Java source structure

• Assume there are four class A, class B, class C, class D. but during the naming of the program there is no restriction to naming the program 
• There is the scope for at most one public class is allowed hence there will one public class or no public class at all
• Once we declare any class as public the program name must be declared with the public class name (name of the public class and program should be matched.) 
• We compile the program but run the dot class file

package com.company;

class First {
    public static void main(String[] args) {
        System.out.println("First class-main");
    }
}

class Second {
    public static void main(String[] argos) {
        System.out.println("Second class-main");
    }
}

class Third {
    public static void main(String[] argos) {
        System.out.println("Third class-main");
    }
}

class Fourth {
}

• We compile the program using cmd

Javac fileName.java 

• compiling the program there will be four dot class file we can run one by one using cmd

Java First

• When we try to run fourth class there will be an error saying 

Error: Main method not found in class Fourth, please define the main method as public static void main (String[] args)

Import statement

Ex 

class Test {
    public static void main(String[] args) {
        ArrayList arrayList = new ArrayList();
    }
}

There will error: cannot find symbol ArrayList arrayList = new ArrayList();

To specify the ArrayList location like 

package com.company;

class Test {
    public static void main(String[] args) {
        java.util.ArrayList arrayList = new java.util.ArrayList();
        System.out.println(arrayList);
    }
}

Let‘s use this ArrayList for many times

package com.company;

class Test {
    public static void main(String[] args) {
        java.util.ArrayList arrayList = new java.util.ArrayList();
        java.util.ArrayList arrayList1 = new java.util.ArrayList();
        java.util.ArrayList arrayList2 = new java.util.ArrayList();
        java.util.ArrayList arrayList3 = new java.util.ArrayList();
        java.util.ArrayList arrayList4 = new java.util.ArrayList();
        java.util.ArrayList arrayList5 = new java.util.ArrayList();
        java.util.ArrayList arrayList6 = new java.util.ArrayList();
        java.util.ArrayList arrayList7 = new java.util.ArrayList();
    }
}

• But when we use the qualified name by default readability will be down hence we use the import statement
• By using import statement length of the code will be reduced and readability will be improved

package com.company;

import java.util.ArrayList;

class Test {
    public static void main(String[] args) {
        ArrayList arrayList = new ArrayList();
    }
}

Types of import statement : 

Two types of import by default available in java

Explicit Import  - recommended 

• We are using 1 or 100 classes  from the same package we should always use an explicit import
• Code writing is one-time activities but readability is frequently required.
• Almost all the IDE generate explicit import only;

Import java.util.ArrayList();

Implicit Import

Import java.util.*;

There are two packages which are by default available for all the program

Java.lang     - the classes present inside the lang package are not required to import

package com.company;

class Test {
    public static void main(String[] args) {
        String s = new String ("Hello World");
        Exception e = new Exception();
        StringBuffer = new StringBuffer("Hello");
    }
}

Default package:  current working directory, or current package

D:/workspace

package com.company;

class Test {
    public static void main(String[] args) {
        String s = new String("Hello World");
        Exception e = new Exception();
        StringBuffer = new StringBuffer("Hello");
        Student s1 = new Student();
    }
}

Another File

package com.company;

class Student{
    public static void main(String[] args) {
        public static void m1(){
        System.out.println("Student class static method");
    }
}

Java > utIl > regex > pattern

import java.util.regex.Pattern;

class Test {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile("Hello");
    }
}

Q. The import statement for Pattern is

• Import java.*;
• Import java.util.*
• Import java.util.regex.*     // valid
• Whenever we import the package all classes and interfaces present inside that package are by default but sub-packages are not available, we have to import explicitly until the subpackage level.

Packages

Group of related classes and interfaces into a single unit are nothing but a package.

Ex 

• Java.sql
• Java.io

Advantages

1. Naming conflicts resolve/uniqueness for each SQL package date, util package date 
2. The modularity of the application 
3. Maintainability of the application
4. Security  - package level access 
5. Every class in java are the part of some package 
6. Highly recommended to use 

How to write the package?

The Internet domain name in reverse order

com.companyName.projectName

ex

com.mesanjay.project;

Package com.mesanjay.project;

package com.company;

import java.util.regex.Pattern;

class Test {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile("Hello");
    }
}

How to compile? 

Javac Test.java     //  cwd > Test.class

Java -d . Test.java    // cwd > com > mesanjay > project > Test.java

dot(.) means current working directory

If the project directory is not created then automatically will be created.

How to run?

Java com.mesanjay.project  Test

Conclusions

• In any java program/source file at most one package statement is allowed

Package pack1;

Package pack2;     //  X     error:class, interface, or enum expected

public class Test{}

Test.java

cmd javac -d .Test.java

• In any java program first non commented statement should be a package statement only

Import java.util.*;     // CE:  package pack1 should be placed in the top

Package pack1;

public class Test{}

Thank you.