![Maharishi Markandeshwar University Page - 20 -
WRITE, COMPILE AND RUN A JAVA PROGRAM
Write source code:
Let us look at a simple code that would print the words Hello World.
public class MyFirstJavaProgram{
/* This is my first java program.
* This will print 'Hello World' as the output
*/
public static void main(String[]args){
System.out.println("Hello World");// prints Hello World
}
}
Let's look at how to save the file, compile and run the program. Please follow the steps given
below:
Open notepad and add the code as above.
Save the file as: MyFirstJavaProgram.java.
Opena commandpromptwindow andgo o the directorywhere yousavedthe class.Assume it's
C:.
Type ' javac MyFirstJavaProgram.java ' and press enter to compile your code. If there are no
errorsin yourcode,the commandprompt will take you to the next line (Assumption: The path
variable is set).](https://image.slidesharecdn.com/trainingreport-170401111300/85/Core-Java-Training-report-20-320.jpg)
![Maharishi Markandeshwar University Page - 26 -
package sumit.java;
public class MyMethodExample {
void tester(String s) {
System.out.println("Hello World");
}
}
Method can be declared with var-args. In this case the method declares a parameter which
accepts from zero to many arguments (syntax: type .. name;) A method can only have one var-
args parameter and this must be the last parameter in the method.
Overwrite of a superclass method: A method must be of the exact same return parameter and the
same arguments. Also the return parameter must be the same. Overload methods: An overloaded
method is a method with the same name, but different arguments. The return type can not be
used to overload a method.
Main method
A public static method with the following signature can be used to start a Java application. Such
a method is typically called main method.
public static void main(String[] args){
}
Constructor
A class contains constructors that are invoked to create objects based on the class definition.
Constructor declarations look like method declarations except that they use the name of the class
and have no return type. A class can have several constructors with different parameters. Each
class must define at least one constructor.
In the following example the constructor of the class expects a parameter.](https://image.slidesharecdn.com/trainingreport-170401111300/85/Core-Java-Training-report-26-320.jpg)
![Maharishi Markandeshwar University Page - 33 -
package com.vogella.javaintro.base;
public class MyStaticExample {
static String PLACEHOLDER = "TEST";
static void test() {
System.out.println("Hello");
}
}
package com.vogella.javaintro.base;
public class Tester {
public static void main(String[] args) {
System.out.println(MyStaticExample.PLACEHOLDER);
MyStaticExample.test();
}
}
If a variable should be defined as constant, you declare it with the static and the final
keyword. The static method runs without any instance of the class, it cannot directly access non-
static variables or methods.](https://image.slidesharecdn.com/trainingreport-170401111300/85/Core-Java-Training-report-33-320.jpg)
![Maharishi Markandeshwar University Page - 38 -
What to do How to do it
}
public Dog getDog() {
return dog;
}
public void setDog(Dog dog) {
this.dog = dog;
}
}
Working With Local Variable
Table 3:
What to do How to do it
Declare a (local) variable of type string. String variable1;
Declare a (local) variable of type string and assign
"Test" to it.
String variable2 = "Test";
Declare a (local) variable of type Person Person person;
Declare a (local) variable of type Person, create a
new Object and assign the variable to this object.
Person person = new Person();
Declare a array of type String String array[];
Declare a array of type Person and create an array
for this variable which can hold 5 Persons.
Person array[]= new Person[5];
Assign 5 to the int variable var1 (which was
already declared);
var1 = 5;](https://image.slidesharecdn.com/trainingreport-170401111300/85/Core-Java-Training-report-38-320.jpg)
![Maharishi Markandeshwar University Page - 40 -
INTEGRATED DEVELOPMENT ENVIRONMENT
The previous chapter explained how to create and compile a Java program on the command line.
