Working With Objects Tonga Institute of Higher Education

Introduction The building block of an object-oriented language is an object. Object - A self-contained entity that contains data and procedures to manipulate the data. An object is like a tool that we can use to do things. In Java, almost everything is an object.  String  Integer  System  PrintStream  Etc. Java has over 2,700 pre-built objects for us to use. You can find a list of pre-built objects in the Java Developer’s Kit documentation.

Advantages of Object Oriented Programming Code is easier to maintain  Code is more readable  Encapsulation Show what the object can do  This is normally what we want to know Hide how the object does it  This can be very complicated  We often don’t care how it is done Code is easier to re-use  A piece of code, once written, should not be thrown away. It is best to re-use the code in other programs. Example: Millions of people use System.out.println(). But it was only written once. Code development is more efficient  You don’t code the same thing over and over again

Advantage of Encapsulation Show what the object can do.  This is accomplished by exposing the signature. Signature – The combination of method name and parameters used to uniquely identify a method. Hide the code that the object uses.  The person using the object doesn’t know how it works. Therefore, we can change the code in a method and don’t need to update the programs using it if we don’t change the signature. If others are using your signature, do not change it!  If you do, you will cause everybody using your object to crash. class Math { public double getPI() { return 3.14; } class Math { public double getPI() { return ; }

Objects Objects are like primitive data types with extra functionality. Generally, use them like primitive data types. Except:  Capitalize the first letter of an object data type Ex: byte vs. String  They have methods that provide extra functionality Look at Java documentation to find functionality. Examples:  String  String.charAt(int index)  String.compareTo(String anotherString)  Date  Date.getHours()  Character  Character.compareTo(Character anotherCharacter)  They have constructors that let you create them. Primitive data types can have object wrappers.  Ex: The object wrapper for the int primitive data type is Integer.

Classes vs. Objects Object - A self-contained entity that contains data and procedures to manipulate the data. Class - The blue print or design for creating an object. Instantiate – The act of creating an object from a class Instance – An instantiated class/object

Using Object Variables 2 Steps to using variables 1. Declare the variable 2. Instantiate the variable / Initialize the variable

Declaring Object Variables – 1 Declare the variable – Tell the computer to reserve a space in memory for the variable. You need to tell the computer 2 things: 1. Name of the variable 2. Type of the variable (What kind of variable you have) Object types  Integer  String This works exactly the same for primitive variables! Type Name

Declaring Object Variables – 2 Use a name that is easy to remember.  Do not use x, y, z Variable names must start with a letter, underscore or dollar sign. Variables should begin with a lowercase character. Then a capital letter for each next word. Examples  firstName  customerID This works exactly the same for primitive variables!

Instantiating Object Variables / Initializing Object Variables Instantiate – The act of creating an object from a class  Use the new keyword Initialize the variable – Assign an initial value to a variable.  A newly instantiated object can be used to initialize a variable  Char values must be enclosed in single quotes.  String values must be enclosed in double quotes. New KeywordType of Object Parameters may not be required

Declaring and Initializing Object Variables in 1 line You can declare and initialize a variable in 1 line.

Strings are Special They are objects, but you can use them like primitives Is the same as

Demonstration Declaring, Instantiating and Initializing Variables

Constructors Constructor – A method that is automatically executed when an object is created.  This allows you to set initial values for the object.  Many objects have multiple constructors. (They are overloaded) You can find a list of constructors in the Java Developer’s Kit documentation.

Demonstration Constructors

Attributes / Fields Attributes / Fields – A variable that a class allows others to see  Use dot notation to access it Example:. Example: JOptionPane.INFORMATION_MESSAGE  You can find a list of fields in the Java Developer’s Kit documentation.

Demonstration Attributes / Fields

Methods Methods - Pieces of code that perform a single function  Use dot notation to access it Example:. ( )  You can find a list of methods in the Java Developer’s Kit documentation. Calling a Method – The act of using a method We’ve already used a lot of methods:  println(…)  main(…)  intValue(…)

Method Inputs Some methods take inputs  Parameter/Arguments – A piece of information that provides additional information to the method as to how it should behave.  Parameters should be in the parenthesis next to the method name  The order they are passed is important  Values are separated by commas  Even if you aren’t passing any parameters, you still need to use () Example: Integer.intValue() Method InputOutput Input InformationMethod Name

Method Outputs Some methods return outputs  When something is returned, it may or may not be used. The programmer chooses what to do with the data returned.  Only one thing may be returned.  Void means nothing is coming back Function – A method that returns a value Method InputOutput Information

Demonstration Methods

Method Overloading If two methods do the same thing, they should have the same name Overloading - Having multiple methods with the same name but different parameters Java determines the correct method to use by matching up the number and type of arguments. Therefore, you can’t have 2 methods with the same name and same number & type of arguments. Without overloading, we would have to remember more function names. That would make code more complicated.

Demonstration Method Overloading

Members Member – An attribute/field or method. Sometimes used to refer to attributes/fields and methods as a whole.