A Mailbox class is to be used by multiple threads . The intended behaviour is: - When the mail box is written, it becomes occupied. When the contents are read, the mailbox becomes empty. - The write method is used to place something in the mailbox. The method should block if the mailbox is already occupied. - The read method is used to extract something from the mailbox. The method should block if the mailbox is empty. Consider the code below, which is not thread safe. Select all statements that are true. Incorrect selections reduce your overall marks for this question. public class Mailbox { private T contents ; public Mailbox() { contents = null ; } public void write(T entry) { contents = entry ; } public T read() { T temp = contents; contents = null; return temp; } }

Question

A Mailbox class is to be used by  multiple threads . The intended behaviour is: 
 - When the mail box is written, it becomes occupied. When the contents are read, the mailbox becomes empty. 
 - The  write  method is used to place something in the mailbox. The method should block if the mailbox is already occupied. 
 - The  read  method is used to extract something from the mailbox. The method should block if the mailbox is empty. 
 Consider the code below, which is not thread safe. Select all statements that are true. Incorrect selections reduce your overall marks for this question. 
 public class Mailbox <T> 
 { 
    private T contents ; 
    public Mailbox() { 
 contents = null ; 
 } 
     public void write(T entry) { 
       contents = entry ; 
    } 
    public T read() { 
       T temp = contents; 
       contents = null; 
       return temp; 
    } 
 }

Accepted Answer

In order to implement the desired functionality,  the  write  method  should include (at least) the following code:

while (contents != null) { 
       wait() ; 
 } 
 contents = entry ; 
 
 notifyAll() ;

Accepted Answer

The ‘ wait ’ method can only be called from inside a synchronized method/block.

Answer

The method ‘ read ’ does not need to be synchronized, only the  write  method needs to be made synchronized.

Answer

The ‘ sleep() ’ method is an alternative to ‘ wait() ’ as either can be used to pause a thread for a defined period of time.

Answer

In the  read  method, we can implement the desired functionality by adding the following control:

if (contents == null) {     
     wait() ; 
 }

Answer

Since the ‘ Mailbox ’ class must contain (at least) a synchronized   write  method to make it thread safe,  Mailbox  should extend the  Thread  class (or implement the ‘ Runnable ’ interface).

Crowdly

A Mailbox class is to be used by multiple threads . The intended behaviour i...

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