Basic Example programs in Java

This section contains some basic examples of java programs, specially for beginners to get some more idea about execution flow in java programs. It also shows you how to pass command line arguments to java programs and how to call method of a program using object. Refer My first java program tutorial first if you don't know how to compile and execute java programs.

Execution flow of java program

Both the programs given below demonstrates the execution flow of 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 see 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 another 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 the line from where this method was called and if a method call is encountered, the execution moves into the new method.

What is command line arguments in Java

A command line argument is an argument which is passed to a program while running that program using java command. This argument get's stored in args (the name can be different) parameter of main method. You can access your argument using args parameter inside main method.

For an example if you want to pass a command line argument "firstArgument" to a program MyFirstProgram.java given below, you need to run the program using java command like below :

java MyFirstProgram firstArgument

Here firstArgument is an argument which is passed to MyFirstProgram and get's stored in args variable of main method. Anything which is passed after class name in java command acts as command line argument and can be accessed using args variable. Let's understand this by the program given below.

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

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

java MyFirstProgram 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 your program, you can do that by using space between arguments. For example command below passes three arguments to MyFirstProgram.

java MyFirstProgram firstArgument secondArgument thirdArgument

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

How to call method using object in java

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. To get more detail about object refer objects in java tutorial and for method refer methods in java tutorial.