A Java Integrated Development Environment (IDE) provides lots of ease of use functionality for
creating java programs. There are other very powerful IDE's available, for example the Eclipse
IDE. . For an introduction on how to use the Eclipse IDE please see Eclipse IDE Tutorial. The
remaining description uses the phrase: Create a Java project called... ". This refers to creating a
Java project in Eclipse. If you are using a different IDE please follow the required steps in this
IDE. Exercises - Creating Java objects and methods Create a Person class and instantiate it
Create a new Java project called com.vogella.javastarter.exercises1 and a package with the same
name. Create a class called Person. Add three instance variables to it, one for storing the first
name of the person, on for storing the last name and one for storing the age of the Person. Use
the constructor of the Person object to set the values to some default value. Write a public
method called writeName() which uses the System.out.println( method to print the first name of
the person to the console. Create a new class called Main with a public static void main(String[]
args). In this method create an instance of the Person class.](https://image.slidesharecdn.com/trainingreport-170401111300/85/Core-Java-Training-report-40-320.jpg)
Core Java Training report
- 1. Maharishi Markandeshwar University Page - 1 - MAHARISHI MARKANDESHWAR UNIVERSITY MULLANA, AMBALA (SESSION 2014-2018) DEPARTMENT OF COMPUTER SCIENCE & ENGINEERING INDUSTRIAL TRAINING REPORT Submitted in Partial Fulfillment of requirements for the Award of Degree of Bachelor of Technology (Computer Science & Engineering) Head of Dept: Submitted By: Mr. Sandeep Goyal Sumit kr. Sharma 11142507
- 2. Maharishi Markandeshwar University Page - 2 - ACKNOWLEDGEMENT The completion of this training work could have been possible with continued & dedicated efforts & guidance of large number of faculty & staff members of the institute .I acknowledge our gratitude to all of them. The acknowledgement however will be incomplete without specific mention as follows I wish to acknowledge my deep gratitude to Md. Danaeyal Hussan, teacher at HP Education Services, Patna for his cooperation and guidance. I am also thankful to his Lab assistant that provided staunch support throughout this training and helped me to complete the training successfully Furthermore I would also like to acknowledge with much appreciation the crucial role of our HOD Prof. Sandeep Goyal for this encouragement & providing all the facilities in the department Finally, I would like to say that I am indebted to my parents for everything that they have done for me. All of this would have been impossible without their constant support. And I also thank to God for being kind to me and driving me through this journey. Sumit Kumar Sharma
- 3. Maharishi Markandeshwar University Page - 3 - MAHARISHI MARKANDESHWAR UNIVERSITY MULLANA, AMBALA Department of Computer Science & Engineering CERTIFICATE PASTE YOUR CERTIFICATE HERE
- 4. Maharishi Markandeshwar University Page - 4 - MAHARISHI MARKANDESHWAR UNIVERSITY MULLANA, AMBALA Department of Computer Science & Engineering APPROVAL CERTIFICATE This dissertation work entitled “Industrial Training Report on CORE JAVA” submitted by Mr. Sumit Kumar Sharma is approved for the award of degree of Bachelor of Technology in Computer Science and Engineering. Internal Examiner External Examiner Date: Date:
- 5. Maharishi Markandeshwar University Page - 5 - INDEX S. No TOPIC NAME PAGE NO. SIGN. 1. Company Profile 6 2. History of Java 7 3. Features of Java 10 4. Introduction To Java 12 5. The Java Platform 14 6. Installation Of Java 16 7. Configuring Variable 18 8. Write, Compile And Run a Java Program 20 9. Package 21 10. Class And Object 22 11. Inheritance 23 12. Variable And Method 25 13. Modifiers 28 14. Import Statement 30 15. More java Languages Constructs 31 16. IDE 40
- 6. Maharishi Markandeshwar University Page - 6 - COMPANY PROFILE Hewlett Packard Enterprise has been conducting Training Program for the last 8 years with great success in India. The Trainings is conducted by Hewlett Packard Enterprise partners. All Trainers who conduct Training are authorized Hewlett Packard Enterprise Partner trainers. At all locations the training is delivered by Hewlett Packard Enterprise Training Partner Professionals only. Hewlett Packard Enterprise is in Education sector for 30 years. You get associated with No.1 IT Company. Every Year approximately 20k students are trained in various programs conducted by Hewlett Packard Enterprise. Qualified Hewlett Packard Enterprise Partner Trainers with good experience deliver such trainings. Specifically Designed Programs based on Industry requirements. We provide adequate conceptual and practical sessions. Additional Modules on Communication Skills, G.D, and P.I will be provided, depending on the program enrolled for. Anybody who is an Undergraduate, Graduate or Post Graduate which includes All B.Tech / MCA , M.Sc IT, B.Sc IT , BCA , PGDCA can opt training from HP. Students studying in 1st year to final year can undergo this program. The Training is also designed to fulfill the requirements of the 6th semester students who wish to do Industrial Training, a mandatory part of their curriculum. Students who wish to upgrade their existing skills can also undergo the program. Students wanting to do International Certifications should do this program.
