Garbage Collection

[Joe San]

Guys,

The question below is from Dan Chisholm,

class I {
  private String name;
  protected void finalize() {System.out.print(name);}
  public I(String s) {name = s;}
}
class J {
  private static void m1(I[] a1) {
    a1[0] = a1[1] = a1[2] = null;
  }
  public static void main (String[] args) {
    I[] a1 = new I[3];   // 1
    a1[0] = new I("A");  // 2
    a1[1] = new I("B");  // 3
    a1[2] = new I("C");  // 4
    m1(a1);
    System.gc();
}}

Now, After method m1 returns, the object created on which line is not eligible for garbage collection? Answer is Line 1

I can understand the above from the point that array references are passed by reference value and any modifications that we do to it's elements will refelect in the calling method as well.

Now considering the question below also from Dan Chisholm,

class I {
  private String name;
  public String toString() {return name;}
  public I(String s) {name = s;}
}
class J {
  private static void m1(I[] a1) {a1 = null;}
  public static void main (String[] args) {
    I[] a1 = new I[3];   // 1
    a1[0] = new I("A");  // 2
    a1[1] = new I("B");  // 3
    a1[2] = new I("C");  // 4
    m1(a1);
    for (int i = 0; i < a1.length; i++) {
      System.out.print(a1[i]);
}}}

After method m1 returns, the object created on which line is eligible for garbage collection? The answer was, None of the above.

I want to know is that what is the difference between the above 2 cases. Why not the statement a1 = null in the m1(I[] a1) method not affect it in the main method?? Anyone please help me with this.

Thanks in advance.

[Pinkal Patel]

Hi,

In the Program 1)

  a1[0] = a1[1] = a1[2] = null;
  

Assigned null to Actual value of the Array
but in the Program 2)

   a1 = null; 

Will Null the Second Refference to Array Object

e.g Lets Change line

  private static void m1(I[] a1) {  

with

  private static void m1(I[] a2) {  

Meance Now
a1 -> OBJECT of Array
and
a2 -> OBJECT of Array
Both Reffer the Same object if you mack a2=null;
meance OBJECT of Array is not aloved for GC

[Joe San]

Ok that is understandable that it will null the second reference. I want to know, won't it affect the reference from the main method?? Why not the reference from the main method be nulled???

Any thoughts on that?

[Barry Gaunt]

Originally posted by Jothi Shankar Kumar Sankararaj:
Ok that is understandable that it will null the second reference. I want to know, won't it affect the reference from the main method?? Why not the reference from the main method be nulled???

Any thoughts on that?

Remember that Java always passes a copy of the reference to the method. So it is a local reference variable that is set to null. The confusion is caused by the fact that the name of the local reference variable (in the parameter list) is the same as the one in the main method, namely a1.

[Joe San]

Ok, I got the concept. Thanks ranchers for the help offered.