Push Operation on Stack in DSA Java - Execution Trace

Concept Flow - Push Operation on Stack

Create new element

↓

Check if stack is full?

Yes→Error: Stack Overflow

No↓

Place new element on top

↓

Update top pointer

↓

Done

This flow shows how a new element is added on top of the stack if there is space, otherwise an overflow error occurs.

Execution Sample

DSA Java

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

Push three elements 10, 20, and 30 onto the stack one by one.

Execution Table

Step	Operation	Nodes in Stack	Top Pointer Index	Visual State
1	Push 10	[10]	0	10 -> null
2	Push 20	[10, 20]	1	20 -> 10 -> null
3	Push 30	[10, 20, 30]	2	30 -> 20 -> 10 -> null
4	Attempt to push when full (if max size 3)	[10, 20, 30]	2	Stack Overflow - no change

💡 Stack stops accepting new elements when top pointer reaches max size - 1 (stack full).

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
stack array	[]	[10]	[10, 20]	[10, 20, 30]	[10, 20, 30]
top pointer	-1	0	1	2	2
stack size	0	1	2	3	3

Key Moments - 3 Insights

Why does the top pointer start at -1 before any push?

The top pointer is -1 to indicate the stack is empty. After the first push (see Step 1 in execution_table), it moves to 0 pointing to the first element.

What happens if we try to push when the stack is full?

As shown in Step 4, pushing when the stack is full causes a 'Stack Overflow' error and the stack state does not change.

Why does the visual state show the newest element first?

Because stack is Last-In-First-Out, the newest pushed element is always on top, so it appears first in the visual state (see Steps 1-3).

Visual Quiz - 3 Questions

Look at the execution_table, what is the top pointer value after pushing 20?

A 0

B 1

C 2

D -1

💡 Hint

Check the 'Top Pointer Index' column at Step 2 in execution_table.

At which step does the stack become full if max size is 3?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'stack size' variable in variable_tracker after each step.

If we push 40 after Step 3, what will happen according to execution_table?

A Stack Overflow error, no change

B Oldest element removed, 40 added

C 40 will be added on top

D Stack size increases to 4

💡 Hint

Refer to Step 4 in execution_table for pushing when stack is full.

Concept Snapshot

Push Operation on Stack:
- Add new element on top if space available
- Update top pointer to new element's index
- If stack full, raise overflow error
- Stack is LIFO: last pushed is first out
- Top pointer starts at -1 when empty

Full Transcript

The push operation adds a new element to the top of the stack. Initially, the top pointer is -1 indicating the stack is empty. When pushing an element, the top pointer moves up by one and the element is placed there. This continues until the stack reaches its maximum size. If a push is attempted when the stack is full, a stack overflow error occurs and no changes are made. The visual state always shows the newest element first because the stack follows last-in-first-out order.

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