- 7. Maharishi Markandeshwar University Page - 7 - HISTORYOF JAVA It is quite interesting to know the development of Java technology, which is widely accepted in the area of networked/distributed computing. Java evolved from a project developing a language for programming consumer electronic devices at Sun Microsystems, USA. Sun’s Stealth Project in 1990 was aimed to do research in the area of application of computers in the consumer electronics market. Bill Joy, James Gosling, Mike Sheridan, Patrick Naughton (formerly the project leader of Sun’s Open Windows user environment), and several other individuals discussed the ideas for the Stealth Project in January 1991.The vision of this project was to develop smart consumer electronic devices that could all be centrally controlled and programmed from a handheld-remote-control-like device. The Stealth group began to work to achieve this goal. It was realized that a platform-independent development-environment was needed. Green Project was initiated by Patrick Naughton, Mike Sheridan, and James Gosling of Sun Microsystems in 1991 (Members of the Stealth Project, which later became known as the Green Project) The tasks of the project were divided amongst the members of Green Project. Mike Sheridan concentrated on business development, Patrick Naughton worked on the graphics system, and James Gosling was to identify the appropriate programming language for the Green Project. Even though creating a new language was not the aim of Green Project, a new language was created as a programming tool in this project since C++ was found unsuitable for the project. An intelligent remote control called *7 (Star Seven) was delivered as the first product of Green Project in 1992. It is a PDA- like device comprising of a new language oak, an operating system called GreenOS, user interface, and hardware. This device was quite impressive to Sun Executives. But no customer was interested in this project. The market was not conducive to this type of device in the consumer electronics industry.
- 8. Maharishi Markandeshwar University Page - 8 - The new language Oak was created by James Gosling, a Green Team member, specifically for *7. Gosling named the new language Oak because that name was struck while looking at an oak three outside of his office window. The name Oak was later renamed to Java in order to avoid legal issues since Oak was the name of an existing language. In November 1992, the Green Project was incorporated under the name FirstPerson. In 1993. Time-Warner was demanding for proposals for set-top box operating systems and video-on- demand technology with an objective of sending the data to consumer all over the country for display on the television sets. At the same time (1993(, NCSA released the first graphical web browser Mosaic 1.0, an easy-to-use front end to the World Wide Web. When FirstPerson was bidding on the Time-Warner TV trial based on video-on-demand technology, Time-Warner chose Silicon Graphics, Inc. (SGI) over Sun. Hence, half of the members of FirstPerson left for SGI and the remaining members continued to work at Sun. Mosaic web browser revolutionized people’s perceptions. The remaining member of FirstPerson returned to work on Java (Oak was renamed Java) to develop Java-based web browser. FirstPerson was dissolved gradually. Naughton and Jonathan Payne developed WebRunner (named after the movie Blade Runner). Later WevRunner was officially announced as the HotJavaTM browser in 1994. This was the turning point for Java. At that time the World Wide Web changed the face of Internet and it was winning the race on Internet. Arthur Ban Hoff implemented the Java compiler in Java itself whereas Gosling implemented it in C. The Java compiler, written in Java made the people to accept Java as a full-featured language. Sun Microsystems officially announced Java environment at Sun World’95 on May 23, 1995. Java entered into the mainstream of the Internet after a struggle for about four years. Netscape Communications incorporated Java into its web browser Netscape Navigator. Within a decade, Java was destined to be the most overreaching technology in the Internet. Java was not restricted to the Internet alone. The initial goal of Sun was also achieved by using Java technology in interactive set-top boxes, hand-held devices and other consumer electronics products.
- 9. Maharishi Markandeshwar University Page - 9 - Sun released the first version 1.0 of Java in 1996. Java is an object-oriented programming language which evolved from C++. It is also a high-level programming language. The different forms of Java versions are discussed in the next section.
- 10. Maharishi Markandeshwar University Page - 10 - FEATURES OF JAVA According to Sun, Java is defined as a simple, object-oriented, distributed, interpreted, robust, secure, architecture –neutral, portable, high performance, multithreaded, and dynamic programming language and platform. Java is simple because the syntax of well-known languages such as C and C++ are used with modifications meant for simplification and improvement. It is easy to read and write Java code if the reader is familiar with C/C++. Almost everything in Java is centered on creating objects, manipulating the objects and making objects work together. Only the primitive operations and data types are at the sub-object level. Hence Java is an object-oriented programming language. An application can be distributed in many systems which are connected together. Since networking capability is incorporated in Java, the data/file can be sent across many systems to run the application. Java is robust because it is more reliable. It ensures the reliability by providing early checking for possible errors. It eliminates error-prone constructs such as pointer. It eliminates error-prone constructs such as pointer. It also supports runtime error handling. Java programs are compiled to a byte code format that can be read and run by interpreters on many platforms including Windows, Linux, etc. Hence, it is architecture neutral. Because of this feature it is portable to new hardware and operating systems.
