main() thread in Java

17 December 2007

The ‘main()’ method in Java is referred to the thread that is running, whenever a Java program runs. It calls the main thread because it is the first thread that starts running when a program begins. Other threads can be spawned from this main thread. The main thread must be the last thread in the program to end. When the main thread stops, the program stops running.

Main thread is created automatically, but it can be controlled by the program by using a Thread object. The Thread object will hold the reference of the main thread with the help of currentThread() method of the Thread class.

Listing shows how main thread can be controlled by a program:

class MainThread {
	public static void main(String args[]) {
		Thread t = Thread.currentThread();
		System.out.println("Current Thread : " + t);
		System.out.println("Name : " + t.getName());
		System.out.println(" ");
		t.setName("New Thread");
		System.out.println("After changing name");
		System.out.println("Current Thread : " + t);
		System.out.println("Name : " + t.getName());
		System.out.println(" ");
		System.out.println("This thread prints first 10 numbers");
		try {
			for (int i = 1; i <= 10; i++) {
				System.out.print(i);
				System.out.print(" ");
				Thread.sleep(1000);
			}
		} catch (InterruptedException e) {
			System.out.println(e);
		}
	}
}

Output:

Current Thread : Thread[main,5,main]
Name : main
 
After changing name
Current Thread : Thread[New Thread,5,main]
Name : New Thread
 
This thread prints first 10 numbers
1 2 3 4 5 6 7 8 9 10

Let us now understand the working of the program in the listing. The program first creates a Thread object called ‘t’ and assigns the reference of current thread (main thread) to it. So now main thread can be accessed via Thread object ‘t’. This is done with the help of currentThread() method of Thread class which return a reference to the current running thread. The Thread object ‘t’ is then printed as a result of which you see the output Current Thread : Thread [main,5,main]. The first value in the square brackets of this output indicates the name of the thread, the name of the group to which the thread belongs. The program then prints the name of the thread with the help of getName() method.

The name of the thread is changed with the help of setName() method. The thread and thread name is then again printed. Then the thread performs the operation of printing first 10 numbers. When you run the program you will that the system wait for sometime after printing each number. This is caused by the statement Thread.sleep (1000). The sleep() method can also used in another way in which it accepts two arguments. The first argument represents the amount of milliseconds and the second argument represents the amount of nanoseconds. Effectively, the thread will sleep for specified milliseconds + nanoseconds. Its (sleep() method’s) prototype is as follows:

static void sleep(long milliseconds, int nanoseconds)

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