HelloJava.md

i# First Program in Java (Hello, Java!)

// To import everything from package directory-[java >> lang ]
import java.lang.*;  
// class name same as that of .java file
class HelloJava{  
    // main method
    public static void main(String args[]){
        System.out.println("Hello, Java!");
    }
}

Import Statement

If programmer wants to use any pre-defined class than it should be imported in the program through import statement

Import statement makes the JVM go to the Java Standard Library, execute the code there and substitute the result ito the program

In this process, no code was copied into source program; hence no wastage of memory and processor time

"""
 To run Program from CLI
 [In: Input] :: [op: Output]
"""
[In]    javac HelloJava.java
[In]    java HelloJava.java
[op]    Hello Java

class

class is a keyword which is used for creating user defined datatype. Each and every java program starts its execution from main()

public

• Since main() method can be accessed by every java programme, hence its access specifier must be public

static

• Since main() method is executing only once throughout the life of entire java program and it is not called with any object hence nature of main() is static

void

• main() method doesn't return anything value hence its datatype is void

String args[ ]

String is a class and each and every main() method of java takes array of objects of String class

println( )

println() is a predefined method present in predefined class called PrintStream class

• To access println() method, we require an object of PrintStream class.

• An object of PrintStream class called out is created by SunMicro System in an another predefined class called System as a static data member.

• Hence, to access println() and print() method, System class is used

• In System.out.println() : System is class; out is object; println() is method

• static public void main (String args[]){} is same as public static void main(){}

• String args[] is same as String something_else[]

Programme to display value of a variable

class Display{
    public static void main(String args[]){
        int a = 100;
        System.out.println("Value of a is: " + a);
    }
}

Output

Value of a is: 100

Programme to display sum of two numbers

class Sum{
    public static void main(String args[]){
        int a = 10;
        int b = 80;
        int c = a + b;
        System.out.println("Sum of" + a "and" + b + "is" + c);
    }
}

Output

Sum of 10 and 80 is 90

Out of the following classes, the same class will run in the name of which the program has been saved

class One{
    public static void main(){
        class Two{
            public static void main(){
                // code here
            }
        }
    }
}

A programme can not have Two main( ) methods in single main class

class Test{
    public static void main(){
        System.out.println("First main method");
    }
    public static void main(){
        System.out.println("Second main method");
    }
}

Class is a collection of data member and methods

// Filename: Demo.java
class First{
    int a, b, c;
    void accept(){
        a = 100;
        b = 900;
    }
    void sum(){
        c = a + b;
    }
    void display(){
        System.out.println("Sum is " + c);
    }
}

class Demo{
    public static void main(String args[]){
        First obj1 = new First();
        obj.accept();
        obj.sum();
        obj.display();
    }
}

Another way

// Filename: Demo.java
class First{
    int a, b, c;  // data members
    void accept(int x, int y){  // (x, y) :: local parameters
        a = x;
        b = y;
    }
    void sum(){
        c = a + b;
    }
    void display(){
        return c;
    }
}

class Demo{
    public static void main(String args[]){
        First obj = new First();
        obj.accept(99, 1);
        obj.sum();
        System.out.println(obj.display());
    }
}

Play with Java classes

class Demo {
  int x;
  public static void main (String args[]){
    System.out.println(x);
  }
}

/*
Output:
Error... 
*/

Note

• 1 non static method can call another static method directly if both belong to same class

• 1 non static method can call another non static method directly if both belong to same class

• 1 static can call another instance method (non-static) w.r.t. object

• 1 instance method can call another static method directly if both belong to same class

  • If both methods are from different class, then calling will be w.r.t. class name