Garbage collection overview in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is garbage collection in Java?

Garbage collection is the process where Java automatically finds and removes objects that are no longer used to free up memory.

beginner

Why is garbage collection important?

It helps prevent memory leaks by cleaning unused objects, so the program runs smoothly without running out of memory.

beginner

What does the term 'heap' mean in Java memory management?

Heap is the part of memory where Java stores objects created during program execution. Garbage collection works mainly on the heap.

intermediate

Name one common garbage collection algorithm used in Java.

One common algorithm is the Mark-and-Sweep algorithm, which marks reachable objects and removes the rest.

intermediate

How does Java know which objects to collect?

Java tracks object references. If no active part of the program can reach an object, it is considered garbage and eligible for collection.

What does Java's garbage collector do?

A Removes objects no longer in use to free memory

B Compiles Java code into bytecode

C Manages user input and output

D Optimizes CPU usage

Garbage collector frees memory by removing objects that are no longer referenced.

Where does Java store objects that garbage collection manages?

A Stack memory

B Register memory

C Cache memory

D Heap memory

Objects are stored in heap memory, which garbage collection cleans up.

Which of these is a sign that an object is ready for garbage collection?

A It has no references pointing to it

B It is still referenced by active code

C It is a primitive data type

D It is stored on the stack

Objects with no references are unreachable and ready for garbage collection.

Which algorithm is commonly used by Java garbage collectors?

A Binary search

B Bubble sort

C Mark-and-Sweep

D Quick sort

Mark-and-Sweep marks reachable objects and sweeps away the rest.

What problem does garbage collection help prevent?

A Syntax errors

B Memory leaks

C Slow internet

D Incorrect output

Garbage collection prevents memory leaks by freeing unused memory.

Explain in simple terms how Java's garbage collection works.

Think about how Java finds objects no longer needed and frees memory.

Why is garbage collection important for Java programs?

Consider what happens if memory is never freed.

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