Garbage Collection in Java

• The programmer doesn't need to be concerned about unused objects in Java as they are automatically handled by the garbage collector. The job of the garbage collector is to make free memory space by destroying objects that are no longer accessible. The garbage collector can be likened to the Daemon thread as it continually operates in the background.
• Java employs an automatic garbage collection system, which involves scanning the heap memory to identify objects that are actively used and those that are not. The unused objects are then deleted. An object is considered in use or referenced if a part of the program still holds a pointer to it.
• The memory that an object has taken up can be freed up when it is no longer referenced or used by any part of the programme.Programmers do not need to explicitly mark objects for deletion because the Java Virtual Machine (JVM) automatically takes care of garbage collection.

How does an object become unreferenced?

1.By nulling a reference

Student stu = new Student();  
stu=null;  

2.By assigning a reference to another

Student stu1 = new Student();  
Student stu2 = new Student();  
stu1=stu2;  

Java garbage collection example

public class Main{
    public void finalize(){
    	System.out.println("Garbage Collected");
    }
    public static void main(String arg[]) {
    	Main main1=new Main();  
    	Main main2=new Main();  
    	main1=null;  
    	main2=null;  
    	System.gc();  
   }
}

Garbage Collected
Garbage Collected 

Important point of Garbage collection

• Garbage collection (GC) in Java is handled by a daemon thread known as the Garbage Collector. The finalise() method is used by this thread just before an object is garbage collected.
• The JVM's garbage collector only remove objects that were created with the "new" keyword. If any objects were created without using "new," the finalize() method can be utilized to perform cleanup operations and destroy those remaining objects.
• Each time an object becomes eligible for garbage collection, the finalize() method is called, providing an opportunity for cleanup processing. This method is defined in the Object class.




• To initiate the garbage collection process, the gc() method is invoked. This method is present in the System and Runtime classes and is responsible for triggering cleanup operations.

Types of Java Garbage Collection Activities

In Java, two types of garbage collection activity are common as given below

• Garbage collection that is minor or incremental : It is thought to have happened when unreachable objects in young generation heap memory were removed.
• Garbage Collection, Major or Complete : It is said to have happened when the objects that survived the minor garbage collection were copied into the old generation or permanent generation heap memory and then removed. Garbage collection occurs less frequently in the elderly than in the younger generation.