Conventions in Java

• Classes start with a capital letter

• Variable names start with a lower case letter

Terminology

• Class and object

• Instance variable

• Methods

• Mutator and accessor methods

• Arguments for a method

• Return type for a method

• Constructors

Classes and Objects

• When we write a class, it is a template for the information we want to keep about an object

• Once we create an object, the mutator methods are one way to store information with the object

• Another way to get information in to the object is to use a constructor with arguments.

• The constructor puts information in the object at the same time it creates it

Constructors for SimpleGradeBook

• The default constructor has no arguments

– If you don’t write a constructor, Java furnishes this automatically

– Whenyou do write the default constructor, the programmer decides what the instant variables should be initially

– Write a default constructor for SimpleGradeBook on the whiteboard:
Two instance variables
String name; double grade;

Constructors with arguments

• Since SimpleGradeBook has two instance variables, it is common to write another constructor with two arguments

– We want to give both instance variables an initial value

– This initial value can be changed by a set method.

• The type of the arguments must be the same as the type of the instance variables

– But the variable names should be different

• Write a two argument constructor for SimpleGradeBook on the whiteboard

Calling the constructors

• A constructor is really a method that is called automatically by the new operator

• It is called in the class with the main( ) method

• First, we will “hard code” the arguments
SimpleGradeBook firstObj = new SimpleGradeBook(“Fred”, 89);

• A better way is to get input from user and store in variables

Scanner in = new Scanner(System.in);
String str = in.next();
double num = nextDouble();

• Then use those variables as the arguments
SimpleGradeBook secObj = new SimpleGradeBook(str, num);

The toString method

• Java likes to make it easy to treat your class objects just like an int or String

• i.e. just use System.out.println(object) to print them

• The toString method is a way for the programmer to easily output an entire object

• public String toString( )
{ /* build a string to output object
usually this is labels for the instance
variables, then the variables themselves,
concatenated together */

• Write a toString( ) method on the whiteboard

• Look at SimpleGradeBook class and test in BlueJ

Out-Lab 2

• This is a one week lab

• It is not much different than the in-lab.

Floating-Point Numbers

• Floating point numbers are those numbers with a fractional part

– i.e. have a decimal point

• They are stored in the computer differently than integers

– Floating point calculations are much slower than integer calculations

• Java has two floating point types

– float

– double—take twice as many bits to store the number as floats

• This makes them twice as accurate.

• There are almost always rounding errors when using floating point numbers

Basic or Primitive types

Four integers type

byte -- 8 bit signed integral value

short -- 16-bit signed

int-- 32 bit signed (most commonly used)

long -- 64 bits

Floats

float – 32 bits (6 to 7 significant decimal digits , exponent to 38)

double – 64 bits (15 significant decimal digits, exponent to 308)

boolean - can only be true or false

char --stored in 16 bit ASCII code

– Java does not use 8-bit ASCII code