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

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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.

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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.

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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

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Variable Tracker

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

key

Key Moments - 2 Insights

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

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

psychology

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

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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.

contract

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.