MULTITHREADING AND PARALLEL PROGRAMMING - Introduction to Java Programming: Comprehensive Version - page 1101

The program creates three tasks (lines 4-6). To run them concurrently, three threads are created
(lines 9-11). The start() method (lines 14-16) is invoked to start a thread that causes the
run() method in the task to be executed. When the run() method completes, the thread
terminates.
Because the first two tasks, printA and printB , have similar functionality, they
can be defined in one task class PrintChar (lines 21-41). The PrintChar class imple-
ments Runnable and overrides the run() method (lines 36-40) with the print-character
action. This class provides a framework for printing any single character a given number
of times. The runnable objects, printA and printB , are instances of the PrintChar
class.
The PrintNum class (lines 44-58) implements Runnable and overrides the run()
method (lines 53-57) with the print-number action. This class provides a framework for print-
ing numbers from 1 to n , for any integer n . The runnable object print100 is an instance of
the class printNum class.

Note
If you don't see the effect of these three threads running concurrently, increase the
number of characters to be printed. For example, change line 4 to

effect of concurrency

Runnable printA = new PrintChar( 'a' , 10000);

Important Note
The run() method in a task specifies how to perform the task. This method is auto-
matically invoked by the JVM. You should not invoke it. Invoking run() directly merely
executes this method in the same thread; no new thread is started.

run() method

30.3

✓ ✓

How do you define a task class? How do you create a thread for a task?

Check

30.4

What would happen if you replace the start() method with the run() method in
lines 14-16 in Listing 30.1?

Point

print100.start();
printA.start();
printB.start();

print100.run();
printA.run();
printB.run();

Replaced by

30.5

What is wrong in the following two programs? Correct the errors.

public class Test implements Runnable {
public static void main(String[] args) {
new Test();

public class Test implements Runnable {
public static void main(String[] args) {
new Test();

} }

public Test() {
Test task = new Test();
new Thread(task).start();
}

public Test() {
Thread t = new Thread( this );
t.start();
t.start();

}

public void run() {
System.out.println( "test" );
}
}

public void run() {
System.out.println( "test" );
}
}

(a) (b)