Java Step By Step Tutorial

Java Packages

20:18:00

Package in Java

A package is a collection of similar types of classes, interfaces and sub-packages.

Packages in Java is a mechanism to encapsulate a group of classes, interfaces and sub packages. Many implementations of Java use a hierarchical file system to manage source and class files. It is easy to organize class files into packages. All we need to do is put related class files in the same directory, give the directory a name that relates to the purpose of the classes, and add a line to the top of each class file that declares the package name, which is the same as the directory name where they reside.

Purpose of package

The purpose of package concept is to provide common classes and interfaces for any program separately. In other words if we want to develop any class or interface which is common for most of the java programs than such common classes and interfaces must be place in a package.

Java Packages are,

java.lang
java.util
java.io
java.net
java.Swing
java.Applet

Packages in Java are the way to organize files when a project has many modules. Same like we organized our files in Computer. For example we store all movies in one folder and songs in other folder, here also we store same type of files in a particular package for example in awt package have all classes and interfaces for design GUI components.

Advantage of package

Package is used to categorize the classes and interfaces so that they can be easily maintained
Application development time is less, because reuse the code
Application memory space is less (main memory)
Application execution time is less
Application performance is enhance (improve)
Redundancy (repetition) of code is minimized
Package provides access protection.
Package removes naming collision.

Type of package

Package are classified into two type which are given below.

1. Predefined or built-in package

2. User defined package

Predefined or built-in package

These are the package which are already designed by the Sun Microsystem and supply as a part of java API, every predefined package is collection of predefined classes, interfaces and sub-package.

User defined package

If any package is design by the user is known as user defined package. User defined package are those which are developed by java programmer and supply as a part of their project to deal with common requirement.

Rules to create user defined package

· package statement should be the first statement of any package program.

· Choose an appropriate class name or interface name and whose modifier must be public.

· Any package program can contain only one public class or only one public interface but it can contain any number of normal classes.

· Package program should not contain any main class (that means it should not contain any main())

· modifier of constructor of the class which is present in the package must be public. (This is not applicable in case of interface because interface have no constructor.)

· The modifier of method of class or interface which is present in the package must be public (This rule is optional in case of interface because interface methods by default public)

· Every package program should be save either with public class name or public Interface name

Compile package programs

For compilation of package program first we save program with public className.java and it compile using below syntax:

Syntax:

javac -d . className.java

Syntax:

javac -d path className.java

Explanations: In above syntax "-d" is a specific tool which is tell to java compiler create a separate folder for the given package in given path. When we give specific path then it create a new folder at that location and when we use . (dot) then it crate a folder at current working directory.

Note: Any package program can be compile but can not be execute or run. These program can be executed through user defined program which are importing package program.

Example of package program

Package program which is save with A.java and compile by javac -d . A.java

package mypack;

public class A

public void show()

System.out.println("Sum method");

Import above class in below program using import packageName.className

import mypack.A;

public class Hello

public static void main(String arg[])

A a=new A();

a.show();

System.out.println("show() class A");

Explanations: In the above program first we create Package program which is save with A.java and compiled by "javac -d . A.java". Again we import class "A" in class Hello using "import mypack.A;" statement.

Difference between Inheritance and package

Inheritance concept always used to reuse the feature within the program between class to class, interface to interface and interface to class but not accessing the feature across the program.
Package concept is to reuse the feature both within the program and across the programs between class to class, interface to interface and interface to class.

Difference between package keyword and import keyword

Package keyword is always used for creating the undefined package and placing common classes and interfaces.
import is a keyword which is used for referring or using the classes and interfaces of a specific package.

Array in Java

20:07:00

Array in java

Array is a collection of similar type of data. It is fixed in size that means you can't increase the size of array at run time. It is collection of homogeneous data elements. It store the value on the basis of index value.

Advantage of Array

One variable can store multiple value: The main advantage of array is we can represent multiple value under the same name.

Code Optimization: No, need to declare a lot of variable of same type data, We can retrieve and short data easily.

Random access: We can retrieve any data from array with the help of index value.

Disadvantage of Array

The main limitation of array is Size Limit when once we declare array there is no chance to increase and decrease the size of array according to our requirement, Hence memory point of view array concept is not recommended to use. To over come this limitation in java introduce collection concept.

Types of Array

There are two types of array in java.

· Single Dimensional Array

· Multidimensional Array

Array Declaration

Single dimension array declaration.

1.   int[]  a;

2.   int  a[];

3.   int  []a;

Note: At the time of array declaration we can not specify the size of array. For Example int[5] a; this is wrong.

2D Array declaration.

1.   int[][]  a;

2.   int  a[][];

3.   int  [][]a;

4.   int[]  a[];

5.   int[]  []a;

6.   int   []a[];

Array creation

Every array in a java is an object, Hence we can create array by using new keyword.

int[] arr = new int[10];    // The size of array is 10.

or

int[] arr = {10,20,30,40,50};

Accessing array elements

Access the elements of array by using index value of an elements.

arrayname[n-1];

Access Array Elements

int[] arr={10,20,30,40};

System.out.println("Element at 4th place"+arr[2]);

Example of Array

public class ArrayEx

public static void main(String []args)

int arr[] = {10,20,30};

for (int i=0; i < arr.length; i++)

 System.out.println(arr[i]);

Output

Note:

1) At the time of array creation we must be specify the size of array otherwise get an compile time error. For Example
int[] a=new int[]; Invalid.
int[] a=new int[5]; Valid

2) If we specify array size as negative int value, then we will get run-time error, NegativeArraySizeException.

