PIC20A Lect2

PIC20A Lecture 2: A simple program

In the section, we will learn how to

write a simple application
use comments
run a java program
print out message on screen

You must read the textbook p32-36 or A Closer Look at HelloWorld.

 1. // name this file HelloWorld.java  
 2.  
 3. /*  Usage :  
 4. type java HelloWorld  
 5. at Dos Command prompt */  
 6.   
 7. /**  Written by Charles Li */  
 8.   
 9. public class HelloWorld { 
10.      public static void main(String[] args) { 
11.             System.out.println("Hello World !!!"); 
12.       } 
13.}

Line 1
// name this file HelloWorld.java
Explanation:
Same as C++, words after // are comments. Every words on the same line after // will be ignored by the compiler
Syntax :
//your comments

Line 3-5
/*.... */
Explanation :
As in C++, all the words between /* and */ will be ignored. /* and */ can be on one single line or on different lines.
Syntax :
/* your comment */
or

         
/* comment 1 
   comment 2 
   comment 3 */

Line 7
/**....*/
Explanation:
This is very simple to /* comments */. Everything between /** and */ will be ignored. Again, the comments can be on either one single line or on multiple lines. The difference between the these 2 types of comments is that the comment between the /** and */ will appear in the documentation generated by the Java tools (javadoc, you will learn this later).
Syntax :
/** your comment */
or

/** comment 1 
    comment 2 
    comment 3 */

Line 9
public class HelloWorld
Explanation:
HelloWorld is the name of the class. You must name the file HelloWorld.java. This is the only possible name of this file.
Syntax :

public class classname {

     .........

}

Line 10
public static void main(String[] args)
Explanation:

Similar to C++, main function is the entry point of the program. The body of main function is the starting point of the program. Codes inside the program will be executed.
You can run the program by java classname
Not every class has a main function. If you want to run the program by java someclass , then you should implements the main function.

Syntax :

public static void main(String[] args) {
        // your codes 
}

Detail Explanation for PIC10B students(don't worry if you don't understand it now)

public indicates the method main is a public method
static indicates the method is a class method.
void is the return type
main is the method name
String[] args means that the argument is an array of String named args.

Line 11.
System.out.println("Hello World !!!");
Explanation:

System.out.println("Hello World !!!") prints Hello World !!! at the command prompt and ends with a newline.
The line is same as
```
cout << "Hello World !!!\n"; 
```
in C++. (The line is different from cout << "Hello World!!!").
Same as C++, every line ends with a semicolon.

Syntax :

     System.out.println("print something with a return at the end"); 


     System.out.print("print something without return");

Detail Explanation for PIC10B students(don't worry if you don't understand it now)

System is a class in java.
out is a class member of the class System.
println(or print) is a member function of out.
The functions have one argument (String).

/* Usage :
type java HelloWho Charles
at Dos Command prompt
The output will be
Hello Charles
*/

public class HelloWho {
public static void main(String[] args) {
System.out.println("Hello " + args[0] );
}
}

Explanation:

args is an array of String
args[0] is the first string after java HelloWho.
If you type java HelloWho Charles Li then args[0] is Charles, args[1] is Li.
If you type java HelloWho "Charles Li", then arg[0] is Charles Li (the whole word).

Question :
What is the output of

java HelloWho Amy
java HelloWho Amy Betty
java HelloWho "Amy Betty"

System.out.println

You can add 2 Strings together by an operator +

Example

System.out.println("Hello " + "Charles");

is same as

cout << "Hello " << "Charles" << endl;

in C++.

You can also print out strings with numbers

Example

int num = 100;
System.out.println("There are " + num + " students.");

is same as

int num = 100;
cout << "There are " << num << " students." << endl ;

Summary

After this lecture, you should know

3 types of comments in Java . (a) // (b) /* comments */ (c) /** comments */
How to write a program with a main function. And how to execute the program.
System.out.println
What is the meaning of args[0], args[1]....