Heap memory in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code related to heap memory?

Consider the following Java code snippet. What will be printed to the console?

Java

public class HeapTest {
    public static void main(String[] args) {
        String a = new String("hello");
        String b = new String("hello");
        System.out.println(a == b);
        System.out.println(a.equals(b));
    }
}

A false\ntrue

B true\ntrue

C false\nfalse

D true\nfalse

💡 Hint

Remember that == compares references, and equals() compares content.

The new String("hello") creates two different objects in heap memory, so a == b is false. But their contents are the same, so a.equals(b) is true.

🧠 Conceptual

intermediate

Which statement about Java heap memory is true?

Choose the correct statement about Java heap memory.

A Heap memory is used to store local variables inside methods.

B Heap memory is shared among all threads and stores objects and arrays.

C Heap memory stores only primitive data types.

D Heap memory is automatically cleared when a method finishes execution.

💡 Hint

Think about where objects and arrays live in Java.

Heap memory stores all objects and arrays and is shared among all threads. Primitive local variables are stored on the stack, not heap.

🔧 Debug

advanced

What error does this code cause related to heap memory?

Analyze the following Java code. What error will it cause when run?

Java

public class HeapOverflow {
    public static void main(String[] args) {
        int i = 0;
        while(true) {
            i++;
            int[] arr = new int[1000000];
            System.out.println("Iteration: " + i);
        }
    }
}

A OutOfMemoryError

B No error, infinite loop runs fine

C NullPointerException

D StackOverflowError

💡 Hint

Think about what happens when you keep creating large arrays without releasing them.

The code keeps creating large arrays without references being freed, filling heap memory until it runs out, causing an OutOfMemoryError.

📝 Syntax

advanced

Which option correctly declares an object stored in heap memory?

Which of the following Java code snippets correctly creates an object that will be stored in heap memory?

A int x = new int(5);

B String s = "hello";

C String s = new String("hello");

D int[] arr = 10;

💡 Hint

Remember how objects are created with new keyword.

Only new String("hello") creates a new object in heap memory. Option C uses a string literal which is stored in the string pool, not heap in the same way. Option C is invalid syntax, and D is invalid assignment.

🚀 Application

expert

How many objects are created in heap memory after running this code?

Consider this Java code snippet. How many distinct objects are created in heap memory after it runs?

Java

public class ObjectCount {
    public static void main(String[] args) {
        String s1 = "test";
        String s2 = new String("test");
        String s3 = s2.intern();
        Integer i1 = 100;
        Integer i2 = new Integer(100);
    }
}

A 3

B 5

C 6

D 4

💡 Hint

Remember string literals, new keyword, and interning behavior.

Objects created: "test" literal (string pool), new String("test"), new Integer(100), and the Integer i1 = 100; uses cached Integer object (not new). The intern() returns the literal object, so no new object there. Total 4 objects.

Practice

1. Which of the following best describes heap memory in Java?

easy

A. It stores method call information and local variables.

B. It stores primitive variables like int and double.

C. It stores all objects created using new keyword.

D. It stores the Java bytecode instructions.

Solution

1. Step 1: Understand what heap memory stores

   Heap memory is used to store objects created dynamically using the new keyword in Java.

2. Step 2: Differentiate from other memory areas

   Primitive variables and method calls are stored on the stack, not heap. Bytecode is stored in method area, not heap.

3. Final Answer:

   It stores all objects created using new keyword. -> Option C

4. Quick Check:

   Heap memory = objects from new [OK]

Hint: Heap = objects created with new keyword [OK]

Common Mistakes:

• Confusing stack with heap memory
• Thinking primitives are stored in heap
• Mixing method area with heap

2. Which of the following Java statements correctly creates an object stored in heap memory?

easy

A. String s = new String("hello");

B. int x = 5;

C. double y = 3.14;

D. boolean flag = true;

Solution

1. Step 1: Identify object creation syntax

   Objects in Java are created using the new keyword, which allocates memory on the heap.

2. Step 2: Check each option

   String s = new String("hello"); uses new String("hello"), which creates a String object on the heap. Other options declare primitives stored on the stack.

3. Final Answer:

   String s = new String("hello"); -> Option A

4. Quick Check:

   Object creation uses new keyword [OK]

Hint: Look for 'new' keyword to spot heap allocation [OK]

Common Mistakes:

• Assuming primitives use heap
• Confusing string literals with new String()
• Ignoring that only new creates heap objects

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

class Test {
  public static void main(String[] args) {
    String a = new String("abc");
    String b = a;
    a = null;
    System.out.println(b);
  }
}

medium

A. null

B. Runtime exception

C. Compilation error

D. abc

Solution

1. Step 1: Understand object references and heap

   Variable a points to a String object "abc" in heap. Then b is assigned the same reference.

2. Step 2: Analyze effect of setting a to null

   Setting a = null removes one reference, but b still points to the object, so it remains accessible.

3. Step 3: Print b

   Printing b outputs the string "abc".

4. Final Answer:

   abc -> Option D

5. Quick Check:

   Reference b still points to "abc" [OK]

Hint: Nulling one reference doesn't delete object if others exist [OK]

Common Mistakes:

• Assuming object is deleted when one reference is null
• Confusing references with objects
• Expecting null output incorrectly

4. Identify the error in the following Java code related to heap memory usage:

public class Demo {
  public static void main(String[] args) {
    int[] arr = new int[5];
    arr = null;
    System.out.println(arr.length);
  }
}

medium

A. No error, prints 5

B. NullPointerException at runtime

C. Compilation error due to null assignment

D. Array size becomes zero

Solution

1. Step 1: Understand array creation and null assignment

   The array is created on heap with size 5, then arr reference is set to null.

2. Step 2: Analyze printing arr.length

   Since arr is null, accessing arr.length causes a NullPointerException at runtime.

3. Final Answer:

   NullPointerException at runtime -> Option B

4. Quick Check:

   Accessing member on null reference throws exception [OK]

Hint: Accessing null reference causes NullPointerException [OK]

Common Mistakes:

• Thinking null assignment causes compile error
• Expecting array length to print after null
• Confusing null with empty array

5. Consider this Java code snippet:

class Node {
  int value;
  Node next;
  Node(int val) { value = val; }
}

public class Test {
  public static void main(String[] args) {
    Node n1 = new Node(1);
    Node n2 = new Node(2);
    n1.next = n2;
    n2.next = n1;
    n1 = null;
    n2 = null;
    // What happens to the Node objects in heap?
  }
}

What happens to the two Node objects in heap after n1 and n2 are set to null?

hard

A. Both objects become unreachable and eligible for garbage collection.

B. Objects remain in heap because they reference each other, preventing garbage collection.

C. Only n1 object is collected, n2 remains.

D. Objects cause a memory leak and crash the program.

Solution

1. Step 1: Analyze object references after null assignments

   Both n1 and n2 references are set to null, so no external references point to these objects.

2. Step 2: Understand garbage collection with cyclic references

   Even though the two objects reference each other, Java's garbage collector detects unreachable cycles and frees them.

3. Final Answer:

   Both objects become unreachable and eligible for garbage collection. -> Option A

4. Quick Check:

   Unreachable objects, even cyclic, are collected [OK]

Hint: Java GC collects unreachable cyclic objects too [OK]

Common Mistakes:

• Thinking cyclic references prevent garbage collection
• Assuming memory leak without external references
• Confusing references with reachability