Multiple choice technology programming languages

Given: 1. class Eco { 2. public static void main(String[] args) { 3. Eco e1 = new Eco(); 4. Eco e2 = new Eco(); 5. Eco e3 = new Eco(); 6. e3.e = e2; 7. e1.e = e3; 8. e2 = null; 9. e3 = null; 10. e2.e = el; 11. e1 = null; 12. } 13. Eco e; 14. } At what point is only a single object eligible for GC?

  1. After line 8 runs

  2. After line 9 runs.

  3. After line 10 runs.

  4. After line 11 runs.

  5. An exception is thrown at runtime.

  6. Never in this program
Reveal answer Fill a bubble to check yourself
E Correct answer
Explanation

Line 10 attempts e2.e = el, but e2 was set to null on line 8. Accessing e2.e throws NullPointerException. Note: el is a typo (should be e1), but the null dereference happens first.

AI explanation

To determine when a single object becomes eligible for garbage collection (GC), we need to understand the concept of object references and the conditions for GC.

In this program, the class Eco has a reference variable e of type Eco. The main method creates three instances of the Eco class: e1, e2, and e3.

Let's go through each line of the code and analyze the changes:

  1. Eco e; - Declares a reference variable e without initializing it.
  2. Eco e1 = new Eco(); - Creates a new object of the Eco class and assigns its reference to e1.
  3. Eco e2 = new Eco(); - Creates a new object of the Eco class and assigns its reference to e2.
  4. Eco e3 = new Eco(); - Creates a new object of the Eco class and assigns its reference to e3.
  5. e3.e = e2; - Assigns the reference of e2 to the instance variable e of the e3 object. This creates a circular reference between e2 and e3.
  6. e1.e = e3; - Assigns the reference of e3 to the instance variable e of the e1 object.
  7. e2 = null; - Sets the e2 reference variable to null. This means that the object originally referenced by e2 is no longer accessible.
  8. e3 = null; - Sets the e3 reference variable to null. This means that the object originally referenced by e3 is no longer accessible.
  9. e2.e = el; - Throws a NullPointerException. Since e2 is null, trying to access e2.e results in an exception.
  10. e1 = null; - Sets the e1 reference variable to null. This means that the object originally referenced by e1 is no longer accessible.

Based on the code execution, let's analyze when each object becomes eligible for GC:

  1. e1 becomes eligible for GC after line 11 runs because the reference to the object is set to null.
  2. e2 becomes eligible for GC after line 8 runs because the reference to the object is set to null.
  3. e3 becomes eligible for GC after line 9 runs because the reference to the object is set to null.
  4. The object referenced by e2 becomes eligible for GC after line 10 runs because it is no longer accessible.

Therefore, at the end of the program, there is no object left that is eligible for GC. Hence, the correct answer is F. Never in this program.

The given correct answer, E. An exception is thrown at runtime, is incorrect because the exception is thrown before any object becomes eligible for GC.

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