Week 1 First Program.pdf

Programming in Java Your first program Hello World Analysis of “Hello World” program • The first line looks like this: public class Hello • The first and most important thing to pay attention to is the word class • A Java program is a class. Every class has a name and the convention is that a cl...

Programming in Java Your first program Hello World Analysis of “Hello World” program • The first line looks like this: public class Hello • The first and most important thing to pay attention to is the word class • A Java program is a class. Every class has a name and the convention is that a class name starts with a capital letter • The simple programs that we are starting with will only contain one class – in this example we have called our class Hello Analysis of “Hello World” program • The first line therefore tells the Java compiler that we are writing a class called Hello • Notice that everything in the class has to be contained between two curly brackets { } - these tell the compiler where the class begins and ends • Note: Every opening curly bracket must have an associated closing bracket Analysis of “Hello World”program Analysis of “Hello World” program • The next line after the curly bracket opening is: Analysis of “Hello World” program • Every program we write will always contain this line • Java applications must have a method called main, which is where execution of the program starts • A method contains a collection of programming instructions that describe how to carry out a particular task • Public makes the application accessible from the outside, e.g. MS-DOS Analysis of “Hello World” program • The rest of this line is beyond your understanding as of yet; all we need to know is that it must be included for the program to compile Analysis of “Hello World” program • A program starts with the first instruction of main, then obeys each instruction in sequence (unless the instruction tells it to jump to some other place in the program) • The program terminates when it has finished obeying the final instruction of main Analysis of “Hello World” program Analysis of “Hello World” program • This now brings us to the important part the line of code that represent our instruction display “Hello World” System.out.print(“Hello World”); • To get anything printed to the screen we use System.out.print and put whatever we want to be displayed in the brackets • For example System.out.print(“Computing”) would print Computing to screen Analysis of “Hello World” program Analysis of “Hello World” program • A sequence of characters enclosed in double quotes as above is known as a String Analysis of “Hello World” program • We can also use another print statement which is println • println is short for print line and the effect of using this statement compared to print is to start a new line after displaying whatever is in the brackets • System.out.print("Hello World"); would print Hello World on screen and then move the cursor to the next line Analysis of “Hello World” program System.out.println("Hello"); System.out.println("World"); Would be displayed on screen as: Hello World Analysis of “Hello World” program System.out.print("Hello"); System.out.print("World"); Would be displayed on screen as: HelloWorld Analysis of “Hello World” program Analysis of “Hello World” program • Note the semi-colon (;) at the end of the print statement • We must include a semi-colon at the end of every instruction we write in Java to indicate that it is a statement • Also note the way the code is indented in this program – this is done to make the code easier to read Errors • Compile time error (syntax error) – Violation of the programming language rules – Detected by the compiler – System.ou.println("Hello"); The error here is a missing ‘t’ in the word out Errors • Run time error (logic error) – Causes program to perform action programmer did not intend – System.out.println("Hello Word"); Program will still compile and run but will print Hello Word instead of Hello World Comments • Comments are lines written by the programmer that convey some type of information concerning the code such as filenames, titles or explanations of what the code is doing • The compiler ignores all comment lines in a program • Comment lines may be placed anywhere in the code Comments • There are two ways to write comments in Java • Comments can start with double forward slashes (//). These are especially convenient for one line comments • A second way to write a comment is by using /* and */ symbols. This comment style is convenient for writing comments with multiple lines because it only needs the /* at the start and the */ at the end of the comment lines Comments // style • Useful for one line comment Comments /* */ style • Useful for block of comments Methods • We have seen that the Hello World program contains a main method • A method is a block of code that performs certain tasks and can return information if necessary • When we refer to a method in Java we place () beside it to indicate that it is a method, eg. main() • Methods allow the programmer to write discrete blocks of code in a logical manner Saving programs • When saving programs in Java one very important fact is that the code file name must be the same name as the class name with a .java extension • When saving the program Hello we must save it as Hello.java This name must match the class name Extension for saving program as Java program Running the program • To test if the program will work properly and is error free we must first compile it • Go to Build on Menu Bar and select Compile or click the green cross • If your program is error free it should compile successfully • If there are errors then messages will be displayed to tell you what these errors are Program compiled successfully Program with Errors Error shown Running the Program • Once your program has compiled successfully go back to the Build menu and select Run or click red run symbol • The output of your program should display in the window at the bottom