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q on garbage collection

JayaSiji Gopal
Ranch Hand

Joined: Sep 27, 2004
Posts: 303
At what point is the object obj eligible for garbage collection?
(Select one correct answer)

public class Test058
public String m()
Object obj = new String("");
String str = null; //1
str = (String)obj; //2
obj = null; //3
return str;


A: After line //1.
B: After line //2.
C: After line //3.
D: Never

The object is returned by the method and not eligible for garbage collection during the lifetime of the method.

I answered b, but the correct answer is d.

could someone please tell me y?

SCJP 1.4, SCWCD 1.4<br /> <br />Thanks in advance!<br />Jayashree.
vidhya jay

Joined: Aug 15, 2004
Posts: 15

Object obj = new String("");
creates a new String object with obj as reference and obj is of Object Type.

str, a String reference is set to null.

obj reference variable is assigned to str and has type casted to String since str is of type String.

Now, str reference and obj reference both point to the String Object.

obj is set to null. But still we have str reference pointing to the String Object,which was created when it had the reference obj.

Hence, the Answer is D : Never.

sanjeevmehra mehra
Ranch Hand

Joined: Aug 21, 2004
Posts: 75

at line 2. object reference "str" and object reference "obj" pointing same object.
at line 3. object reference "obj" = null.
but object reference "str" still refering to (have address of) that object.

hope it's helpfull to understand.

thanks & regards,<br />Sanjeev.
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subject: q on garbage collection