Why Check if Stack is Empty or Full in DSA Java?

The Big Idea

What if your program crashes just because it tried to add to a full stack or remove from an empty one?

The Scenario

Imagine you have a stack of plates in your kitchen. You want to add a new plate or take one out, but you need to know if the stack is already full or completely empty before you do anything.

The Problem

If you try to add a plate without checking if the stack is full, the plates might fall and break. If you try to take a plate when the stack is empty, you will be left empty-handed and confused. Doing this manually every time is slow and can cause mistakes.

The Solution

By having simple checks to see if the stack is empty or full, you can safely add or remove plates without any accidents. These checks act like traffic lights, guiding your actions and preventing errors.

Before vs After

✗ Before

if (top == -1) {
  System.out.println("Stack is empty");
} else if (top == maxSize - 1) {
  System.out.println("Stack is full");
}

✓ After

public boolean isEmpty() {
  return top == -1;
}

public boolean isFull() {
  return top == maxSize - 1;
}

What It Enables

This lets you safely and quickly manage your stack without worrying about errors or crashes.

Real Life Example

When you use the undo button in a text editor, the program checks if there are actions to undo (stack not empty) before going back, preventing errors.

Key Takeaways

Manual checks can cause mistakes and slow you down.

Simple methods to check empty or full keep your stack safe.

These checks make your programs reliable and easy to use.

Practice

1. What does the isEmpty() method in a stack typically check?

easy

A. If the stack has exactly one element

B. If the stack is full

C. If the stack has no elements

D. If the stack is initialized

Solution

1. Step 1: Understand the purpose of isEmpty()

   The isEmpty() method checks whether the stack contains any elements or not.

2. Step 2: Identify the condition checked

   If the stack has no elements, it is empty, so isEmpty() returns true.

3. Final Answer:

   If the stack has no elements -> Option C

4. Quick Check:

   Empty stack means no elements = true [OK]

Hint: Empty means zero elements in stack [OK]

Common Mistakes:

• Confusing isEmpty() with isFull()
• Thinking isEmpty() checks for one element
• Assuming isEmpty() checks initialization

2. Which of the following is the correct syntax to check if a stack named stack is full in Java?

easy

A. stack.isEmpty();

B. stack.isFull();

C. stack.full();

D. stack.isFilled();

Solution

1. Step 1: Identify the standard method name for full check

   In stack implementations, the method to check if the stack is full is usually named isFull().

2. Step 2: Match the correct syntax

   The correct syntax to call this method on a stack object named stack is stack.isFull();.

3. Final Answer:

   stack.isFull(); -> Option B

4. Quick Check:

   Method to check full = isFull() [OK]

Hint: Full check method is isFull() [OK]

Common Mistakes:

• Using isEmpty() to check full
• Using non-existent methods like full()
• Misspelling method names

3. What will be the output of the following Java code?

class Stack {
  int maxSize = 3;
  int top = -1;
  int[] arr = new int[maxSize];

  boolean isFull() {
    return top == maxSize - 1;
  }

  boolean isEmpty() {
    return top == -1;
  }

  void push(int val) {
    if (!isFull()) {
      arr[++top] = val;
      System.out.println("Pushed " + val);
    } else {
      System.out.println("Stack is full");
    }
  }
}

public class Main {
  public static void main(String[] args) {
    Stack stack = new Stack();
    stack.push(10);
    stack.push(20);
    stack.push(30);
    stack.push(40);
  }
}

medium

A. Pushed 10
Pushed 20
Pushed 30
Stack is full

B. Pushed 10
Pushed 20
Pushed 30
Pushed 40

C. Stack is full
Stack is full
Stack is full
Stack is full

D. Pushed 10
Pushed 20
Stack is full
Stack is full

Solution

1. Step 1: Trace push operations and stack state

   Initially, top = -1. Each push increments top and adds value if stack not full. Max size is 3, so max top index is 2.

2. Step 2: Check each push call

   Push 10: top becomes 0, prints "Pushed 10".
   Push 20: top becomes 1, prints "Pushed 20".
   Push 30: top becomes 2, prints "Pushed 30".
   Push 40: top is 2 (full), so prints "Stack is full".

3. Final Answer:

   Pushed 10 Pushed 20 Pushed 30 Stack is full -> Option A

4. Quick Check:

   Stack max size 3, 4th push fails [OK]

Hint: Stack full when top == maxSize-1 [OK]

Common Mistakes:

• Assuming push always succeeds
• Ignoring top index limits
• Confusing isEmpty() with isFull()

4. Find the error in this stack implementation snippet that checks if the stack is empty:

public boolean isEmpty() {
  if (top == 0) {
    return true;
  } else {
    return false;
  }
}

medium

A. No error, the code is correct

B. The method should return false when top == 0

C. The method should check if top == maxSize

D. The condition should be top == -1 to check empty

Solution

1. Step 1: Understand stack empty condition

   In stack, empty means no elements pushed, so top is usually -1.

2. Step 2: Identify incorrect condition

   The code checks top == 0, which means one element is present, so it is wrong for empty check.

3. Final Answer:

   The condition should be top == -1 to check empty -> Option D

4. Quick Check:

   Empty stack top = -1 [OK]

Hint: Empty stack top is -1, not 0 [OK]

Common Mistakes:

• Checking top == 0 for empty
• Confusing full and empty conditions
• Returning wrong boolean values

5. You have a stack with max size 5 and current top = 3. Which of the following statements is true about the stack's state?

hard

A. The stack is neither full nor empty

B. The stack is empty

C. The stack is full

D. The stack has exactly 5 elements

Solution

1. Step 1: Understand stack indexing and size

   Stack max size is 5, so valid top indices are 0 to 4. top = 3 means 4 elements are present (indices 0,1,2,3).

2. Step 2: Determine if stack is full or empty

   Full means top == maxSize - 1 which is 4 here, so not full.
   Empty means top == -1, so not empty.
   Therefore, stack is neither full nor empty.

3. Final Answer:

   The stack is neither full nor empty -> Option A

4. Quick Check:

   top=3, maxSize=5 means partial fill [OK]

Hint: Full if top = maxSize-1, empty if top = -1 [OK]

Common Mistakes:

• Assuming top=3 means full
• Confusing top index with element count
• Thinking top=3 means empty