Garbage collection overview in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new object that will be eligible for garbage collection.

Java

MyObject obj = new [1](); obj = null;

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

AMyObject

BInteger

CString

DObject

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong class name like String or Integer.

Not using the new keyword.

Creating an instance of MyObject and then setting the reference to null makes the object eligible for garbage collection.

2fill in blank

medium

Complete the code to explicitly request garbage collection.

Java

System.[1]();

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

ArunFinalization

Bgc

Cexit

DcollectGarbage

Attempts:

3 left

💡 Hint

Common Mistakes

Using exit which terminates the program.

Using non-existent methods like collectGarbage.

The method System.gc() is used to suggest that the Java Virtual Machine performs garbage collection.

3fill in blank

hard

Fix the error in the code to avoid preventing garbage collection.

Java

MyObject obj = new MyObject();
obj.[1] = obj;

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

Athis

BselfReference

Creference

Dparent

Attempts:

3 left

💡 Hint

Common Mistakes

Using this which is a keyword, not a field.

Using selfReference which is not a standard field name.

Assigning obj.reference = obj; creates a self-reference which can prevent garbage collection if not handled properly.

4fill in blank

hard

Fill both blanks to create a weak reference that allows garbage collection.

Java

WeakReference<MyObject> weakRef = new WeakReference<[1]>(obj);
obj = [2];

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

AMyObject

Bnull

CObject

Dnew MyObject()

Attempts:

3 left

💡 Hint

Common Mistakes

Using Object instead of MyObject as generic type.

Assigning obj to a new object instead of null.

Using WeakReference allows the object to be garbage collected when no strong references exist. Setting obj = null; removes the strong reference.

5fill in blank

hard

Fill all three blanks to implement a finalize method that prints a message before garbage collection.

Java

protected void [1]() throws [2] {
    System.out.[3]("Object is being garbage collected");
}

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

Afinalize

BThrowable

Cprintln

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using print instead of println.

Using wrong method name like finalizeObject.

The finalize() method is called by the garbage collector before reclaiming the object. It can throw Throwable. Using println prints the message with a new line.

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