Push Operation on Stack in DSA Java - Build from Scratch

Push Operation on Stack

📖 Scenario: Imagine you have a stack of plates in your kitchen. You want to add a new plate on top of the stack. This is called a push operation in computer science.

🎯 Goal: You will create a simple stack using an array and perform the push operation to add elements on top of the stack.

📋 What You'll Learn

Create an integer array called stack with size 5

Create an integer variable called top initialized to -1

Write a method called push that adds an integer to the stack if there is space

Print the stack elements after pushing new values

💡 Why This Matters

🌍 Real World

Stacks are used in many real-world applications like undo features in text editors, browser history, and expression evaluation.

💼 Career

Understanding stack operations is fundamental for software development roles, especially those involving algorithms, system design, and debugging.

1

Create the stack array and top variable

Create an integer array called stack with size 5 and an integer variable called top initialized to -1.

DSA Java

// Create stack array and top variable
// Your code here

Hint

Use int[] stack = new int[5]; to create the array and int top = -1; to track the top position.

2

Create the push method

Create a static method called push that takes an integer parameter value. Inside the method, check if top is less than 4. If yes, increment top by 1 and assign value to stack[top]. If the stack is full, print "Stack Overflow".

DSA Java

public class StackExample {
    static int[] stack = new int[5];
    static int top = -1;

    // Create push method below
    // Your code here

    public static void main(String[] args) {
    }
}

Hint

Remember to check if top is less than 4 before adding a new element.

3

Push values onto the stack

Inside the main method, call the push method three times to add the values 10, 20, and 30 to the stack.

DSA Java

public class StackExample {
    static int[] stack = new int[5];
    static int top = -1;

    static void push(int value) {
        if (top < 4) {
            top++;
            stack[top] = value;
        } else {
            System.out.println("Stack Overflow");
        }
    }

    public static void main(String[] args) {
        // Push 10, 20, 30 onto the stack
        // Your code here
    }
}

Hint

Call push three times with the values 10, 20, and 30.

4

Print the stack elements

After pushing the values, use a for loop to print all elements in the stack array from index 0 to top. Print each element separated by a space on the same line.

DSA Java

public class StackExample {
    static int[] stack = new int[5];
    static int top = -1;

    static void push(int value) {
        if (top < 4) {
            top++;
            stack[top] = value;
        } else {
            System.out.println("Stack Overflow");
        }
    }

    public static void main(String[] args) {
        push(10);
        push(20);
        push(30);
        // Print stack elements from index 0 to top
        // Your code here
    }
}

Hint

Use a for loop from 0 to top and print each element with a space.

Practice

1. What does the push operation do in a stack?

easy

A. Adds an element to the top of the stack

B. Removes the top element from the stack

C. Checks if the stack is empty

D. Returns the bottom element of the stack

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, so new elements are added on top.

2. Step 2: Define push operation

   The push operation adds a new element to the top of the stack, increasing its size by one.

3. Final Answer:

   Adds an element to the top of the stack -> Option A

4. Quick Check:

   Push = Add top element [OK]

Hint: Push always adds element on top of stack [OK]

Common Mistakes:

• Confusing push with pop
• Thinking push removes element
• Assuming push adds at bottom

2. Which of the following is the correct syntax to push an integer 10 onto a stack named stack in Java?

easy

A. stack.add(10);

B. stack.push(10);

C. stack.insert(10);

D. stack.put(10);

Solution

1. Step 1: Recall Java Stack API

   Java's Stack class uses the method push() to add elements.

2. Step 2: Match correct method

   Among options, only push() is the correct method to add elements to a stack.

3. Final Answer:

   stack.push(10); -> Option B

4. Quick Check:

   Java stack push method = push() [OK]

Hint: Use push() method to add elements in Java stack [OK]

Common Mistakes:

• Using add() which is for lists
• Using insert() which doesn't exist
• Using put() which is for maps

3. What will be the output after executing the following Java code?

Stack<Integer> stack = new Stack<>();
stack.push(5);
stack.push(10);
stack.push(15);
System.out.println(stack);

medium

A. [5, 15, 10]

B. [15, 10, 5]

C. [10, 15, 5]

D. [5, 10, 15]

Solution

1. Step 1: Trace push operations

   Elements 5, 10, and 15 are pushed in order, so stack stores them as [5, 10, 15].

2. Step 2: Understand Stack's toString()

   Java's Stack prints elements from bottom to top in list format, so output is [5, 10, 15].

3. Final Answer:

   [5, 10, 15] -> Option D

4. Quick Check:

   Stack print shows bottom to top [OK]

Hint: Stack prints bottom to top in Java [OK]

Common Mistakes:

• Assuming print shows top element first
• Confusing stack with queue order
• Expecting reversed list output

4. Identify the error in the following code snippet that tries to push an element onto a stack:

Stack<String> stack = new Stack<>();
stack.push("Hello");
stack.push(100);

medium

A. Stack must be initialized with size

B. Stack class does not have push method

C. Cannot push integer 100 into a Stack of Strings

D. Missing semicolon after push statements

Solution

1. Step 1: Check stack type and pushed elements

   The stack is declared to hold Strings only, but 100 is an integer.

2. Step 2: Identify type mismatch error

   Pushing an integer into a String stack causes a compile-time type error.

3. Final Answer:

   Cannot push integer 100 into a Stack of Strings -> Option C

4. Quick Check:

   Type mismatch error = pushing wrong data type [OK]

Hint: Stack type must match pushed element type [OK]

Common Mistakes:

• Ignoring generic type mismatch
• Thinking push method missing
• Assuming size needed at init

5. Given an empty stack, which sequence of push operations will result in the stack containing elements in this order from bottom to top: 3 -> 7 -> 5 -> null?

hard

A. push(3), push(7), push(5)

B. push(5), push(7), push(3)

C. push(7), push(3), push(5)

D. push(5), push(3), push(7)

Solution

1. Step 1: Understand stack order after pushes

   Elements pushed first are at bottom, last pushed at top.

2. Step 2: Match push order to desired stack

   To get 3 at bottom, then 7, then 5 on top, push in order: 3, then 7, then 5.

3. Final Answer:

   push(3), push(7), push(5) -> Option A

4. Quick Check:

   Push order = bottom to top [OK]

Hint: Push elements in bottom-to-top order [OK]

Common Mistakes:

• Reversing push order
• Confusing push with pop order
• Assuming stack inserts at bottom