Check if Stack is Empty or Full in DSA Java - Build from Scratch

Check if Stack is Empty or Full

📖 Scenario: You are building a simple stack to manage plates in a kitchen. The stack can hold only a limited number of plates. You want to check if the stack is empty (no plates) or full (maximum plates) before adding or removing plates.

🎯 Goal: Create a stack with a fixed size, add a configuration for the maximum size, write methods to check if the stack is empty or full, and print the results.

📋 What You'll Learn

Create an integer array called stack with exactly 5 elements.

Create an integer variable called top and set it to -1 to represent an empty stack.

Create an integer variable called maxSize and set it to 5.

Write a method called isEmpty that returns true if top is -1, otherwise false.

Write a method called isFull that returns true if top is equal to maxSize - 1, otherwise false.

Print the results of calling isEmpty() and isFull().

💡 Why This Matters

🌍 Real World

Stacks are used in many real-life situations like managing plates, undo actions in software, and browser history.

💼 Career

Understanding stack operations is important for software development roles, especially when working with algorithms and memory management.

1

Create the stack data structure

Create an integer array called stack with exactly 5 elements and an integer variable called top set to -1.

DSA Java

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

Hint

Use int[] stack = new int[5]; to create the stack array and int top = -1; to mark it empty.

2

Add the maximum size configuration

Create an integer variable called maxSize and set it to 5.

DSA Java

int[] stack = new int[5];
int top = -1;
// Create maxSize variable
// Your code here

Hint

Use int maxSize = 5; to store the maximum stack size.

3

Write methods to check if stack is empty or full

Write a method called isEmpty that returns true if top == -1, otherwise false. Also write a method called isFull that returns true if top == maxSize - 1, otherwise false.

DSA Java

int[] stack = new int[5];
int top = -1;
int maxSize = 5;

// Write isEmpty method
// Your code here

// Write isFull method
// Your code here

Hint

Use return top == -1; in isEmpty() and return top == maxSize - 1; in isFull().

4

Print the results of isEmpty and isFull

Print the results of calling isEmpty() and isFull() using System.out.println.

DSA Java

int[] stack = new int[5];
int top = -1;
int maxSize = 5;

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

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

// Print the results
// Your code here

Hint

Use System.out.println(isEmpty()); and System.out.println(isFull()); to print the results.

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