Interview Question : What is garbage collection in Java?

What is garbage collection?
The definition say that garbage collection is a process that is responsible for making space in a computer's memory by removing data that is no longer required or in use.
Java Garbage Collection:
In java, garbage means unreferenced objects.
Garbage Collection is process of reclaiming the runtime unused memory automatically. In other words, it is a way to destroy the unused objects.
To do so, we were using free() function in C language and delete() in C++. But, in java it is performed automatically. So, java provides better memory management.
Advantage of Garbage Collection
It makes java memory efficient because garbage collector removes the unreferenced objects from heap memory.
It is automatically done by the garbage collector(a part of JVM) so we don't need to make extra efforts.
How can an object be unreferenced?
1) By nulling a reference:
Employee e=new Employee();
e=null;

2) By assigning a reference to another:
Employee e1=new Employee();
Employee e2=new Employee();
e1=e2;//now the first object referred by e1 is available for garbage collection

3) By annonymous object:
new Employee();

There are different types of references in Java. Instances eligibility for garbage collection depends on the type of reference it has.

Reference	Garbage Collection
Strong Reference	Not eligible for garbage collection
Soft Reference	Garbage collection possible but will be done as a last option
Weak Reference	Eligible for Garbage Collection
Phantom Reference	Eligible for Garbage Collection

In Java destruction of object from memory is done automatically by the JVM. When there is no reference to an object, then that object is assumed to be no longer needed and the memory occupied by the object are released. This technique is called Garbage Collection. This is accomplished by the JVM.the Garbage Collection can not be forced explicitly. We may request JVM for garbage collection by calling System.gc() method. But This does not guarantee that JVM will perform the garbage collection.
Sometime an object will need to perform some specific task before it is destroyed such as closing an open connection or releasing any resources held. To handle such situation finalize() method is used. finalize() method is called by garbage collection thread before collecting object. Its the last chance for any object to perform cleanup utility.
Signature of finalize() method

protected void finalize()
{
 //finalize-code
}

Java provide one method in system class to call the garbage collector explicitly. we have following example to demonstrate the process.

public class Test
{
public static void main(String[] args)
{
Test t = new Test();
t=null;
System.gc();
}
public void finalize()
{
System.out.println("Garbage Collector called...");
}
}

gc() method is used to call garbage collector explicitly. However gc() method does not guarantee that JVM will perform the garbage collection. It only request the JVM for garbage collection. This method is present in System and Runtime class.

Points to Remember:

• The Garbage collector of JVM collects only those objects that are created by new keyword. So if you have created any object without new, you can use finalize method to perform cleanup processing (destroying remaining objects).
• Garbage collection is performed by a daemon thread called Garbage Collector(GC). This thread calls the finalize() method before object is garbage collected.
• Neither finalization nor garbage collection is guaranteed.