Java puzzle 30: Simple garbage collection

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Try to use the following principle to solve this puzzle!

When there is no references pointing to an object, that object can be eligible for garbage collection.

Java puzzle 30: Simple garbage collection

Level: Normal

Question: After which line number, the object bird created at line //1 is eligible for garbage collection?

public class TestBird
  public static void main(String[] args)
    Bird bird = new Bird("Eagle"); // 1
    Bird bird2 = new Bird("Falcon"); // 2
    bird2 = bird; // 3
    bird = null; // 4
    Bird bird3 = bird; // 5
    bird2 = null; // 6
class Bird
  String name;
  Bird(String name)
  { = name;
Author: Sar Maroof

For the answer and the explanation you can join the discussion below!

Some members thought that it should be line 4. That is understandable, but I already posted the explanation with the arguments why it should be after line 6. See the comments below!

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Add a Comment
  1. Answer explanation

    bird2 at line 2 is also pointed to the same object bird at line 1.
    At line 4 the object bird points to null, but remember that the object bird 2 is still points to the object bird at line 1.
    After the statement bird2 = null; at line 6, there is no object points to the object bird at line 1.
    The object on line 1 is now eligible for garbage collection.

  2. Thanks for this nice post, I am not familiar with Garbage collection, but the way you demonstrate it is really interesting!

    1. You are welcome!

  3. I really don’t get it. I thought after line 4. Can anyone explain me why line 6?

      1. I try to give you a hint! Is there any reference pointing to the object bird after line 4? If you can answer this question, you can simply solve the puzzle.

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