"Hello world"

// 0[bB][01][01_]*[lL]?

// 0|[1-9][0-9_]*[lL]?

// ([0-9][0-9_]*\.([0-9][0-9_]*)?|\.[0-9][0-9_]*)([eE][+\-]?[0-9][0-9_]*)?[fFdD]?|[0-9][eE][+\-]?[0-9][0-9_]*[fFdD]?|[0-9]([eE][+\-]?[0-9][0-9_]*)?[fFdD]|0[xX]([0-9a-fA-F][0-9a-fA-F_]*\.?|([0-9a-fA-F][0-9a-fA-F_]*)?\.[0-9a-fA-F][0-9a-fA-F_]*)[pP][+\-]?[0-9][0-9_]*[fFdD]?

// 0[xX][0-9a-fA-F][0-9a-fA-F_]*[lL]?

// 0[0-7_]+[lL]?

/* A comment
*/

// A comment

// Package = directory. Java classes can be grouped together in packages. A package name is the same as the directory (folder) name which contains the .java files. You declare packages when you define your Java program, and you name the packages you want to use from other libraries in an import statement.
// The first statement, other than comments, in a Java source file, must be the package declaration.
// Following the optional package declaration, you can have import statements, which allow you to specify classes from other packages that can be referenced without qualifying them with their package.
// This source file must be Drawing.java in the illustration directory.
package illustration;
import java.awt.*;
public class Drawing {
 // ...
}

Iterator iter = list.iterator();
//Iterator iter = list.iterator();    in J2SE 5.0
while (iter.hasNext()) {
    System.out.print(iter.next());
    if (iter.hasNext())
        System.out.print(", ");
}

interface MyInterface{  
   /* This is a default method so we need not
    * to implement this method in the implementation 
    * classes  
    */
   default void newMethod(){  
       System.out.println("Newly added default method");  
   }  
   /* Already existing public and abstract method
    * We must need to implement this method in 
    * implementation classes.
    */
   void existingMethod(String str);  
}  
public class Example implements MyInterface{ 
  // implementing abstract method
    public void existingMethod(String str){           
        System.out.println("String is: "+str);  
    }  
    public static void main(String[] args) {  
      Example obj = new Example();
      
      //calling the default method of interface
        obj.newMethod();     
        //calling the abstract method of interface
        obj.existingMethod("Java 8 is easy to learn"); 
  
    }  
}

import javax.swing.*;
import javax.swing.JOptionPane;
// use fully qualified name without import:
javax.swing.JOptionPane.showMessageDialog(null, "Hi");
// There are 166 packages containing 3279 classes and interfaces in Java 5.
// import java.io.*; Input-output classes.

List v = new ArrayList();
v.add("test");
Integer i = v.get(0); // (type error)  compilation-time error

// By default, assertions are disabled
// java –enableassertions Test
int score = 10;
assert score >= 10 : " Below";
System.out.println("score is "+score);