Garbage collection overview in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of garbage collection in Java?

Choose the best description of what garbage collection does in Java.

A Prevents the creation of new objects to save memory.

B Manually deletes objects when the programmer calls a delete method.

C Automatically frees memory by removing objects that are no longer reachable.

D Optimizes CPU usage by pausing all running threads.

💡 Hint

Think about how Java manages memory without programmer intervention.

Garbage collection automatically frees memory by removing objects that are no longer reachable, helping prevent memory leaks.

❓ query_result

intermediate

What output does this code produce regarding object finalization?

Consider the following Java code snippet. What will be printed when the program runs?

Java

class Test {
  protected void finalize() {
    System.out.println("Finalized");
  }
  public static void main(String[] args) {
    Test t = new Test();
    t = null;
    System.gc();
    System.out.println("End of main");
  }
}

A End of main

B

Finalized
End of main

C

End of main
Finalized

D No output

💡 Hint

Garbage collection and finalization timing is not guaranteed immediately after System.gc().

Calling System.gc() suggests garbage collection but does not guarantee immediate finalization. The program prints "End of main" and may finalize later.

📝 Syntax

advanced

Identify the syntax error in this garbage collection related code snippet

Which option contains a syntax error preventing compilation?

Java

public class GCExample {
  public static void main(String[] args) {
    Object obj = new Object();
    obj = null;
    System.gc();
  }
}

A finalize() { System.out.println("Cleaning up"); }

B protected void finalize() { System.out.println("Cleaning up"); }

C void finalize() { System.out.println("Cleaning up"); }

D public void finalize() { System.out.println("Cleaning up"); }

💡 Hint

Check method declaration syntax in Java.

Method declarations must include a return type. Option A lacks a return type, causing a syntax error.

❓ optimization

advanced

Which approach helps reduce garbage collection overhead in Java?

Select the best practice to minimize the impact of garbage collection on application performance.

A Create many short-lived objects frequently to keep memory usage high.

B Avoid using local variables to reduce memory allocation.

C Call System.gc() explicitly after every object creation.

D Reuse objects when possible instead of creating new ones repeatedly.

💡 Hint

Think about how object reuse affects memory and garbage collection frequency.

Reusing objects reduces the number of new objects created, lowering the frequency and cost of garbage collection.

🔧 Debug

expert

Why does this Java program cause a memory leak despite garbage collection?

Examine the code below and select the reason it causes a memory leak.

Java

import java.util.ArrayList;
public class LeakExample {
  static ArrayList<Object> list = new ArrayList<>();
  public static void main(String[] args) {
    while(true) {
      Object obj = new Object();
      list.add(obj);
    }
  }
}

A The infinite loop causes the program to crash before garbage collection can run.

B Objects are never dereferenced because they are stored in a static list, preventing garbage collection.

C System.gc() is not called, so garbage collection never happens.

D The Object class does not support garbage collection.

💡 Hint

Consider what keeps objects reachable and how that affects garbage collection.

Storing objects in a static list keeps references alive, so garbage collector cannot free them, causing a memory leak.

Practice

1. What is the main purpose of garbage collection in Java?

easy

A. Increase the size of the heap memory

B. Automatically free memory used by objects no longer needed

C. Manually delete objects from memory

D. Compile Java code into bytecode

Solution

1. Step 1: Recall the purpose of garbage collection and evaluate options

   Garbage collection automatically frees memory used by objects that are no longer referenced in the program. Only "Automatically free memory used by objects no longer needed" matches this definition.

2. Final Answer:

   Automatically free memory used by objects no longer needed -> Option B

3. Quick Check:

   Garbage collection = automatic memory freeing [OK]

Hint: Garbage collection frees unused memory automatically [OK]

Common Mistakes:

• Thinking garbage collection deletes objects manually
• Confusing garbage collection with compiling code
• Believing it increases heap size

2. Which of the following is the correct way to suggest garbage collection in Java?

easy

A. GarbageCollector.run()

B. System.collect()

C. Runtime.gcCollect()

D. System.gc()

Solution

1. Step 1: Recall and verify the method to suggest garbage collection

   Java provides System.gc() to suggest that the JVM runs garbage collection. Only System.gc() is a valid Java method; the others are incorrect or do not exist.

2. Final Answer:

   System.gc() -> Option D

3. Quick Check:

   Suggest GC = System.gc() [OK]

Hint: Use System.gc() to suggest garbage collection [OK]

Common Mistakes:

• Using System.collect() which does not exist
• Confusing Runtime methods with System methods
• Assuming garbage collection runs immediately after calling System.gc()

3. Consider the following Java code snippet:

public class TestGC {
  public static void main(String[] args) {
    String s = new String("hello");
    s = null;
    System.gc();
    System.out.println("Done");
  }
}

What is the expected output when this code runs?

medium

A. Done

B. hello

C. No output, program crashes

D. Done followed by garbage collection logs

Solution

1. Step 1: Trace code execution and determine output

   The string object "hello" is created, then the reference s is set to null, making it eligible for GC. System.gc() suggests collection but does not guarantee immediate action or logs. The only guaranteed output is "Done" from the print statement.

2. Final Answer:

   Done -> Option A

3. Quick Check:

   Print statement output = Done [OK]

Hint: System.gc() does not print anything; only explicit prints show [OK]

Common Mistakes:

• Expecting garbage collection logs to print automatically
• Thinking the string "hello" prints after null assignment
• Assuming program crashes due to null reference

4. Identify the error in the following Java code related to garbage collection:

public class GCExample {
  public static void main(String[] args) {
    Object obj = new Object();
    obj = null;
    System.gc;
  }
}

medium

A. Cannot assign null to an Object

B. Object cannot be garbage collected

C. Missing parentheses in System.gc call

D. System.gc is not a valid method

Solution

1. Step 1: Locate the syntax error in the code

   System.gc is a method call requiring parentheses (System.gc()); the code has System.gc without them, causing a compilation error. Assigning null to Object and the method's existence are valid.

2. Final Answer:

   Missing parentheses in System.gc call -> Option C

3. Quick Check:

   Method calls need parentheses [OK]

Hint: Always use parentheses when calling methods like System.gc() [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Thinking null assignment is invalid
• Believing System.gc is not a method

5. You have a Java program that creates many temporary objects inside a loop. Which approach best helps ensure efficient memory use with garbage collection?

hard

A. Set object references to null when no longer needed inside the loop

B. Call System.gc() inside every loop iteration

C. Increase heap size to avoid garbage collection

D. Avoid creating any objects inside the loop

Solution

1. Step 1: Identify best practice for efficient memory use in loops

   Many temporary objects in a loop can pressure memory; explicitly setting references to null when no longer needed helps the GC identify and free them sooner. Frequent System.gc() is inefficient and not guaranteed; larger heap delays issues; avoiding all objects may not be feasible.

2. Final Answer:

   Set object references to null when no longer needed inside the loop -> Option A

3. Quick Check:

   Null references help GC free memory efficiently [OK]

Hint: Nullify unused references to help garbage collection [OK]

Common Mistakes:

• Calling System.gc() too often expecting instant cleanup
• Thinking increasing heap size solves memory leaks
• Believing you must avoid all object creation in loops