Java Tutorial - Java Hello World

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Let's start by compiling and running the following short sample program.

/* //from  w w w  .  j  ava  2s. c o m
 This is a simple Java program. Call this file "Main.java". 
 */

public class Main {
  // Your program begins with a call to main().
  public static void main(String args[]) {
    System.out.println("Java.");
  }
}

In Java, a source file is called a compilation unit. It is a text file that contains one or more class definitions. The Java compiler requires that a source file use the .java filename extension.

In Java, all code must reside inside a class. By convention, the name of the public class should match the its file name. And Java is case-sensitive.

The code above generates the following result.

Compiling the Program

To compile the program, execute the compiler, javac, specifying the name of the source file on the command line:

C:\>javac Main.java 

The javac compiler creates a file called Main.class. Main.class contains the byte code version of the program.

To run the program, use the Java interpreter, called java. Pass the class name Main as a command-line argument, as shown here:

C:\>java Main 

When the program is run, the following output is displayed:

When Java source code is compiled, each individual class is put into its own file named classname.class.

A Closer Look at the Main.java

The first part is a comment.

/* 
   This is a simple Java program. Call this file "Main.java". 
*/ 

Comment is a remark for a program. The contents of a comment are ignored by the compiler. The next line of code in the program is shown here:

public class Main { 

The keyword class declares that a new class is being defined. Main is the name of the class. The entire class definition is between the opening curly brace ({) and the closing curly brace (}). The next line in the program is the single-line comment, shown here:

// Your program begins with a call to main(). 

A single-line comment begins with a // and ends at the end of the line. The next line of code is shown here:

public static void main(String args[]) { 

Java applications begin execution by calling main(String args[]). Java is case-sensitive. Thus, Main is different from main.

A Short Program with a variable

A variable is a memory location that may be assigned a value. The value of a variable is changeable.

The following code defines a variable and change its value by assigning a new value to it.

public class Main {
  public static void main(String args[]) {
    int num; // a variable called num
    num = 100;//  w  w w  .ja v a 2s  .com

    System.out.println("This is num: " + num);
    num = num * 2;

    System.out.print("The value of num * 2 is ");
    System.out.println(num);
  }
}

When you run this program, you will see the following output:

The following snippet declares an integer variable called num. Java requires that variables must be declared before they can be used.

int num; // this declares a variable called num 

Following is the general form of a variable declaration:

type var-name; 

In the program, the line assigns to num the value 100.

num = 100; // this assigns num the value 100 

Define more than one variable with comma

To declare more than one variable of the specified type, you may use a comma-separated list of variable names.

public class Main {
  public static void main(String args[]) {
    int num, num2;
    num = 100; // assigns num the value 100
    num2 = 200;//from  w  w w.  j av a2 s  .c o  m
    System.out.println("This is num: " + num);
    System.out.println("This is num2: " + num2);

  }
}

When the program is run, the following output is displayed:

Using Blocks of Code

Java can group two or more statements into blocks of code. Code block is enclosing the statements between opening and closing curly braces({}).

For example, a block can be a target for Java's if and for statements. Consider this if statement:

public class Main {
    public static void main(String args[]) {
        int x, y;
        x = 10;/* w  w w . ja  va  2 s  .c o m*/
        y = 20;
        if (x < y) { // begin a block
            x = y;
            y = 0;
            System.out.println("x=" + x);
            System.out.println("y=" + y);
        } // end of block
    }
}

Here is the output of the code above:

Example

A block of code as the target of a for loop.

public class Main {
  public static void main(String args[]) {
    int i, y;//from  ww  w  .  j a  v  a  2  s. c o m
    y = 20;
    for (i = 0; i < 10; i++) { // the target of this loop is a block
      System.out.println("This is i: " + i);
      System.out.println("This is y: " + y);
      y = y - 1;

    }
  }
}

The output generated by this program is shown here:

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