blocks|key|5357556|text|c3不是对象。它是一个引用null的变量。变量不能符合GC的条件。只有对象才有资格进行GC。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5357557|5357558|entityMap^0|0|2|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@$9|J|A|K|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|L|8|@]|D|@]|E|$]]|$1|G|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|H|$]]

<code>c3</code> is not an object. It's a variable referencing null. A variable can't be eligible to GC. Only objects can be eligible to GC.

The objects eligible to GC are the Cardboard which was initially referenced by <code>c1</code>, and the Short instance initially referenced by the CardBoard initially referenced by <code>c1</code>.

blocks|key|3144878|text|c3是一个具有空引用的本地句柄，它不指向(并且hever已经指向)已分配的对象。因此，GC不存在任何问题。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3144879|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

c3 is a local handle with a null reference, it does not point (and hever has pointed) to an allocated object. Therefore there's nothing to GC.

blocks|key|3628333|text|垃圾收集器扫描内存中的对象，当它找到一个对象时，检查指向它的句柄(反之亦然)，当调用垃圾收集器时，没有指向它们的句柄的对象(在内存中)将从内存中删除。在这种情况下，代码中只有2个对象在内存中创建，但有3个句柄。垃圾回收器最多只能删除2个对象(因为只存在两个对象)+C3仅指向空位置。不会删除C1、C2和C3。它们用来指向的对象将被移除(以防没有其他句柄点指向它们)。因此，在这一点上，由于C3从未指向内存中的对象，因此将其设置为null对垃圾收集器没有影响。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3628334|希望我能帮上忙|3628335|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Garbage collector scans for objects in memory and when it finds one checks for handles pointing at it (and not vice verse)
When Garbage Collector is called the objects (inside memory) that have no handle pointing to them are removed from memory. In this occasion of code you only have 2 objects created in memory but three handles. Garbage Collector will only delete at max 2 objects (since only two objects exist) C3 was only pointing at a null position. C1,C2,C3 are not removed. The objects that they used to point are removed (in case no other handle points at them). So at this point since C3 never pointed to an object in memory setting it to null does not make a difference for the Garbage Collector.

Hope I was helpful

blocks|key|3628360|text|您的CardBoard#go()方法什么也不做。它接收对某个对象的引用，立即忘记它，替换为null，并返回这个null值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3628361|所以，这句话|3628362|CardBoard+c3+=+c1.go(c2);|code-block|syntax|javascript|3628363|也不执行任何操作，即不创建任何对象。仅分配给c3的值为空。所以它不是垃圾收集，因为它已经不存在了。|3628364|entityMap^0|2|E|19|4|1J|4|0|0|0|M|2|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|Z|8|@$9|10|A|11|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|P|$]]

Your <code>CardBoard#go()</code> method does nothing. It receives reference to some object, immediately forgets it, replacing by <code>null</code>, and returns this <code>null</code> value. 

So, the line 

<pre><code>CardBoard c3 = c1.go(c2);
</code></pre>

also does nothing, i.e. no object is created. Just null assigned to <code>c3</code>. So it is not garbage collected because it is already not exist.

blocks|key|5357641|text|c3本身只是一个引用变量，它不在堆空间中(即不在垃圾收集范围内)|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|5357642|CardBoard+c3+=+c1.go(c2);行仅将c3引用设置为null，因此c3不会影响垃圾收集，因为它不会使任何对象符合条件。|CODE|5357643|entityMap^0|0|2|0|0|P|0^^$0|@$1|2|3|4|5|6|7|K|8|@$9|L|A|M|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|N|8|@$9|O|A|P|B|H]]|D|@]|E|$]]|$1|I|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|J|$]]

The c3 itself is just a reference variable, which is not in the heap space (i.e. not in the scope of garbage collection)

The line <code>CardBoard c3 = c1.go(c2);</code> only sets the c3 reference to null
Thus c3 has not impact on garbage collection as it doesn't make any object eligible.

blocks|key|5267005|text|c3没有对对象的引用，它只是为了制造一些混乱。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5267006|我们创建了两个对象c1和c2，我们遗漏了Short类型的包装器对象，所以基本上我们总共创建了四个对象。|5267007|当我们设置c1+=+null时，这意味着我们使用c1访问的短对象可用于GC，而不是我们可以使用c2访问的短对象。|5267008|这可以使用以下代码进行验证：|5267009|公共类CardBoard+{|5267010|Short+story+=+200;

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

public+static+void+main(String[]+args)+{
++++Runtime+run+=+Runtime.getRuntime();

++++CardBoard+c1+=+new+CardBoard();
++++CardBoard+c2+=+new+CardBoard();
++++CardBoard+c3+=+c1.go(c2);

++++//+System.out.println(run.freeMemory());

++++run.gc();
++++//+System.out.println(run.freeMemory());

++++c1.story+=+100;

++++System.out.println(c1.story);
++++c1+=+null;
++++run.gc();

++++System.out.println(c2.story);

}|code-block|syntax|javascript|5267011|}|5267012|输出:+100+200|5267013|这意味着我们可以从c1访问的c1和Short+story可用于垃圾回收，而不是c2的Short+story。|5267014|entityMap^0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Y|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|Z|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|10|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|11|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|12|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|14|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|V|$]]

c3 has no references to an object,it is just to make some confusion.

We have created two objects c1 and c2, what we are missing out is wrapper objects of type Short, so basically we have created four objects in total.

When we are making c1 = null it means that short object which we are accessing using c1 is available for GC, not the short object which we can access using c2.

This can be verified using following code:

public class CardBoard {

<pre><code>Short story = 200;

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

public static void main(String[] args) {
 Runtime run = Runtime.getRuntime();

 CardBoard c1 = new CardBoard();
 CardBoard c2 = new CardBoard();
 CardBoard c3 = c1.go(c2);

 // System.out.println(run.freeMemory());

 run.gc();
 // System.out.println(run.freeMemory());

 c1.story = 100;

 System.out.println(c1.story);
 c1 = null;
 run.gc();

 System.out.println(c2.story);

}
</code></pre>

}

output: 
100
200

which means that c1 and Short story which we can access from c1 is available for garbage collection not Short story of c2.

In my studying for OCJP I came across the following question:

<pre><code>class CardBoard {
 Short story = 200;
 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 
}}
</code></pre>

When //doStuff is reached, how many objects are eligible for GC? 
The correct answer is 2, meaning <code>c1</code> and its <code>story</code> object.

When line //doStuff is reached, c3 is also null. Why isn't it eligible for GC too?

java garbage collection and null reference

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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在我学习OCJP的过程中，我遇到了以下问题：class CardBoard {           Short story = 200;           CardBoard go(CardBoard cb) {                cb = null;                return cb; ...

问java垃圾收集和空引用
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