Basic Program Examples

This section contains some basic examples of java programs, specially for beginners to get some more idea about execution flow in java programs.

 class FirstProgramFlow
   {
      public static void main(String [] args)
         {
            System.out.println("first ");
            System.out.print("second ");
            System.out.println("third ");
         }
   }	

Output:

As you can observe from the output, the program executes from top to bottom starting from main method. It executes first line then moves to second line, executes second line then move to third line and so on.

Let's see one more program related to execution flow. This program calls method inside other method which makes it a bit more interesting for beginners to understand the execution flow. This program will give you more idea about execution flow in java.

 class SecondProgramFlow
   {
      public static void main(String [] args)
         {
            // calling firstMethod 
            firstMethod(); 
            System.out.println("main method");
         }
      public static void firstMethod()
         {
            System.out.println("first method");
            // calling secondMethod and assigning return value in i 
            int i = secondMethod(); 
            System.out.println("After second method call, i = "+i);
         }
     public static int secondMethod()
         {
            System.out.println("second method");
            return 1;
         }
   }

Output:

Once a method is called, it finishes all it's statement until and unless a return or method call statement is encountered. if a return statement is encountered, the execution moves back to caller of method and if a method call is encountered, the execution moves into the new method.

Command line arguments

A command line argument is an argument that is passed to program while running the program. This argument get's stored in args variable of main method which you can use in main method.

For example if you want to pass a command line argument to a program CommandLineArgument.java given below, you need to write java command like below at the time of execution :

java CommandLineArgument firstArgument

Here "firstArgument" is an argument which is passed to CommandLineArgument program and get's stored in args variable of main method. So anything that is passed after class name in java command acts as command line argument. Let's see the program of command line argument.

 class CommandLineArgument
   {
      public static void main(String [] args)
         {
            System.out.println("args = "+args[0]);
         }
   }   	

Compile the above program as javac CommandLineArgument.java and run it using the command below.

java CommandLineArgument firstArgument

Output:

As you can see the "firstArgument" is passed to program and get's printed in output. If you will not pass any argument to above program, it will throw ArrayIndexOutOfBoundsException at runtime since you are accessing args[0] in your program.

If you want to pass multiple arguments to program, you can do that by using space between arguments. For example command below passes multiple arguments to program.

java CommandLineArgument firstArgument secondArgument thirdArgument

These arguments will be stored in args variable and can be accessed using args[0], args[1], args[2] and so on respectively.

Calling method using object

Since java is an object oriented programming language, you can create object in java and can call method using object. Program below illustrates how to create an object and call the method using that object.

 class MethodCallByObject
   {
      public static void main(String [] args)
         {
            /* creating object obj of class MethodCallByObject */
            MethodCallByObject obj = new MethodCallByObject();
            /* calling firstMethod using object obj */ 
            obj.firstMethod(); 
            System.out.println("first");
         }
      void firstMethod()
         {
      	    System.out.println("second");
      	    System.out.println("third");
         }
   }

Output:

In java dot(.) operator is used to access the member variable and method of an object. In above example we are accessing firstMethod using object obj. The program above is used to give you a brief idea about how to create an object and call method using that. We will get to know more about this in later sections.