- 11. Maharishi Markandeshwar University Page - 11 - Although the speed of execution of Java programs is slower compared to C/C++ programs, Java is meant for distributed applications. Whenever the file/data is transmitted from one system to another, the performance is not affected. Hence, it is having high performance. Java supports multithreaded programming to perform several tasks simultaneously. Any code can be loaded without recompilation at runtime. Hence it is dynamic.
- 12. Maharishi Markandeshwar University Page - 12 - INTRODUCTION TO JAVA With the invention of microprocessors, the world is scientifically developed with sophisticated equipments, systems, and devices. Microprocessors are used in computers, televisions, and fax machines. Even the hand-held devices such as pagers, PDAs (Personal Digital Assistant), and cell phones make use of microprocessors. All these electronic devices are helpful because of their communication capabilities. With the increasing capabilities and decreasing cost of information processing and networking technologies, the network is growing rapidly for transmitting information through electronic systems. Internet is the network of networks between different types of computers located at different places to transmit information. Information can reach to any place in the world quickly at a cheaper rate through the Internet. Thus, the Internet has made the world a global village for information exchange. The emerging infrastructure of electronic devices and interconnected computer networks create an environment that presents new challenges to software industries. for this emerging computing environment, Java process to be a well – suited programming language. it is found suitable for networked environments involving a great variety of computer and devices. Java has many characteristics that have contributed to its popularity: Platform independence - Many languages are compatible with only one platform. Java was specifically designed so that it would run on any computer, regardless if it was running Windows, Linux, Mac, Unix or any of the other operating systems. Simple and easy to use - Java's creators tried to design it so code could be written efficiently and easily. Multi-functional - Java can produce many applications from command-line programs to applets to Swing windows (basically, sophisticated graphical user interfaces).
- 13. Maharishi Markandeshwar University Page - 13 - Java does have some drawbacks. Since it has automated garbage collection, it can tend to use more memory than other similar languages. There are often implementation differences on different platforms, which have led to Java being described as a "write once, test everywhere" system. Lastly, since it uses an abstract "virtual machine", a generic Java program doesn't have access to the Native API's on a system directly. None of these issues are fatal, but it can mean that Java isn't an appropriate choice for a particular piece of software.
- 14. Maharishi Markandeshwar University Page - 14 - THE JAVA PLATFORM One thing that distinguished Java from some other languages is its ability to run the same compiled code across multiple operating systems.In other languages, the source code (code that is written by the programmer), is compiled by a compiler into an executable file. This file is in machine language, and is intended for a single operating system/processor combination, so the programmer would have to re-compile the program seperately for each new operating system/processor combination.Java is different in that it does not compile the code directly into machine language code. Compilation creates bytecode out of the source code. Bytecode generally looks something like this: a7 f4 73 5a 1b 92 7d When the code is run by the user, it is processed by something called the Java Virtual Machine (JVM). The JVM is essentially an interpreter for the bytecode. It goes through the bytecode and runs it. There are different versions of the JVM that are compatible with each OS and can run the same code. There is virtually no difference for the end-user, but this makes it a lot easier for programmers doing software development. Java and Open Source:- In 2006 Sun started to make Java available under the GNU General Public License (GPL). Oracle continues this project called OpenJDK. Java Virtual machine :- The Java virtual machine (JVM) is a software implementation of a computer that executes programs like a real machine.
- 15. Maharishi Markandeshwar University Page - 15 - The Java virtual machine is written specifically for a specific operating system, e.g. for Linux a special implementation is required as well as for Windows. Java programs are compiled by the Java compiler into bytecode. The Java virtual machine interprets this bytecode and executes the Java program. Java Runtime Environment vs. Java Development Kit A Java distribution comes typically in two flavors, the Java Runtime Environment (JRE) and the Java Development Kit (JDK). The Java runtime environment (JRE) consists of the JVM and the Java class libraries and contains the necessary functionality to start Java programs. The JDK contains in addition the development tools necessary to create Java programs. The JDK consists therefore of a Java compiler, the Java virtual machine, and the Java class libraries.
