CSA - Unit 2

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Objects

Objects

• Something being created/ manipulated by the program

• Characterized by it’s state and behavior

• Instance of a class

Object Reference

• A variable that represents an object

Class

• Encapsulates data

• An object is an instance of a class

Public class

• Usable by all client programs (pieces of code outside the class)

  • All classes are public in AP Java subset

Public Methods

• Accessible to all client programs

Private Methods/Variables

• Can only be accessed by methods in the same class

Static Variable (Class Variable)

• Memory allocation only happens once

  • Shared by ALL INSTANCES of the class

Static Final Variable (constant)

• Cannot be changed

• Public

• Single value that applies to the whole class instead of a new instance for each object of the class

  • BankAccount.PENALTY for client

  • PENALTY in own class

Method Header

Public void Withdraw (String password, double amount)

Access Specifier | Return Type | Method Name | Parameter List

• Access specifier = who has access

• Void = method doesn’t return a value

  • Parameters are separated by commas, require data type

• Implementation follows header and is enclosed by {}

Constructor

• Method that creates an object of the class

  • Name is same as class, no return type

No-Argument Constructor

• No parameters

  • Provides initial values for an object

Reference to an Object

• Variable assigned values of the object

Constructor with parameters

• Sets instance variables to values of those parameters

  • They can be changed in different references

Accessor Method

• Public method that accesses a class object without altering it

  • .getBalance

Mutator Method

• Changes state of an object by modifying at least one instance variable

Instance Methods

• Perform operations on individual objects of a class

• Constructors, accessors, mutators

Static Methods

• Perform an operation for the entire class

• Keyword Static in header

  • Can use a static variable in its code

Driver Class

• Main() method is always static

  • Tests other classes

Method Overloading

• 2 or more methods in the same class with the same name but different parameter lists

• Distinguished by method SIGNATURE

  • Can’t have 2 methods with same signature but different return types, so return type is irrelevant

Scope

• Region in which the variable/ method is visible and can be accessed

• A class’s instance + static variables and methods belong to the class’s scope (enclosed by {})

  • DOT OPERATOR NOT NEEDED

Local Variable

• Defined inside a method or statement

  • Scope is from declaration to end of its block

• Precedence over instance variables with same name

Block

• Piece of code enclosed by {}

This Keyword

• Instance methods are called for a particular object, which is an IMPLICIT PARAMETER for the method

  • Referred to with keyword this

Reference Data Types

• Objects and arrays

• String, Random, int[], string [] [], Cat, etc

• STORES MEMORY ADDRESSES

  • If object is changed, any references aliased to it will also change

Primitive Data Types

• Double, int, char, boolean

  • STORES VALUE DIRECTLY

Null Reference

• Uninitialized object variable

• Null pointer

• Doesn’t refer to an object

Formal Parameters

• Placeholders for actual parameters

• String acctPassword, Double amount

Actual Parameters

• Actual values given to a method call

• Match up with formal parameters

Primitive Types as Parameters

• Changes made to parameters of method will not affect the values of the arguments in the calling program

• Basically goes back to what it was before

• Passed by value

Objects as Parameter