作为参数传递并在方法中设置为 null 的对象的垃圾收集

Question

I have was trying to understand this question but could not get settled with the answer provided in the book. Can somebody explain to me? Question is given bellow.

class CardBoard {
  Short story = 5;

  CardBoard go(CardBoard cb) {
    cb = null;
    return cb;
  }

  public static void main(String[] args) {
    CardBoard c1 = new CardBoard();
    CardBoard c2 = new CardBoard();
    CardBoard c3 = c1.go(c2);
    c1 = null;
    // do stuff
  }
}

When // do stuff is reached, how many objects are eligible for garbage collection?

Given answer is 2 objects. Only one CardBoard object (c1) is eligible, but it has an associated Short wrapper object that is also eligible

I dont understand why object c1 alone is eligible for garbage collection.

Two object are created, c1 and c2.

For object c3 we are passing c2 to c1.go() method. And that that is set to null and returned object is asigned to c3.

In the method c1.go(), object that cb reffering to (which by my undertanding, the same object that c2 reffering to ) is set as null. Won't that make the object that c2 reffering to will also be available for garbage collection?

Answer 1

Java is pass-by-value . When you do CardBoard c3 = c1.go(c2); , the instance pointed to by c2 is passed to go() but the moment you do cb=null , the original c2's cb is NOT set to null , a new null reference aconst_null is pushed onto the stack and returned. Hence, there are only 2 objects ( Short and c1 ) that are eligible for GC.

PS : As Andreas says, Short values are cached for values between -128 and 127 and hence in your case, the same Short instance will be reused. So only one object will be eligible for GC. If your Short value is out of that range, then 2 Objects will be eligible for GC.

Answer 2

Java is pass-by-value, so when you null out parameter cb , that does not change the value of variable c2 .

The go() method call simply ends up returning null , leaving c1 and c2 unchanged.

So, when execution get to // do stuff , c1 is null, c2 refers to second object, and c3 is null. Which means the first object is eligible for GC.

---

Since each object has a Short reference, you'd think that two objects will be freed (first object and its story ), and two objects will remain (second object and its story ), however that is not really true.

A Short value of 5 will not create a new instance of a Short object. Values between -128 and 127 (inclusive) are cached , and the cache values are permanent.

This means that the true answer is that only 2 objects were allocated, and the first object is eligible for GC. The Short story references don't allocate anything.