Garbage Collection in Java – Simple Explanation with Example & Output

 

Garbage Collection in Java

Automatic Memory Management Explained Simply

Introduction to Garbage Collection

In Java, memory management is automatic. This means you don't have to manually allocate and free memory like in C or C++. The Java Virtual Machine (JVM) has a built-in system called Garbage Collection (GC) that takes care of cleaning up unused objects.

Why Garbage Collection is Needed

When a program runs, it creates objects in memory. Over time, some objects are no longer needed, but they still occupy memory. Without cleanup, memory would fill up, causing your program to slow down or crash. Garbage Collection solves this problem by automatically removing unused objects.

How Memory Works in Java

Stack Memory

• Stores method calls and local variables
• Automatically cleared when methods finish

Heap Memory

• Stores objects created using new
• Managed by the Garbage Collector

What is Garbage?

Garbage means any object in heap memory that is no longer referenced by any part of the program.

What is Garbage Collection?

Garbage Collection is the process of automatically finding and deleting unused objects from heap memory so that space can be reused.

How Garbage Collection Works

Automatic Memory Management

Java tracks which objects are in use. When an object is no longer referenced, it becomes eligible for GC.

Mark and Sweep Process

1. Mark - The GC marks all objects that are still in use
2. Sweep - The GC removes all unmarked (unused) objects

✓ Advantages

• No manual memory management
• Reduces memory leaks
• Improves program stability

✗ Limitations

• Can't control exactly when GC will run
• May cause slight pauses in program execution

Requesting GC Manually

Although GC runs automatically, you can request it using:

System.gc();

Note: This is only a request - the JVM may ignore it

Example of Garbage Collection

Code

public class GarbageCollectionExample {
  public static void main(String[] args) {
    GarbageCollectionExample obj1 = new GarbageCollectionExample();
    GarbageCollectionExample obj2 = new GarbageCollectionExample();

    obj1 = null; // Eligible for GC
    obj2 = null; // Eligible for GC

    System.gc(); // Request GC
  }

  @Override
  protected void finalize() {
    System.out.println("Garbage Collector called for object");
  }
}

Output

Garbage Collector called for object
Garbage Collector called for object

Explanation

• Two objects are created
• References set to null
• System.gc() requests GC
• finalize() runs before object removal

Types of Garbage Collectors

Serial GC

• Best for small applications
• Works in single thread

Parallel GC

• Uses multiple threads
• Good for multi-core systems

CMS Collector

• Reduces application pause time
• Runs alongside application

G1 Collector

• Divides heap into regions
• Prioritizes garbage-heavy regions

Best Practices

➤

Avoid unnecessary object creation

➤

Use local variables when possible

➤

Nullify references when no longer needed

Common Misconceptions

Myth:

GC deletes objects immediately when unused

Truth:

GC runs only when JVM decides it's needed

Myth:

System.gc() always runs GC

Truth:

It's just a request - JVM may ignore it

Conclusion

Garbage Collection in Java makes memory management easier and safer by automatically cleaning up unused objects. While it's powerful, understanding how it works helps you write more efficient and reliable programs.

Frequently Asked Questions

Q1: Can we force GC in Java?

No, you can only request it using System.gc()

Q2: What without GC?

Memory leaks occur, leading to crashes or slow performance

Q3: Is finalize() guaranteed?

Not always - it's not guaranteed to run

Q4: Default GC?

Depends on JVM - usually G1 GC for newer versions

Q5: Performance impact?

Causes short pauses but improves memory efficiency

Java Memory Management System | Automatic Garbage Collection