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2.3 Creating and Storing Objects

Unit 2 – Using Objects

AP Computer Science A

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2.3 Creating and Storing Objects

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Learning Objectives

You will be able to…

  • create class diagrams to represent the attributes and behavior of a class.
  • use a constructor to create an instance of a class.

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Activity - Java Zoo

  • You are going to create an Animal class..
    • choose the animal for this class
    • must have at least 4 attributes
      • your attributes need to have a data type
        • string, boolean, int, double...
    • must have at least 4 behaviors
      • name them using camelCasing

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Class Diagram

A class diagram is an organized template that allows you to preview the contents of class, such as the attributes and methods.

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Mr. Potato Head Blueprint

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Characteristics:

Number of eyes

Color of eyes

Color of Hat

Number of Arms

Facial hair

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Behaviors:

Talk

Move

Yell

Change color of nose

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Class Diagrams

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Mr. Potato Head Blueprint

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Characteristics:

Number of eyes

Color of eyes

Color of Hat

Number of Arms

Facial hair

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Behaviors:

Talk

Move

Yell

Change color of nose

iPod blueprint

attributes:� current song� volume� battery life

behavior:� power on/off� change station/song� change volume� choose random song

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Creating a New Object

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Turtle mertle = new Turtle(habitat)

<Class> <variable name> = new <Class>(argument)

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  • This is what is referred to as a constructor - a statement that constructs the object from the class.

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Constructors

A constructor is a special method (function) used to create an instance of a class

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  • The constructor of a class is a method that allows us to initialize the attributes(variables) of an object when it is first created.
  • Constructors always have the same name as the class and are used with the keyword new.
  • An object variable is created using the keyword new followed by a call to a constructor.

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Constructors

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  • The constructor with nothing placed in the parenthesis may set certain attributes to a default value.

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  • creates a world with a default size
  • this size is 640 by 480
  • the Turtle class needs something in the parenthesis!
  • will not work otherwise….

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Arguments

An argument is a value that is passed into a method or constructor.

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  • An argument passed into a constructor allows us to choose the values of particular attributes for that object...

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  • This can always be changed later...

Animal tiger = new Animal(“orange”);

//create animal with orange fur

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Arguments

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  • A constructor with values placed make it easier to construct objects with particular attributes.

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creates a World object with length 100 and height 300.

creates a Turtle object on the world called w with a default location (required).

creates a Turtle object on the world called w with a location (200,100).

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Overloaded Constructors

A constructor is said to be overloaded if there are multiple constructors with the same name but different arguments can be placed in the parenthesis.

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Activity Cont. - Java Zoo

  • Create three different constructors for your animal that you made earlier
    • label what should go in the parenthesis with the variables you used for your attributes

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Class Diagram

  • Constructors also have a place in the class diagram.

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Turtle

Turtle(World w)

Turtle(World w, int xpos, int ypos)

  • String name
  • Color bodyColor
  • int width
  • int height
  • int xpos
  • int ypos
  • forward()
  • backward()
  • turnLeft()
  • turnRight()
  • penUp()
  • penDown()

Class Name

Constructors

Attributes

Methods

2.3 Creating and Storing Objects