Creating Instances

• Think of a class as a blueprint.

• The blueprint isn’t the house. It’s the plans for a house.

• From that blueprint (or class) you can make many instances(objects) of that house.

• Let’s say we have our Rectangle class.

• We can create multiple “instances” of that class like this:

Rectangle box1 = new Rectangle(); • Now we have a Rectangle object named “box1”

Rectangle box2 = new Rectangle();• Now we have a Rectangle object named “box2”

• Each Rectangle object (or instance) has access to the Rectangle class’ procedures and data fields.

• However, each Rectangle object has its own INSTANCE of those procedures and data fields.

• Ex:Data (Fields)

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Methods That Operate on the Data

-dblLength = 5

-dblWidth = 10

-setLength( )

-setWidth( )

-getLength( )

-getWidth( )

-calcArea( )

Data (Fields)

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Methods That Operate on the Data

-dblLength = 3

-dblWidth = 7

-setLength( )

-setWidth( )

-getLength( )

-getWidth( )

-calcArea( )

box1 box2- This variable holds the memory address of this Rectangle object

- This variable holds the memory address of this Rectangle object

• Instance Variables:

– Also known as instance fields.

– The variables in each separate instance of the class.

– For example, each instance of the Rectangle class has its own dblLength and dblWidth variables.

• Instance Methods:

– Also known as instance procedures.

– The methods in each separate instance of the class.

– For Example:• box1 This instance has its own setLength method.• box2 This instance has its own setLength method.

– Instance methods do not have the keyword “static” in their method headers.

• Main methods usually are written like this:public static void main( )

• Methods in other classes are written like this:

public void getLength( )

Why is static there?

Why do we not put static there?

• Static:

– A header declaration that tells Java whether or not multiple instances of a variable or method are allowed.

– If a data member(variables and methods) is static, that means that it is the only ONE.

– There can not be multiple instances of something if it is static.

– For example, if you see this:

public static void main( )

– There can only be ONE instance of the main method. It is static.

Main

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Main ( )

– If you see this:

public void getLength( )

– There can be multiple instances of the getLength method. It is NOT static.

box2

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getLength ( )

getWidth( )

box3

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getLength ( )

getWidth( )

box1

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getLength ( )

getWidth( )