MULTITHREADING AND PARALLEL PROGRAMMING - Introduction to Java Programming: Comprehensive Version - page 1104

You can use the join() method to force one thread to wait for another thread to finish. For
example, suppose you modify the code in lines 53-57 in Listing 30.1 as follows:

join()

public void run() {

Thread thread4 =

Thread
print100

Thread
thread4 new Thread(

new PrintChar( 'c', 40 ));

thread4.start();
try {

for ( int i = 1 ; i <= lastNum; i++) {
System.out.print ( "" + i);
if (i == 50 ) thread4.join();
}

thread4.join()

thread4
to finish

Wait for

}
catch (InterruptedException ex) {
}

thread4 finished

}

A new thread4 is created and it prints character c 40 times. The numbers from 50 to 100
are printed after thread thread4 is finished.
Java assigns every thread a priority. By default, a thread inherits the priority of the
thread that spawned it. You can increase or decrease the priority of any thread by using the
setPriority method, and you can get the thread's priority by using the getPriority
method. Priorities are numbers ranging from 1 to 10 . The Thread class has the int constants
MIN_PRIORITY , NORM_PRIORITY , and MAX_PRIORITY , representing 1 , 5 , and 10 , respec-
tively. The priority of the main thread is Thread.NORM_PRIORITY .
The JVM always picks the currently runnable thread with the highest priority. A lower-
priority thread can run only when no higher-priority threads are running. If all runnable
threads have equal priorities, each is assigned an equal portion of the CPU time in a circular
queue. This is called round-robin scheduling . For example, suppose you insert the following
code in line 16 in Listing 30.1:

setPriority(int)

round-robin scheduling

thread3.setPriority(Thread.MAX_PRIORITY);

The thread for the print100 task will be finished first.

Tip
The priority numbers may be changed in a future version of Java. To minimize the impact
of any changes, use the constants in the Thread class to specify thread priorities.

Tip
A thread may never get a chance to run if there is always a higher-priority thread run-
ning or a same-priority thread that never yields. This situation is known as contention
or starvation . To avoid contention, the thread with higher priority must periodically
invoke the sleep or yield method to give a thread with a lower or the same priority
a chance to run.

contention or starvation

30.6

✓ ✓

Which of the following methods are instance methods in java.lang.Thread ?
Which method may throw an InterruptedException ? Which of them are depre-
cated in Java?
run , start , stop , suspend , resume , sleep , interrupt , yield , join

Check

Point

30.7

If a loop contains a method that throws an InterruptedException , why should
the loop be placed inside a try-catch block?

30.8

How do you set a priority for a thread? What is the default priority?