- 16. Maharishi Markandeshwar University Page - 16 - INSTALLATION OF JAVA Check installation: Java might already be installed on your machine. You can test this by opening a console (if you are using Windows: Win+R, enter cmd and press Enter) and by typing in the following command: java -version If Java is correctly installed, you should see some information about your Java installation. If the command line returns the information that the program could not be found, you have to install Java. Installing the Java Development Kit: Before installing the Java Development Kit (JDK), you should probably know what it is. It is distributed by Oracle. It contains the core libraries and compiler required to develop Java. The JDK should not be confused with the JRE (Java Runtime Environment). The JRE is a JVM for running, as opposed to compiling, Java programs. Downloading and Installing: To download the JDK, go to http://www.oracle.com/technetwork/java/javase/downloads/index.html. Click on "JDK with Net Beans Bundle". Follow the instructions for downloading the JDK installation file. Windows: If you are running Windows, simply run the executable file and follow the installation instructions. UNIX, Solaris, or Linux: For Linux and UNIX, download the "jdk1 6.0" for Linux systems. Save the downloaded file in any drive. Once you have saved the file, extract it to a place that you can remember, by using Terminal or by double clicking on the file.
- 17. Maharishi Markandeshwar University Page - 17 - When you have finished extracting the file, copy the JDK 1.6.0 folder and paste it in the user/local (To paste to the user/local directory, you have to be in root) so that every user can use the java files. You can delete the downloaded zip file so that it doesn't take up space on your drive. Macintosh: The latest available JDK is automatically installed by the operating system. Because Java for Macintosh is developed and maintained by Apple, in coordination with Sun, the current version on the Macintosh may not be the current version that is available Note on Editions: The JDK comes in three editions. Java Standard Edition (JSE) – This version is the basic platform for Java. The course will focus on this edition. Java Enterprise Edition (JEE) – This edition is mainly for developing and running distributed multitier architecture Java applications, based largely on modular software components running on an application server. We will not be covering this version in the course. Java Micro Edition (JME) – This edition is primarily for developing programs to run on consumer appliances, such as PDAs and cell phones.
- 18. Maharishi Markandeshwar University Page - 18 - CONFIGURING VARIABLES Before writing code, it is recommended that you set the Path variable on your system so you can compile your code more easily. For Windows Users: From the Control Panel, double click "System" (System and Maintenance in Vista) For Windows 7 or Vista, click on "System," "Advanced System Settings" on the left, and then on "Environment Variables." For XP and 2000, click on the "Advanced" tab and click on "Environment Variables" For NT, click on the "Environment" tab. Select the Path variable and click "Edit" Add the path to the bin directory of where Java is installed on your hard drive. It should probably be: C:Program FilesJavajdk1.6.0_20bin unless you changed it during installation. Click OK Validate installation: Switch again to the command line and run the following command. java -version The output should be similar to the following output. java version "1.7.0_25" OpenJDK Runtime Environment (IcedTea 2.3.10) (7u25-2.3.10-1ubuntu0.13.04.2) OpenJDK 64-Bit Server VM (build 23.7-b01, mixed mode)
- 19. Maharishi Markandeshwar University Page - 19 - Development Process with Java: Java source files are written as plain text documents. The programmer typically writes Java source code in an Integrated Development Environment (IDE) for programming. An IDE supports the programmer in the task of writing code, e.g. it provides auto-formatting of the source code, highlighting of the important keywords, etc. At some point the programmer (or the IDE) calls the Java compiler (javac). The Java compiler creates the byte code instructions. These instructions are stored in .class files and can be executed by the Java Virtual Machine. Garbage collector The JVM automatically re-collects the memory which is not referred to by other objects. The java garbage collector checks all object references and find the objects which can be automatically released. While the garbage collector releases the programmer from the need to explicitly manage memory the programmer still need to ensure that he does not keep unneeded object references otherwise the garbage collector cannot release the associated memory. Keeping unneeded object references are typically called memory leaks. Class path The class path defines where the Java compiler and Java runtime look for .class files to load. This instruction can be used in the Java program. For example if you want to use an external Java library you have to add this library to your class path to use it in your program.
- 20. Maharishi Markandeshwar University Page - 20 - WRITE, COMPILE AND RUN A JAVA PROGRAM Write source code: Let us look at a simple code that would print the words Hello World. public class MyFirstJavaProgram{ /* This is my first java program. * This will print 'Hello World' as the output */ public static void main(String[]args){ System.out.println("Hello World");// prints Hello World } } Let's look at how to save the file, compile and run the program. Please follow the steps given below: Open notepad and add the code as above. Save the file as: MyFirstJavaProgram.java. Opena commandpromptwindow andgo o the directorywhere yousavedthe class.Assume it's C:. Type ' javac MyFirstJavaProgram.java ' and press enter to compile your code. If there are no errorsin yourcode,the commandprompt will take you to the next line (Assumption: The path variable is set).
