Check if Stack is Empty or Full in DSA Java - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java stack check?

Consider a stack of size 3. After pushing two elements, what does the isFull() method return?

DSA Java

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

  boolean isFull() {
    return top == arr.length - 1;
  }

  void push(int x) {
    if (!isFull()) {
      arr[++top] = x;
    }
  }

  public static void main(String[] args) {
    Stack s = new Stack();
    s.push(10);
    s.push(20);
    System.out.println(s.isFull());
  }
}

A Compilation error

B true

C false

D ArrayIndexOutOfBoundsException

💡 Hint

Think about how many elements the stack can hold and how many are pushed.

The stack size is 3, but only 2 elements are pushed. So, top is 1, and isFull() checks if top == 2, which is false.

❓ Predict Output

intermediate

What does this code print about stack emptiness?

After pushing and popping elements, what does isEmpty() return?

DSA Java

class Stack {
  int[] arr = new int[2];
  int top = -1;

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

  void push(int x) {
    if (top < arr.length - 1) {
      arr[++top] = x;
    }
  }

  void pop() {
    if (!isEmpty()) {
      top--;
    }
  }

  public static void main(String[] args) {
    Stack s = new Stack();
    s.push(5);
    s.push(10);
    s.pop();
    s.pop();
    System.out.println(s.isEmpty());
  }
}

A ArrayIndexOutOfBoundsException

B true

C Compilation error

D false

💡 Hint

Check how many elements remain after popping twice.

After pushing two elements and popping twice, the stack is empty, so isEmpty() returns true.

❓ Predict Output

advanced

What error occurs when pushing on a full stack?

What happens when you push an element onto a full stack without checking if it is full?

DSA Java

class Stack {
  int[] arr = new int[2];
  int top = -1;

  void push(int x) {
    arr[++top] = x;
  }

  public static void main(String[] args) {
    Stack s = new Stack();
    s.push(1);
    s.push(2);
    s.push(3);
  }
}

A ArrayIndexOutOfBoundsException

B StackOverflowError

C No error, element added successfully

D NullPointerException

💡 Hint

Consider what happens when the array index goes beyond its size.

The array size is 2, but push is called 3 times without checking. The third push tries to access index 2, causing ArrayIndexOutOfBoundsException.

🧠 Conceptual

advanced

Which condition correctly checks if a stack is empty?

Given a stack with a top pointer, which condition correctly returns true if the stack is empty?

A top == -1

B top == 0

C top == arr.length

D top > 0

💡 Hint

Think about the initial value of top before any push.

The top is initialized to -1 when the stack is empty. So, top == -1 means empty stack.

❓ Predict Output

expert

What is the printed output after these stack operations?

Analyze the stack operations and determine the printed output.

DSA Java

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

  boolean isFull() {
    return top == arr.length - 1;
  }

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

  void push(int x) {
    if (!isFull()) {
      arr[++top] = x;
    }
  }

  int pop() {
    if (!isEmpty()) {
      return arr[top--];
    }
    return -1;
  }

  public static void main(String[] args) {
    Stack s = new Stack();
    s.push(7);
    s.push(14);
    s.push(21);
    System.out.print(s.pop() + " ");
    s.push(28);
    System.out.print(s.pop() + " ");
    System.out.print(s.pop() + " ");
    System.out.print(s.isEmpty());
  }
}

A 14 21 28 false

B 21 28 14 true

C 28 21 14 false

D 21 28 14 false

💡 Hint

Trace each push and pop step carefully.

Push 7,14,21 fills stack. Pop returns 21. Push 28. Pop returns 28. Pop returns 14. Stack still has 7, so isEmpty() is false.

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