push() method.

import java.util.Stack;

public class StackDemo {

    public static void main(String[] args) {

        // Creating an empty Stack
        Stack<String> stack = new Stack<>();

        // Adding elements using push()
        stack.push("Welcome");
        stack.push("To");
        stack.push("Geeks");
        stack.push("For");
        stack.push("Geeks");

        // Displaying the initial Stack
        System.out.println("Initial Stack: " + stack);

        // Adding more elements using push()
        stack.push("Hello");
        stack.push("World");

        // Displaying the final Stack
        System.out.println("Final Stack: " + stack);
    }
}

Initial Stack: [Welcome, To, Geeks, For, Geeks]
Final Stack: [Welcome, To, Geeks, For, Geeks, Hello, World]

Explanation:

• The program creates an empty Stack<String> named stack.
• The push() method adds "Welcome", "To", "Geeks", "For", and "Geeks" to the stack.
• The initial stack is displayed using System.out.println().
• "Hello" and "World" are then added to the top of the stack using push().
• Finally, the updated stack is displayed, showing all elements in the order they were added.

Example: Adding Integer Elements to a Stack

The following program demonstrates how to add integer elements to a Stack, including a null value.

import java.util.Stack;

public class StackDemo {
    public static void main(String[] args) {

        // Creating an empty Stack
        Stack<Integer> stack = new Stack<>();

        // Adding elements to the Stack using push()
        stack.push(10);
        stack.push(15);
        stack.push(30);
        stack.push(20);
        stack.push(5);
        stack.push(null);

        // Displaying the Stack
        System.out.println("Initial Stack: " + stack);

        // Pushing additional elements into the Stack
        stack.push(1254);
        stack.push(4521);

        // Displaying the final Stack
        System.out.println("Final Stack: " + stack);
    }
}

Initial Stack: [10, 15, 30, 20, 5, null]
Final Stack: [10, 15, 30, 20, 5, null, 1254, 4521]

Explanation:

• The program creates an empty Stack<Integer> named stack.
• The push() method adds integer values 10, 15, 30, 20, 5, and null to the stack.
• The initial stack is displayed using System.out.println().
• The values 1254 and 4521 are then added to the top of the stack using push().
• Finally, the updated stack is displayed, including the newly added elements.