- 21. Maharishi Markandeshwar University Page - 21 - BASICS: PACKAGE, CLASS AND OBJECT It is important to understand the base terminology of Java in terms of packages, classes and objects. This section gives an overview of these terms. Package: Java groups classes into functional packages. Packages are typically used to group classes into logical units. For example all graphical views of an application might be placed in the same package called com.vogella.webapplication.views. It is common practice to use the reverse domain name of the company as top level package. For example the company might own the domain, vogella.com and in this example the Java packages of this company starts with com.vogella. Other main reason for the usage of packages is to avoid name collisions of classes. A name collision occurs if two programmers give the same fully qualified name to a class. The fully qualified name of a class in Java consists out of the package name followed by a dot (.) and the class name. Without packages, a programmer may create a Java class called Test. Another programmer may create a class with the same name. With the usage of packages you can tell the system which class to call. For example if the first programmer puts the Test class into package report and the second programmer puts his class into package xmlreader you can distinguish between these classes by using the fully qualified name, e.g. xmlreader.Test or report.Test.
- 22. Maharishi Markandeshwar University Page - 22 - Class: Def.: Template that describes the data and behavior associated with an instance of that class. In Java source code a class is defined by the class keyword and must start with a capital letter. The body of a class is surrounded by {}. package test; class MyClass { } The data associated with a class is stored in variables ; the behavior associated to a class or object is implemented with methods. A class is contained in a Java source file with the same name as the class plus the .java extension. Object Def.: An object is an instance of a class. The object is the real element which has data and can perform actions. Each object is created based on the class definition. Inheritance A class can be derived from another class. In this case this class is called a subclass. Another common phrase is that a class extends another class. The class from which the subclass is derived is called a superclass. Inheritance allows a class to inherit the behavior and data definitions of another class. The following codes demonstrate how a class can extend another class. In Java a class can extend a maximum of one class.
- 23. Maharishi Markandeshwar University Page - 23 - package sumit.java; class MyBaseClass { void hello(){ System.out.println("Hello from MyBaseClass"); } } package com.vogella.javaintro.base; class MyExtensionClass extends MyBaseClass { } Override methods and the @override annotation If a class extends another class it inherits the methods from its superclass. If it wants to change these methods it can override these methods. To override a method you use the same method signature in the source code of the subclass. To indicate to the reader of the source code and the Java compiler that you have the intention to override a method you can use the @override annotation. The following code demonstrates how you can override a method from a superclass. package sumit.java; class MyBaseClass { void hello(){ System.out.println("Hello from MyBaseClass"); } }
- 24. Maharishi Markandeshwar University Page - 24 - package sumit.java; class MyExtensionClass2 extends MyBaseClass { } Object has superclass Every object in Java implicitly extends the Object class. The class defines the following methods for every Java object: equals(o1) allows to check if the current object is equal to o1 getClass() returns the class of the object hashCode() returns an identifier of the current object toString() Give a string representation of the current object
- 25. Maharishi Markandeshwar University Page - 25 - VARIABLES AND METHODS Variable Variables allow the Java program to store values during the runtime of the program. A variable can either be a primitive variable or a reference variable. A primitive variable contains value while the reference variable contains a reference (pointer) to the object. Hence if you compare two reference variables, you compare if both point to the same object. To compare objects use the object1.equals(object2) method call. Instance variable Instance variable is associated with an instance of the class (also called object). Access works over these objects. Instance variables can have any access control and can be marked final or transient. Instance variables marked as final cannot be changed after assigned to a value. Local variable Local (stack) variable declarations cannot have access modifiers. Final is the only modifier available to local variables. This modifier defines that the variable cannot be changed after first assignment. Local variables do not get default values, so they must be initialized before use. Methods A method is a block of code with parameters and a return value. It can be called on the object.
- 26. Maharishi Markandeshwar University Page - 26 - package sumit.java; public class MyMethodExample { void tester(String s) { System.out.println("Hello World"); } } Method can be declared with var-args. In this case the method declares a parameter which accepts from zero to many arguments (syntax: type .. name;) A method can only have one var- args parameter and this must be the last parameter in the method. Overwrite of a superclass method: A method must be of the exact same return parameter and the same arguments. Also the return parameter must be the same. Overload methods: An overloaded method is a method with the same name, but different arguments. The return type can not be used to overload a method. Main method A public static method with the following signature can be used to start a Java application. Such a method is typically called main method. public static void main(String[] args){ } Constructor A class contains constructors that are invoked to create objects based on the class definition. Constructor declarations look like method declarations except that they use the name of the class and have no return type. A class can have several constructors with different parameters. Each class must define at least one constructor. In the following example the constructor of the class expects a parameter.
