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Island of Isolation in Java

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  • Difficulty Level : Medium
  • Last Updated : 12 Dec, 2021
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In Java, object destruction is taken care of by the garbage Collector module and the objects which do not have any references to them are eligible for garbage collection. Garbage Collector is capable to identify this type of object. Now let us do discuss what the exactly Island of Isolation stands for as follows:

  • Object 1 references Object 2 and Object 2 references Object 1. Neither Object 1 nor Object 2 is referenced by any other object. That’s an island of isolation.
  • Basically, an island of isolation is a group of objects that reference each other but are not referenced by any active object in the application. Strictly speaking, even a single unreferenced object is an island of isolation too.

Example:

Java




// Java Program to Illustrate Island of Isolation
 
// Main class
public class GFG {
 
    GFG i;
 
    // Method 1
    // Main driver method
    public static void main(String[] args)
    {
 
        // Creating object of class inside main() method
        GFG t1 = new GFG();
        GFG t2 = new GFG();
 
        // Object of t1 gets a copy of t2
        t1.i = t2;
 
        // Object of t2 gets a copy of t1
        t2.i = t1;
 
        // Till now no object eligible
        // for garbage collection
        t1 = null;
 
        // Now two objects are eligible for
        // garbage collection
        t2 = null;
 
        // Calling garbage collector
        System.gc();
    }
 
    // Method 2
    // overriding finalize() Method
    @Override protected void finalize() throws Throwable
    {
        // Print statement
        System.out.println("Finalize method called");
    }
}

Output:

Output Explanation: 

  • Before destructing an object, Garbage Collector calls finalize method at most one time on that object. It is because finalize() method is called two times in the above example is because two objects are eligible for garbage collection. This is because we don’t have any external references to t1 and t2 objects after executing t2=null.
  • All we have is only internal references(which is in instance variable i of class Test) to them of each other. There is no way we can call instance variable of both objects. So, none of the objects can be called again tillt2.i  =  t1

  • Both the objects have external references t1 and t2.
  • t1 = null : Both the objects can be reached via t2.i and t2 respectively.
  • t2 = null: No way to reach any of the objects. 
     

Now, both the objects are eligible for garbage collection as there is no way we can call them. This is popularly known as Island of Isolation.

This article is contributed by Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.


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