Understanding GC

Garbage collection is an important topic in Java, and can come up in interviews and exams.

• A candidate will be asked when a certain object in a method can be garbage collected.

Consider the following code.

// After which line is the object created at line 5
// eligible for garbage collection?
3:  public static void main(String... args){
4:   String one, two;
5:   one = new String("1");
6:   two = new String("2");
7:   one = two;
8:   String three = one;
9:   one = null;
10: }

The best way to handle this question is to draw it on paper or a whiteboard:

• Draw the heap, and as an object is created and assigned to a reference, draw arrows between the two.

• As references change, cross out the arrows and redraw as necessary.

An object is eligible for garbage collection once it no longer has arrows pointing to it from outside the heap.

Garbage Collection Example

We will go through an example, line by line.

// After which line is the object created at line 5
// eligible for garbage collection?
3:  public static void main(String... args){
4:   String one, two;
5:   one = new String("1");
6:   two = new String("2");
7:   one = two;
8:   String three = one;
9:   one = null;
10: }

First, line 4:

4: String one, two;

Draw the reference variables:

5: one = new String("1");

For each new, draw the object in the heap and connect its reference:

6: two = new String("2");

7: one = two;

This means that one no longer points to the the previous object, instead now pointing to whatever two points to:

The arrow (reference) to "1" was broken at line 7.

• No references point to "1" so it is eligible for garbage collection after line 7.

The rest of the example:

8: String three = one;

9: one = null;

"2" is not eligible for garbage collection until the method finishes.

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