First Program Of Java
we will learn how to write the simple / first program of java. We can write a simple hello java program easily after installing the JDK.
To create a simple / first java program, you need to create a class that contains main method. Lets understand the requirement first.
Requirement for Hello Java Example
For executing any java program, you need to
- install the JDK if you dont have installed it, download the JDK and install it.
- set path of the jdk/bin directory.
- create the java program
- compile and run the java program
Creating hello java example
Lets create the hello java program: FirstProgram.java
class FirstProgram{
public static void main(String args[]){
System.out.println("Welcome To java First Program");
}
}
To compile: javac FirstProgram.java
To execute: java FirstProgram
Output : Welcome To java First Program
Understanding first java program
Lets see what is the meaning of class, public, static, void, main, String[], System.out.println().
- class keyword is used to declare a class in java.
- public keyword is an access modifier which represents visibility, it means it is visible to all.
- static is a keyword, if we declare any method as static, it is known as static method. The core advantage of static method is that there is no need to create object to invoke the static method. The main method is executed by the JVM, so it doesnt require to create object to invoke the main method. So it saves memory.
- void is the return type of the method, it means it doesnt return any value.
- main represents startup of the program.
- String[] args is used for command line argument. We will learn it later.
- System.out.println() is used print statement. We will learn about the internal working of System.out.println statement later.
How many ways can we write a java program
There are many ways to write a java program. The modifications that can be done in a java program are given below:
1) By changing sequence of the modifiers, method prototype is not changed.
Lets see the simple code of main method.
static public void main(String args[])
2) subscript notation in java array can be used after type, before variable or after variable.
Lets see the different codes to write the main method.
public static void main(String[] args)
public static void main(String []args)
public static void main(String args[])
3) You can provide var-args support to main method by passing 3 ellipses (dots)
Lets see the simple code of using var-args in main method. We will learn about var-args later in Java New Features chapter.
public static void main(String... args)
4) Having semicolon at the end of class in java is optional.
Lets see the simple code.
class A{
static public void main(String... args){
System.out.println("hello World");
}
};
Valid java main method signature
public static void main(String[] args)
public static void main(String []args)
public static void main(String args[])
public static void main(String... args)
static public void main(String[] args)
public static final void main(String[] args)
final public static void main(String[] args)
final strictfp public static void main(String[] args)
Invalid java main method signature
public void main(String[] args)
static void main(String[] args)
public void static main(String[] args)
abstract public static void main(String[] args)
Resolving an error "javac is not recognized as an internal or external command"?
If there occurs a problem like displayed in the below figure, you need to set a path. Since DOS does not know javac or java, we need to set a path. Path is not required in such a case if you save your program inside the JDK/bin folder. But its a good approach to set path. Click here for How to set path in java.
