8.4.3

Method Modifiers

CLASSES

has exactly the same effect as:

class BumpTest {

int count;

void bump() {

synchronized (this) {

count++;

}

}

static int classCount;

static void classBump() {

try {

synchronized (Class.forName("BumpTest")) {

classCount++;

}

} catch (ClassNotFoundException e) {

...

}

}

}

The more elaborate example:

public class Box {

private Object boxContents;

public synchronized Object get() {

Object contents = boxContents;

boxContents = null;

return contents;

}

public synchronized boolean put(Object contents) {

if (boxContents != null)

return false;

boxContents = contents;

return true;

}

}

defines a class which is designed for concurrent use. Each instance of the class

Box

 has an instance variable

contents

 that can hold a reference to any object.

You can put an object in a

Box

 by invoking

put

, which returns

false

 if the box is

already full. You can get something out of a

Box

 by invoking

get

, which returns a

null reference if the

box

 is empty.

If

put

 and

get

 were not

synchronized

, and two threads were executing

methods for the same instance of

Box

 at the same time, then the code could misbe 

have. It might, for example, lose track of an object because two invocations to

put

occurred at the same time.

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