week2. Using Objects (Basic Java Concept part1)

◆ Types : a set of values and the operations that can be carried out on the value

- Java has separate types for integers and floating-point numbers

- A value such as 13 or 1.3 that occurs in a Java program is called a number literal

◆ Number Types

- A type defines a set of values and the operations that can be carried out on the values

- Number types are primitive types (most basic, can stand alone / ※ it's NOT object)

- can be combined by arithmetic operators (+,-,*...)

◆ Variables

- Purpose : store a value that you want to see at a later time

- variable has a type, a name and a value

EX) int     width  = 13;

　　type   name    value

※NAME※

- Must be a char

- Cannot be a number

- Cannot use symbol

- _ is allowed but not advisible

- No reserved word

- variables can be used in place if the values that they store

- not match the type -> error to store a value

◆ Identifier : name of a variable, method, or class

- Rules for identifier in Java

* OK : letters, digits, _ (underscore), $ 

* NO : starting with a digit // other symbol (?,%...) // spaces // reserved word

- By convention...

* variable names start with a lower case

* class names start wit/ a upper case

- DO NOT use $ : it is intended for names that are automatically generated by tools

◆ The Assignment Operator, = : used to change the value

◆ Uninitialized Value

- use a variable that has never had a value assigned to it is an ERROR

-> assign a value to the variable before you use it

-> Even better, initialize the variable when you declare it

◆ Objects and Classes

- Object : entity that you can manipulate in your programs (by calling method)

- Each objects belongs to a class

◆ Methods

- Method : Sequence of instructions that accesses the data of object

* you manipulate objects by calling its methods 

- Class : declares the methods that you can apply to its objects

* Class determines legal methods

- Public Interface : specifies what you can do with the objects of a class

 

◆ Overloaded Method : when a class declares two methods with the same name, but different parameters

 

◆ String Methods

- length : counts the number of characters in a string

- toUpperCase : creates another String objects that contains the characters of original string, with lowercase letters converted to uppercase

- When applying a method to an object, make sure method is defined in the appropriate class

 

◆ Parameters : an input to a method

- Implicit parameter : the object on which a method is invoked

- Explicit parameter : all parameters except the implicit

- Not all methods have explicit parameter

 

◆ Constructing Objects

- Construction : the process of creating a new object

ex. Rectangle box = new Rectangle(5,10,20,30) ~ 5,10,20,30 is construction parameters

 

◆ Accessor and Mutator Methods

- Accessor method : does not change the state of its implicit parameter

- Mutator method : changes the state of its implicit parameter

 

◆ The API Documentation

- API : Application Programming Interface

- API documentation : lists classes and methods in the Java library.

-http://java.sun.com/javase/7/docs/api/index.html

 

◆ Packages

- Package : a collection of classes with a related purpose

- Import library classes by spectifying the package and class name

 

◆ Object Reference : describes the location of an object

- The new operator returns a reference to a new object

- Multiple object variables can refer to the same object

Rectangle box = new Rectangle(5,10,20,30);

Rectangle box2 = box;

- Primitive type variables ≠ object variables