Stack memory in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a stack of integers using Java's Stack class.

Java

Stack<Integer> numbers = new [1]<>();

Drag options to blanks, or click blank then click option'

AArrayList

BStack

CLinkedList

DHashSet

Attempts:

3 left

💡 Hint

Common Mistakes

Using ArrayList or LinkedList instead of Stack.

Forgetting to specify the generic type.

We use Stack to create a stack data structure in Java.

2fill in blank

medium

Complete the code to add an element to the top of the stack.

Java

numbers.[1](10);

Drag options to blanks, or click blank then click option'

Apush

Badd

Cinsert

Doffer

Attempts:

3 left

💡 Hint

Common Mistakes

Using add which works for lists but is not the stack method.

Using offer which is for queues.

The push method adds an element to the top of the stack.

3fill in blank

hard

Fix the error in the code to remove the top element from the stack.

Java

int top = numbers.[1]();

Drag options to blanks, or click blank then click option'

Apoll

Bremove

Cpop

Ddelete

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove which requires an index or object.

Using poll which is for queues.

The pop method removes and returns the top element of the stack.

4fill in blank

hard

Fill both blanks to check if the stack is empty and print a message.

Java

if (numbers.[1]()) {
    System.out.println("Stack is [2].");
}

Drag options to blanks, or click blank then click option'

AisEmpty

Bempty

Cfull

Dclear

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty() method which exists but returns a boolean differently.

Using full() or clear() which are not correct here.

The isEmpty() method checks if the stack has no elements, so the message says "empty".

5fill in blank

hard

Fill all three blanks to create a stack, push two elements, and pop one.

Java

Stack<String> stack = new [1]<>();
stack.[2]("apple");
stack.[3]();

Drag options to blanks, or click blank then click option'

AStack

Bpush

Cpop

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using add instead of push.

Using remove or delete instead of pop.

We create a Stack, add an element with push, and remove the top element with pop.

Practice

1. What does stack memory primarily store in Java?

easy

A. Method calls and local variables

B. Objects and class variables

C. Static variables and constants

D. Files and network connections

Solution

1. Step 1: Understand stack memory role

   Stack memory stores data related to method execution like local variables and method calls.

2. Step 2: Differentiate from heap memory

   Heap stores objects, while stack stores method calls and local variables.

3. Final Answer:

   Method calls and local variables -> Option A

4. Quick Check:

   Stack memory = method calls + local variables [OK]

Hint: Stack = method calls + local variables [OK]

Common Mistakes:

• Confusing stack with heap memory
• Thinking objects are stored in stack
• Mixing static variables with stack data

2. Which of the following is the correct way to declare a local variable inside a method in Java?

easy

A. static int x = 10;

B. local int x = 10;

C. var int x = 10;

D. int x = 10;

Solution

1. Step 1: Recall local variable syntax

   Local variables inside methods are declared simply with type and name, e.g., int x = 10;

2. Step 2: Identify invalid keywords

   Keywords like 'local' or 'var int' are invalid in Java, and 'static' is not allowed for local variables.

3. Final Answer:

   int x = 10; -> Option D

4. Quick Check:

   Local variable syntax = type + name + value [OK]

Hint: Local vars: type name = value; no extra keywords [OK]

Common Mistakes:

• Using 'local' keyword which doesn't exist
• Trying to use 'static' inside methods
• Confusing 'var' usage in older Java versions

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

public class Test {
  public static void main(String[] args) {
    int a = 5;
    int b = 10;
    int c = add(a, b);
    System.out.println(c);
  }
  public static int add(int x, int y) {
    return x + y;
  }
}

medium

A. 15

B. 510

C. Compilation error

D. Runtime error

Solution

1. Step 1: Trace method calls and parameters

   Main calls add(5, 10), passing values 5 and 10 as local variables on the stack.

2. Step 2: Calculate return value

   Add method returns 5 + 10 = 15, which is stored in c and printed.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add method returns sum; print result [OK]

Common Mistakes:

• Concatenating numbers as strings
• Confusing method parameters with global variables
• Expecting runtime or compile errors

4. Identify the error in this Java code related to stack memory usage:

public class Demo {
  public static void main(String[] args) {
    int result = factorial(4);
    System.out.println(result);
  }
  public static int factorial(int n) {
    if (n == 0) return 1;
    else return n * factorial(n - 1);
  }
}

medium

A. Stack overflow due to too many local variables

B. Missing base case causing infinite recursion

C. No error, code runs correctly

D. Incorrect method return type

Solution

1. Step 1: Check recursion base case

   The factorial method has a base case: if n == 0 return 1, stopping recursion.

2. Step 2: Confirm recursion correctness

   Each recursive call reduces n by 1, so recursion ends properly without stack overflow.

3. Final Answer:

   No error, code runs correctly -> Option C

4. Quick Check:

   Base case present, recursion safe [OK]

Hint: Check for base case to avoid stack overflow [OK]

Common Mistakes:

• Assuming missing base case when it exists
• Confusing local variables with stack overflow cause
• Thinking return type is wrong

5. Consider this recursive method that calculates the sum of numbers from n down to 1:

public static int sum(int n) {
  if (n <= 0) return 0;
  else return n + sum(n - 1);
}

What will happen if you call sum(1000000) in Java, and why?

hard

A. It will return the correct sum without issues

B. It will cause a stack overflow error due to deep recursion

C. It will run infinitely without stopping

D. It will cause a compile-time error

Solution

1. Step 1: Analyze recursion depth

   Calling sum(1000000) causes 1,000,000 recursive calls, each adding a stack frame.

2. Step 2: Understand stack memory limits

   Java's stack size is limited; deep recursion like this exceeds it, causing stack overflow error.

3. Final Answer:

   It will cause a stack overflow error due to deep recursion -> Option B

4. Quick Check:

   Deep recursion > stack size = stack overflow [OK]

Hint: Very deep recursion causes stack overflow error [OK]

Common Mistakes:

• Assuming recursion always works regardless of depth
• Thinking it runs infinitely without base case
• Confusing runtime errors with compile errors