当一个对象符合垃圾收集器的条件时？ [英] when an object is eligible for a garbage collector?

问题描述

考虑这个示例代码：

  1。 public class GC {
 2. private Object o; 
 3. private void doSomethingElse（Object obj）{o = obj; } 
 4. public void doSomething（）{
 5. Object o = new Object（）; 
 6. doSomethingElse（o）; 
 7. o = new Object（）; 
 8. doSomethingElse（null）; 
 9. o = null; 
 10.} 
 11.} 
  

当调用doSomething方法时，之后在
5行中创建的对象是否可用于垃圾回收？

A。第5行

B。第6行

C。第7行

D。第8行

E。第9行

F。第10行

答案：D

为什么D？确实，当执行第6行时，第5行中创建的对象现在由实例var o和本地var o引用，并且当执行第8行时，对象现在仅由本地ref var o引用，为什么答案是D，执行第9行后会发生什么？感谢。

解决方案

这个问题让IMO感到困惑的主要原因是有两个变量名为 o 。一个是实例变量 o ，另一个是局部变量 o 方法 doSomething（ ）。

 时间实例var o本地变量o 
在行5之前：null 
行5：null对象＃1 
行6：对象＃1对象＃1 
行7：对象＃1对象＃2 
行8：空对象＃2 <没有更多的引用对象＃1 
  

所以在（或执行后）第8行，对象＃1有资格收藏。

consider this sample code:

1.  public class GC {
2.      private Object o;
3.      private void doSomethingElse(Object obj) { o = obj; }
4.      public void doSomething() {
5.          Object o = new Object();
6.          doSomethingElse(o);
7.          o = new Object();
8.          doSomethingElse(null);
9.          o = null;
10.     }
11. }

When the doSomething method is called, after which line does the Object created in line 5 become available for garbage collection?

A. Line 5

B. Line 6

C. Line 7

D. Line 8

E. Line 9

F. Line 10

Answer: D

why D? it's true that when Line 6 is executed the object created in Line 5 is now referenced by the instance var o and the local var o and when Line 8 is executed the object now is referenced by only the local ref var o, so why the answer is D and what happens after Line 9 is executed?? thanks.

解决方案

The main reason this question is confusing IMO is that there are 2 variables named o. One is the instance variable o and the other is the local variable o inside method doSomething().

Time            instance var o    local var o
Before Line 5:            null               
Line 5:                   null       Object#1
Line 6:               Object#1       Object#1
Line 7:               Object#1       Object#2
Line 8:                   null       Object#2   <- No more references to Object#1

So on (or after executing) line 8, Object#1 is eligible for collection.

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