IT 114: Introduction to Java

IT 114: Introduction to Java
Class 4

Topics

Homework 2

Homework 2 is posted here.

I want you to write a Java program that will draw a rectangle.

Be sure to read the specification carefully and draw a rectangle of exactly the right size.

If the rectangle is not the right size you will lose points.

It is due this coming Sunday, at 11:59 PM.

Announcements

The first graded quiz will be given next week.

The questions will be taken from Class Quizzes 2 - 6.

Tips and Examples

Fixing Problems with Accounts

If you have problems with your Unix account you need to speak to the CS Operators in the Unix Lab
See this page for details

Review

Structure of Java Programs

All Java programs must have the following format


public class CLASS_NAME {
    public static void main(String[] args) {
        ...
    }
}

Copying A File in Unix

The Unix command to copy a file is cp which stands for "copy"
It has the following format
```
cp  ORIGINAL_FILE_NAME  NEW_FILE_NAME
```

Using `nano` in This Class

There are two types of assignments in this class
- Class Exercises
- Homework Assignments
In each case you will make a copy of the template file xxx.java to your current directory
Then you will use nano to change the file
To run nano use the following format
```
nano  FILE_NAME
```
You can use nano to create a new source code file
Or to change a file that already exists

Compiling the Source Code

All source code files need to be compiled
You do this by running javac the Java compiler
```
javac Ex3.java
```
If the compilation works you should see two files in your directory
```
$ ls
Ex3.class  Ex3.java
```
Ex3.class is the object file
It contains bytecodes that the Java interpreter understands

Running a Java Program

To run your program you need to use the Java interpreter
The name of this program is java
You need to tell it which object file to run
But you don't use the full name of the file
Instead you give it the filename without the .class extension
```
$ java Ex3
Hello world!
```

New Material

Why You Do Java Program Have Their Unusual Structure?

All Java program have the following structure


public class CLASS_NAME {
    public static void main(String[] args) {
        ...
    }
}

Why?
To understand this we have to talk about two important ideas
- Objects
- Classes

Objects

Java is an object oriented language
But what are objects
An object is a section of a computer's RAM
An object can hold many values
Along with code that can work with these values
The values are usually connected in some way
For example a car object might hold the following values
- Make
- Model
- Manufacturer
- Year
- Vehicle Identification Number (VIN)
There is no limit to the number of values an object can contain
Objects only exist when the program that created them is running

Classes

Objects have to be defines before you can create them
The definition are a type of software called a class
Classes are software written in Java
Objects are the contents of RAM created from a class when a Java program is run
There is only one class for each type of object
A single class can create many objects
A class is a template or blueprint for an object
Think of the molds that children use with clay
Let's say they are molds of dinosaurs
And each mold is of a different dinosaur
A child can use one mold to stamp out many copies of that dinosaur
A class is like the mold
And the clay dinosaurs are like the objects

Statements

A Java program consists of a number of statements
A statement is a line or lines of code that represent a complete action
There are two kinds of statement
- Simple statements
- Compound statements
A simple statement can be written on a single line and contains no other statements
A compound statement contains other statements within it
All statements must end with a semicolon, ;
If you don't you will get an error when you compile it

Printing a Line in Java

The first Java statement you need to learn prints a line of text
It has the following form
```
System.out.println(STRING);
```
This may seem like a lot of characters to perform a simple action
The statement looks like this due to Java's object oriented design
Don't worry about why the extra stuff is needed
Just memorize it like some sort of magical incantation

If you leave out System.out, you will get an error message

$ cat BadHello.java
public class BadHello {
    public static void main(String[ ] args)  {
        println("Hello,  world!") ;
    } 
}

$ javac BadHello.java
    BadHello.java:3: cannot find symbol
    symbol  : method println(java.lang.String)
    location: class BadHello
        println("Hello,  world!") ;
        ^
    1 error

The javac compiler objects because it does not know println
it only knows System.out.println
System.out.println statement prints whatever is inside the parentheses
And then advances to the next line
If the parentheses are empty it simply prints a blank line

Data Types

The basic job of any program is to create and manipulate data
And data comes in many forms
Each of these forms must be represented in RAM in the most efficient way possible
To do this, each computer language has different data types
Each of these data types specifies how a value is to be represented in RAM
Some languages, like Python and Perl, are very easy going with respect to data types
But Java is not one of them
Java is a type-safe language
Another term for this type of language is strongly typed
In a type-safe language, you have to assign a data type to each value
The data type of a value determines what you can do with it
For example, the following is illegal
```
'c'/'e'
```
because this is dividing one character by another

The Java Data Types

There are 8 data types in Java
But we will only use 4 of them

int integer

double decimal

bool boolean

char character
int values are the counting numbers
They can be positive or negative
```
-3, -2, -1, 0, 1, 2, 3
```
double values can also be positive or negative
```
8.75, -.7654, 5.1836456
```
The only two values a bool can have are true and false
char values are a single character
They must be enclosed in single quotes
```
'a' 'q' '5' '@'
```
Notice that a char value does not have to be a letter

`int`	integer
`double`	decimal
`bool`	boolean
`char`	character

Topics

Tips and Examples

Review

New Material

Homework 2

Announcements

Tips and Examples

Fixing Problems with Accounts

Review

Structure of Java Programs

Copying A File in Unix

Using nano in This Class

Compiling the Source Code

Running a Java Program

New Material

Why You Do Java Program Have Their Unusual Structure?

Objects

Classes

Statements

Printing a Line in Java

Data Types

The Java Data Types

Class Quiz

Attendance

Class Exercise

Using `nano` in This Class