Programming with Java

Transcription

1 Programming with Java Variables and Output Statement Lecture 03 First stage Software Engineering Dep. Saman M. Omer

2 Objectives ü Declare and assign values to variable ü How to use eclipse ü What is the steps for writing a program ü The rules of programming language ü How to start writing a program in java ü How data will be printed ü Join messages in output commands by using the concatenation (+) operator

3 Outlines Ø What is a Variable? Ø Declaring a Variable Ø Assignment statement Ø Eclipse Ø Programming Languages and Programming in Java Ø A Java program Ø Output statement Ø Examples

4 The computer's memory q Think of the computer like a calculator for a moment. o We have already seen how to calculate values. q A flexible calculator has "memory" keys to store and retrieve a computed value. o In what situation(s) is this useful? q How can we save and restore a value that our Java program previously calculated?

5 What is a Variable? q Variable: A piece of your computer's memory that is given a name and type, and can store a value. q A variable is a placeholder used for storing data. q Data may be numeric, or string of characters. q The data inside variable called Value, it can be change or delete. q Named location that stores a value of one particular type

6 Declaring a Variable q Syntax: datatype variablename; q Example: ² int score ; ² int book ; q Names of variables should be meaningful and reflect the data they will store o This makes the logic of the program clear q score and book, are two variables of type integer, they may hold values like 1, 50, 200. q note that we use int type, because score or book, both are measured in integer numbers, (i.e, there is no 3.6 books!!!!!)

7 Declaring a Variable v For values that contain fraction parts like 3.5, or 7.21, we use type double: ² double weight; ² double w; ² double x; v Note: all java statements must be ended with a semicolon. (;) v Think of variables as an empty box that you can put values in. v Example: int book ; v This statement will reserve a location in computers memory. Name: book Type : int

8 Java Variable Naming rules Java variable names are case sensitive. The variable name money is not the same as Money or MONEY. Characters allowed to be used in names are: 1 All English Capital Letters (Upper Case Letters ) 2 All English Small Letters (Lower Case Letters) 3 Underscore ( _ ) 4 Currency symbols ( $,, ) 5 Digits( 0, 1, 2,,8,9). Variable name must begin with either a letter or the dollar sign "$", or the underscore character "_". Digits are not allowed to be used at the beginning of the name, Spaces are also not allowed, Java keywords are not allowed to be used as variable name.

9 Assignment statement q Assignments allow values to be put into variables. q Simple assignment: q datatype variablename = value ; q Examples: 1. int score = 7 ; Name: score Type : int Value : 7 2. int x,y ; x = 3 ; y = 5 ; Name: y Type : int Value : 5 Name: x Type : int Value : 3

10 Assignment statement The variable must be declared before use.

11 Eclipse How to create a project and classes for writing a program In

12 ² To create a project: File à New à Java Project ² Enter a name for the project, then click Finish

13 ² To create a class, click on the File à New à Class ² Enter a name then finish

14 Programming Languages q Programming languages have a set of rules : o Syntax: refers to the structure (or form) of code that a programming language specifies. If a program contains the syntax error that will not be compiled. ² Things that can cause a compiler error are incorrect keywords, missing brackets, semicolons and many others. o Semantics: refer to the meaning of the program, symbols, characters and words are arranged with that structure. ² The most common semantic error is one in which the code uses a variable that isn t initialized properly.

15 Programming in Java q To create a working program in Java, you need to three steps. : Ø Create a program by typing it into a file named, say, MyCode.java. Ø Compile the program which is translate a source language into a machine language (0 and 1). Ø Run (or execute) the program.

16 A Java program public class CLASS-NAME { public static void main (String args[]) { // your program goes here } // end of main } // end of class

17 Hello world program Here is the Hello world program in Java. public class Hello { public static void main (String args[]) { System.out.println( Hello world! ); } // end of main } // end of class Output Hello word

18 Analyses the program q That piece of program is a part of a complete program. q public class CLASS-NAME o o Java is referred to as an Object-Oriented Programming(OOP) language. The OOP languages require the program to be written in separate unit called classes. q All statements, and braces are needed to be written inside a class (any name) and a method called main. q public static void main (String args[]) o The main method is the method that's defined to be called at the start of an application. Without it, there is no place to start running. q What is method, what is class, both of them will be explained in next lectures.

19 Output statement q This is the way, always stuffs will printed on a sample text screen. ² System.out.println(variable-name); prints the value of variable <variable-name> to the user ² System.out.println( any message ); prints the message within quotes to the user ² System.out.println( hello + world + a + plus + b); assuming the value of a is 3 and b is 7, it prints helloworld3plus7 q println( ) is used for breaking current line, and starting a new one. q method print ( ) is used for printing any string or variable, directly after last print. q \n read as Slash n, can be used with method print( ), and work exactly as prinln ( ) method.

20 Example 1 public class Hello { public static void main (String args[]) { System.out.println( This is my first Java program! ); System.out.println( I like Java. ); System.out.println( I think Java is cool. ); } // end of main } // end of class Output This is my first Java program! I like Java. I think Java is cool. Ø Exercise: change the println to print. Lets see what is the difference?

21 Example 2 public class MyFirstJavaProgram { public static void main (String args[]) { int myage, myfriendage; myage = 20; myfriendage = myage + 1; System.out.println( Hello, \n I am + myage + years old,\n and \n my friend is + myfriendage + years old ); System.out.println( Goodbye ); } // end of main } // end of class Output Hello, I am 20 years old, and my friend is 21 years old Goodbye

22 Example 3 q What will be output from the following program?

23 The next lecture will explain an Input statement, Identifiers, Naming rules and Data Types

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