Java Fundamentals

1. Packages in Java

A package contains a group of related classes and interfaces. It is an encapsulation mechanism that binds related types and provides a clear physical directory structure for project modules.

• Groups related classes and interfaces.
• Provides separation between project modules.
• Improves organization, reuse and maintainability.
• Creates a namespace and helps prevent naming conflicts.
• Makes related classes and interfaces easier to locate and use.
• Provides access protection.

Package Statement

package com.example.demo;

The package statement should be the first statement in the source file. Only one package statement can occur in a source file, and it applies to all types in that file. Without a package statement, the types belong to the default package.

2. Types of Packages

3. Predefined Packages

Java predefined packages are supplied as part of the Java platform and contain ready-to-use classes and interfaces.

• java.lang
• java.io
• java.awt
• java.util
• java.net

4. java.lang Package

The java.lang package contains commonly required classes and interfaces.

java
└── lang
    ├── String (class)
    ├── StringBuffer (class)
    ├── Object (class)
    ├── Runnable (interface)
    └── Cloneable (interface)

Note: java.lang is available by default whether or not it is explicitly imported.

5. java.io Package

The java.io package contains classes used for input/output operations.

java
└── io
    ├── FileInputStream (class)
    ├── FileOutputStream (class)
    ├── FileReader (class)
    ├── FileWriter (class)
    └── Serializable (interface)

6. User-Defined Packages

Packages declared by the programmer are called user-defined packages.

• A source file can contain only one package declaration.
• The package declaration should be at the beginning of the source file.
• Package names should normally use lowercase letters.

Valid

package pack1;
import java.lang.*;

Another Valid Example

package pack2;
import java.io.*;
import java.net.*;

Invalid

package pack1;
package pack2;

import java.io.*;
import java.lang.*;
package pack3;

7. Compiling Java Programs with Packages

When a source file contains a package statement, use the -d option with javac to create the required package directory structure and place compiled class files in the specified destination.

Syntax

javac -d Destination_folder file_name.java

Example

javac -d . Test.java

8. Package Directory Structure

After compilation, the directory structure follows the package hierarchy.

Com
└── dss
    └── bank
        └── deposit
            └── *.class

The total number of generated .class files depends on the number of classes present in the source file.

Online Exam Example

Com
└── tcs
    └── OnlineExam
        └── corejava
            ├── Test.class
            ├── A.class
            ├── B.class
            └── C.class

Execution

java com.tcs.onlineexam.Test

9. Import Session

The main purpose of the import statement is to make Java predefined or other package types available to the program by their simple names.

Example 1

import java.lang.String;

String is a predefined class. Normally java.lang is automatically available.

Example 2

import java.awt.*;

The wildcard form imports accessible types from the specified package.

10. Set CLASSPATH System Variable

The CLASSPATH variable specifies locations that Java tools can use to find classes and resources.

Display Current CLASSPATH

Delete Current Contents

Set CLASSPATH

11. Quick Revision

Package Workflow

Source File
    ↓
package statement
    ↓
javac -d Destination file.java
    ↓
Package directory structure
    ↓
.class files
    ↓
java fully.qualified.ClassName