- 27. Maharishi Markandeshwar University Page - 27 - package sumit.java; public class MyConstructorExample2 { String s; public MyConstructorExample2(String s) { this.s = s; } } If no explicit constructor is defined the compiler adds implicitly a constructor. If the class is sub- classed then the constructor of the super class is always implicitly called in this case. In the following example the definition of the constructor without parameters (also known as the empty constructor) is unnecessary. If not specified the compiler would create one. package sumit.java; public class MyConstructorExample { // Unnecessary, would be created by the compiler if left out public MyConstructorExample() { } } The naming conversion for creating a constructor is the following: classname (Parameter p1, ..) {} . Every object is created based on a constructor. This constructor method is the first statement called before anything else can be done with the object.
- 28. Maharishi Markandeshwar University Page - 28 - MODIFIERS Access modifiers There are three access modifiers keywords available in Java. public, protected and private. There are four access levels: public, protected, default and private. They define how the corresponding element is visible to other components. If something is declared public, e.g. classes or methods can be freely created or called by other Java objects. If something is declared private, e.g. a method, it can only be accessed within the class in which it is declared. protected and default are similar. A protected class can be accessed from the package and sub-classes outside the package while a default class can get only accessed via the same package. The following table describes the visibility: Table1. Access Level Modifier Class Package Subclass World Public Y Y Y Y protected Y Y Y N no modifier Y Y N N Private Y N N N Other modifiers final methods: cannot be overwritten in a subclass abstract method: no method body synchronized method: threat safe, can be final and have any access control
- 29. Maharishi Markandeshwar University Page - 29 - native methods: platform dependent code, apply only to methods strictfp: class or method
- 30. Maharishi Markandeshwar University Page - 30 - IMPORT STATEMENTS Usage of import statements In Java you have to access a class always via its full-qualified name, e.g. the package name and the class name. You can add import statements for classes or packages into your class file, which allow you to use the related classes in your code without the package qualifier. Static imports Static import is a feature that allows members (fields and methods) which are defined in a class with the public static access modifier to be used in Java code without specifying the class in which the member is defined. The feature provides a typesafe mechanism to include constants into code without having to reference the class that originally defined the field.
- 31. Maharishi Markandeshwar University Page - 31 - MORE JAVA LANGUAGE CONSTRUCTS Interface Interfaces are contracts for what a class can do but they say nothing about the way in which the class must do it. An interface is a type similar to a class. Like a class an interface defines methods. An interface can have only abstract methods, no concrete methods are allowed. Methods defined in interfaces are by default public and abstract – explicit declaration of these modifiers is optional. Interfaces can have constants which are always implicitly public, static and final. A class can implement an interface. In this case it must provide concrete implementations of the interface methods. If you override a method defined by an interface you can also use the @override annotation. The following code shows an example implementation of an interface and its usage within a class. package sumit.java; public interface MyDefinition { // constant definition String URL="http://www.vogella.com"; // define several method stubs void test(); void write(String s); } package com.vogella.javaintro.base; public class MyClassImplementation implements MyDefinition {
- 32. Maharishi Markandeshwar University Page - 32 - @Override public void test() { // TODO Auto-generated method stub } @Override public void write(String s) { // TODO Auto-generated method stub } } Class methods and class variables Class methods and class variables are associated with the class and not an instance of the class, i.e. objects. To refer to these element you can use the classname and a dot (".") followed by the class method or class variable name. Class methods and class variables are declared with the static keyword. Class methods are also called static methods and class variables are also called static variables or static fields. An example for the usage of a static field is println of the following statement: System.out.println("Hello World"). out is a static field, an object of type PrintStream, and you call the println() method on this object. If you define a static variable the Java runtime environment associates one class variable for a class no matter how many instances (objects) exists. The static variable can therefore be seen as a global variable. The following code demonstrates the usage of static fields.
- 33. Maharishi Markandeshwar University Page - 33 - package com.vogella.javaintro.base; public class MyStaticExample { static String PLACEHOLDER = "TEST"; static void test() { System.out.println("Hello"); } } package com.vogella.javaintro.base; public class Tester { public static void main(String[] args) { System.out.println(MyStaticExample.PLACEHOLDER); MyStaticExample.test(); } } If a variable should be defined as constant, you declare it with the static and the final keyword. The static method runs without any instance of the class, it cannot directly access non- static variables or methods.
