How to make object eligible for garbage collection

In the language like C, a programmer need to manually do the memory management himself/herself by allocating memory programmatically and then de-allocating memory programmatically, but Java provides automatic memory management facility, wherein a programmer need not take care of allocating/de-allocating memory, but there is utility which is part of JVM which do it for programmer automatically and we call is as Mr. Garbage Collector. Don't go on its name, it's a big guy actually, which does pretty decent job for programmer :)

What is Garbage collection ?

Employee emp = new Employee();

emp = null;

Employee emp1 = new Employee();

Employee emp2 = new Employee();

emp1 = emp2

emp1 has also started referring to object which emp2 was referring. Basically we are assigning reference to second object, to reference variable of first object, such that reference variable of first object has also started pointing to second object.

package com.blogspot.javasolutionsguide;

Class TestGC {
    public static void main(String args[]) {
        testMethod();  // Line1
    }

    public static void testMethod() {
        Employee emp1=new Employee();
    }
}

package com.blogspot.javasolutionsguide;                                                                                                                          Class TestGC{

  public static void main(String args[]){
     Employee emp  = testMethod();              // Line 1
     System.out.println("Name is :"+emp.getName());  //Line 2
  }

public static Employee testMethod(){
      Employee emp1 = new Employee();      //Line 3
      emp1.setName("JavaSolutionsGuide"); //Line 4
      return emp1  ;   // Line 5
   }
}

Output : JavaSolutionsGuide

package com.blogspot.javsolutionsguide

Class TestGC{
  public static void main(String args[]){
     testMethod();              // Line 1
  }

public static Employee testMethod(){
      Employee emp1 = new Employee();      //Line 3
      emp1.setName("JavaSolutionsGuide"); //Line 4
      return emp1  ;   // Line 5
   }
}

package com.blogspot.javasolutionsguide

Class TestGC{
          static Employee emp;      //Line 1
   public static void main(String args[]){
      testMethod();              // Line 2
      System.out.println("Name is :"+emp.getName());  //Line 3
   }

   public static void testMethod(){
        emp = new Employee();  // Line 4
        emp.setName("GB");     // Line 5
   }
}

package com.blogspot.javasolutionsguide

class TestGC{
          TestGC tgc ;    //Line 1
   public static void main(String args[]){
             TestGC tgc1 = new TestGC();   //Line 2
             TestGC tgc2 = new TestGC();   //Line 3
             TestGC tgc3 = new TestGC();   //Line 4
             tgc1.tgc = tgc2;      //Line 5
             tgc2.tgc =tgc3;       //Line 6
             tgc3.tgc =tgc1;       //Line 7

            tgc1 = null;    //Line 8
            tgc2 = null;    //Line 9
            tgc3 = null;    //Line 10
   }
}

Here after Line 4,we have three objects created in heap and they are referred by reference variables tgc1,tgc2 and tgc3.

After Lines 5,6,7

After Line 8

Although tgc1 = null ,still first object can be referred by tgc3.tgc ,so none of three objects are eligible for garbage collection at this time.

After Line 9

Here both tgc1 and tgc2 are assigned null, but still first object can be accessed by tgc3.tgc and second object by tgc3.tgc.tgc ,hence none of the three objects are eligible for garbage collection.

After Line 10

As all three tgc1,tgc2 and tgc3 are set to null ,it means we are left with no external reference to access any of these three objects. So although there are internal references through which these objects can access each other, but they are totally isolated from outside world, hence the name Island of isolation. So here all three objects are eligible for garbage collection.