3) To specify array size the allowed data types are byte, short, int, char If we use other data type then we will get an Compile time error.

4) The maximum allowed size of array in java is 2147483647 (It is maximum value of int data type)

Difference Between Length and Length() in Java

length: It is a final variable and only applicable for array. It represent size of array.

Example:

int[] a=new int[10];

System.out.println(a.length);  // 10

System.out.println(a.length());  // Compile time error

length(): It is final method applicable only for String objects. It represent number of character present in String.

String s="Java";

System.out.println(s.length());  // 4

System.out.println(s.length);  // Compile time error

Note: Array is presently not used in Java due to limitations, You need not waste too much time in this area, Consecrate in Java Collections.

Important Keywords in Java

20:00:00

In the Java programming language, a keyword is one of 51 reserved words that have a predefined meaning in the language; because of this, programmers cannot use keywords as names for variables, methods, classes, or as any other identifier.

But some important keywords should known as java developer,

There are,

Final

Static

This

super

static

Synchronize

volatile

Final keyword in java

It is used to make a variable as a constant, Restrict method overriding, Restrict inheritance. It is used at variable level, method level and class level. In java language final keyword can be used in following way.

• Final at variable level

• Final at method level

• Final at class level

Static keyword in java

The static keyword is used in java mainly for memory management. It is used with variables, methods, blocks and nested class. It is a keyword that are used for share the same variable or method of a given class. This is used for a constant variable or a method that is the same for every instance of a class. The main method of a class is generally labeled static.

No object needs to be created to use static variable or call static methods, just put the class name before the static variable or method to use them. Static method can not call non-static method.

In java language static keyword can be used for following

• variable (also known as class variable)

• method (also known as class method)

• block

• nested class

Difference between static and final keyword

static keyword always fixed the memory that means that will be located only once in the program where as final keyword always fixed the value that means it makes variable values constant.

Note: As for as real time statement there concern every final variable should be declared the static but there is no compulsion that every static variable declared as final.

This keyword in java

this is a reference variable that refers to the current object. It is a keyword in java language represents current class object

Usage of this keyword

• It can be used to refer current class instance variable.

• this() can be used to invoke current class constructor.

• It can be used to invoke current class method (implicitly)

• It can be passed as an argument in the method call.

• It can be passed as argument in the constructor call.

• It can also be used to return the current class instance.

Why use this keyword in java ?

The main purpose of using this keyword is to differentiate the formal parameter and data members of class, whenever the formal parameter and data members of the class are similar then jvm get ambiguity (no clarity between formal parameter and member of the class)

To differentiate between formal parameter and data member of the class, the data member of the class must be preceded by "this".

"this" keyword can be use in two ways.

• this . (this dot)

• this() (this off)

this . (this dot)

which can be used to differentiate variable of class and formal parameters of method or constructor.

"this" keyword are used for two purpose, they are

• It always points to current class object.

• Whenever the formal parameter and data member of the class are similar and JVM gets an ambiguity (no clarity between formal parameter and data members of the class).

Super keyword in java

Super keyword in java is a reference variable that is used to refer parent class object. Super is an implicit keyword create by JVM and supply each and every java program for performing important role in three places.

• At variable level

• At method level

• At constructor level

Need of super keyword:

Whenever the derived class is inherits the base class features, there is a possibility that base class features are similar to derived class features and JVM gets an ambiguity. In order to differentiate between base class features and derived class features must be preceded by super keyword.

Synchronized Keyword in Java

Synchronized Keyword is used for when we want to allowed only one thread at a time then use Synchronized modifier. If a method or block declared as a Synchronized then at a time only one thread is allowed to operate on the given object.

Synchronized is a Modifier which is applicable for the method or block, we can not declare class or variable with this modifier.

Advantage of Synchronized

The main advantage of Synchronized keyword is we can resolve data inconsistency problem.

Dis-Advantage of Synchronized

The main dis-advantage of Synchronized keyword is it increased the waiting time of thread and effect performance of the system, Hence if there is no specific requirement it is never recommended to use synchronized keyword.

Volatile Keyword in Java

If the variable keep on changing such type of variables we have to declare with volatile modifier. Volatile is a modifier applicable only for variables but not for method and class.

If a variable declared as volatile then for every thread a separate local copy will be created. Every intermediate modification performed by that thread will takes place in local copy instead of master copy. Once the value got finalized just before terminating the thread the master copy value will be updated with local stable value.

Advantage of Volatile

The main advantage of Volatile keyword is we can resolve data inconsistency problems.

Dis-Advantage of Volatile

The main dis-advantage of Volatile keyword is, crating and maintaining a separate copy for every thread, increases complexity of the programming and effects performance of the system. Hence if there is no specific requirement it is never recommended to use volatile keyword, and it is almost outdated keyword.

Note: Volatile variable means its value keep on changing where as final variable means its value never changes. Hence final-Volatile combination is illegal combination for variables.

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Java Step By Step Tutorial

Java Packages

Advantage of package

Array in Java

Important Keywords in Java

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