Objektorienteeritud programmeerimine (Narva Kolledž) 2017/18 kevad

In this session, we are going to use Notepad and Terminal.

1. Compile and run the program in the command line

When you start writing a piece of code in Java in any editor (e.g. Notepad, Gedit, etc.), the program looks like a simple text file. Unlike Python, the program cannot be executed at once. In Java, the program has to written, complied and after that executed.

1. Create a folder to keep the files (e.g., "c:\myProject", or "d:\myProject").

2. Launch a programming text editor (e.g. NotePad, or NotePad++, or TextPad, DO NOT use MS Word) and enter the source code:

 /*
  * First Java program to say Hello
  */
 public class HelloWorld {   // Save as "HelloWorld.java" under "d:\myProject"
    public static void main(String[] args) {
      System.out.println("Hello, world!");
    }
 }

3. Save the file as HelloWorld.java into your folder (e.g. d:\myProject).

4. Start a cmd: click the "Start" button ⇒ "run..." ⇒ Enter "cmd").

5. Set the Current Drive to the drive where you have saved your source file HelloWorld.java. (e.g. if the source file is saved in drive d, enter d: as follow:

 C:> d:
 D:>

6. Set the Current Working Directory to the directory with the source file via the cd (Change Directory) command. (e.g. if the source file is saved into d:\myProject:

 D:> cd \myProject
 D:\myProject>

7. Issue a dir (List Directory) command to confirm that the source file is present in the current directory:

 D:\myProject> dir
 ......
 xx-xxx-xx  06:25 PM               277 HelloWorld.java
 ......

8. Invoke the JDK compiler javac to compile the source code HelloWorld.java.

The compilation is successful if the command prompt returns. Otherwise, error messages would be shown.

9. The output of the compilation is a Java class called HelloWorld.class. Issue a dir command again to check for the output.

 D:\myProject> dir
 ......
 xx-xxx-xx  01:53 PM               416 HelloWorld.class
 xx-xxx-xx  06:25 PM               277 HelloWorld.java
......

10. To run the program, invoke the Java Runtime java:

 D:\myProject> java HelloWorld
 Hello, world!

Conclusion: Computers do not directly understand Java. We need a compiler between the source code and the computer. When we are programming using the command line interface, the command javac HelloWorld.java will compile the HelloWorld.java file into bytecode, which can be executed using the Java interpreter. To run the compiled program, you can use the command java HelloWorld where HelloWorld is the name of the original source code file.

2. Change the first program

1. Add one more statement System.out.println("Hello, World!") into the program.
2. Change the output text.
3. Compile the program in cmd and run it.
4. Delete the last two characters ln in the first statement System.out.println. What has changed in the output?

3. Kitty

Write a program that prints a kitty speaking a greeting, similar to (but different from) the following:

 /\_/\     -----
( ‘ ’ )  / Hello, \
(  -  ) <  Junior  |
 | | |   \ Coder! / 
 (_|_)     -----

4. A random number

Study the following example:

 double randomDouble = Math.random()*5+15;
 long roundedRandomLong = Math.round(Math.random()*5+15);
 int invalidInt = (int)Math.random()*5+15;

What is the output of the statements? What data type are the outputs?

Write a program that generates two random integers in the range of 21 and 90 (not inclusive), and prints out their fraction accurate to third decimal place.