- 34. Maharishi Markandeshwar University Page - 34 - ABSTRACT CLASS AND METHODS A class and method can be declared as abstract. An abstract class can not be directly instantiated. If a class has at least one method which only contain the declaration of the method but not the implementation then this class is abstract and can not be instantiated. Sub-classes need then to define the methods except if they are also declared as abstract. If a class contains an abstract method it also needs to get defined with the keyword abstract. The following example shows an abstract class. package sumit.java; public abstract class MyAbstractClass { abstract double returnDouble(); } Working With ClassesTable 2_ What to do How to do it Create a new class called MyNewClass. package test; public class MyNewClass { } Create a new attribute (instance variable) called var1 of type String in the MyNewClass class package test; public class MyNewClass { private String var1; } Create a Constructor for your MyNewClass package test;
- 35. Maharishi Markandeshwar University Page - 35 - What to do How to do it class which has a String parameter and assigns the value of it to the var1 instance variable. public class MyNewClass { private String var1; public MyNewClass(String para1) { var1 = para1; // or this.var1= para1; } } Create a new method called doSomeThing in your class which does not return a value and has no parameters package test; public class MyNewClass { private String var1; public MyNewClass(String para1) { var1 = para1; // or this.var1= para1; } public void doSomeThing() { } } Create a new method called doSomeThing2 in your class which does not return a value and has two parameters, a int and a Person package test; public class MyNewClass {
- 36. Maharishi Markandeshwar University Page - 36 - What to do How to do it private String var1; public MyNewClass(String para1) { var1 = para1; // or this.var1= para1; } public void doSomeThing() { } public void doSomeThing2(int a, Person person) { } } Create a new method called doSomeThing2 in your class which returns an int value and has three parameters, two Strings and a Person package test; public class MyNewClass { private String var1; public MyNewClass(String para1) { var1 = para1; // or this.var1= para1; }
- 37. Maharishi Markandeshwar University Page - 37 - What to do How to do it public void doSomeThing() { } public void doSomeThing2(int a, Person person) { } public int doSomeThing3(String a, String b, Person person) { return 5; // Any value will do for this example } } Create a class called MyOtherClass with two instance variables. One will store a String, the other will store a Dog. Create getter and setter for these variables. package test; public class MyOtherClass { String myvalue; Dog dog; public String getMyvalue() { return myvalue; } public void setMyvalue(String myvalue) { this.myvalue = myvalue;
- 38. Maharishi Markandeshwar University Page - 38 - What to do How to do it } public Dog getDog() { return dog; } public void setDog(Dog dog) { this.dog = dog; } } Working With Local Variable Table 3: What to do How to do it Declare a (local) variable of type string. String variable1; Declare a (local) variable of type string and assign "Test" to it. String variable2 = "Test"; Declare a (local) variable of type Person Person person; Declare a (local) variable of type Person, create a new Object and assign the variable to this object. Person person = new Person(); Declare a array of type String String array[]; Declare a array of type Person and create an array for this variable which can hold 5 Persons. Person array[]= new Person[5]; Assign 5 to the int variable var1 (which was already declared); var1 = 5;
- 39. Maharishi Markandeshwar University Page - 39 - What to do How to do it Assign the existing variable pers2 to the exiting variable pers1; pers1 = pers2; Declare a ArrayList variable which can hold objects of type Person ArrayList<Person> persons; Create a new ArrayList with objects of type Person and assign it to the existing variable persons persons = new ArrayList<Person>(); Declare a ArrayList variable which can hold objects of type Person and create a new Object for it. ArrayList<Person> persons = new ArrayList<Person>();
- 40. Maharishi Markandeshwar University Page - 40 - INTEGRATED DEVELOPMENT ENVIRONMENT The previous chapter explained how to create and compile a Java program on the command line. A Java Integrated Development Environment (IDE) provides lots of ease of use functionality for creating java programs. There are other very powerful IDE's available, for example the Eclipse IDE. . For an introduction on how to use the Eclipse IDE please see Eclipse IDE Tutorial. The remaining description uses the phrase: Create a Java project called... ". This refers to creating a Java project in Eclipse. If you are using a different IDE please follow the required steps in this IDE. Exercises - Creating Java objects and methods Create a Person class and instantiate it Create a new Java project called com.vogella.javastarter.exercises1 and a package with the same name. Create a class called Person. Add three instance variables to it, one for storing the first name of the person, on for storing the last name and one for storing the age of the Person. Use the constructor of the Person object to set the values to some default value. Write a public method called writeName() which uses the System.out.println( method to print the first name of the person to the console. Create a new class called Main with a public static void main(String[] args). In this method create an instance of the Person class.