Garbage collection overview in Java - Step-by-Step Execution

Concept Flow - Garbage collection overview

Start Program

↓

Create Objects

↓

Objects Used?

No→Mark for Garbage Collection

↓

Remove Unused Objects

↓

Continue Using Objects

↓

Free Memory

↓

Program Ends

This flow shows how Java creates objects, checks if they are still used, and if not, marks and removes them to free memory.

Execution Sample

Java

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

This code creates an object, removes its reference, and requests garbage collection.

Execution Table

Step	Action	Object Reference	Garbage Collection Status	Memory Status
1	Create new Object	obj -> Object1	Not marked	Memory allocated for Object1
2	Set obj to null	obj -> null	Object1 becomes unreachable	Memory still allocated
3	Call System.gc()	obj -> null	Garbage collector runs	Memory freed for Object1
4	Program ends	obj -> null	No more objects	Memory clean

💡 Program ends, all unreachable objects are collected and memory is freed

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
obj	null	Object1 reference	null	null	null

Key Moments - 2 Insights

Why does setting obj to null make the object eligible for garbage collection?

Because after setting obj to null (see execution_table step 2), no references point to the object, so it is unreachable and marked for collection.

Does calling System.gc() guarantee immediate garbage collection?

No, calling System.gc() (step 3) only suggests the JVM to run garbage collection, but it may not happen instantly.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the status of the object reference?

A obj is set to null

B obj points to a new object

C obj still points to the object

D obj is undefined

💡 Hint

Check the 'Object Reference' column at step 2 in execution_table

At which step does the garbage collector free the memory?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Memory Status' column in execution_table for when memory is freed

If we never set obj to null, what would happen to the object?

A It would cause an error

B It would be collected immediately

C It would never be eligible for garbage collection

D It would be collected only at program end

💡 Hint

Refer to variable_tracker and execution_table step 2 where obj is set to null to become unreachable

Concept Snapshot

Garbage Collection in Java:
- Objects created use memory.
- When no references point to an object, it becomes unreachable.
- Unreachable objects are marked for garbage collection.
- JVM frees memory by removing unreachable objects.
- System.gc() suggests JVM to run GC but does not guarantee immediate action.

Full Transcript

This visual execution shows how Java manages memory with garbage collection. First, an object is created and referenced by a variable. When the variable is set to null, the object has no references and becomes unreachable. The garbage collector then marks this object and frees its memory when System.gc() is called. This process helps keep memory clean and prevents leaks by removing objects no longer in use.

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