Creating Instances

1. Creating Instances

2. 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.

3. 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”

4. 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.

5. Data (Fields) Data (Fields) -dblLength = 3 -dblWidth = 7 -dblLength = 5 -dblWidth = 10 • Ex: ---------------------------------- ---------------------------------- -setLength( ) -setLength( ) -setWidth( ) -setWidth( ) -getLength( ) -getLength( ) -getWidth( ) -getWidth( ) -calcArea( ) -calcArea( ) Methods That Operate on the Data Methods That Operate on the Data box2 - This variable holds the memory address of this Rectangle object box1 - This variable holds the memory address of this Rectangle object

6. 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.

7. 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.

8. 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?

9. 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.

10. For example, if you see this: public static void main( ) • There can only be ONE instance of the main method. It is static. Main ------------------ Main ( )

11. If you see this: public void getLength( ) • There can be multiple instances of the getLength method. It is NOT static. box1 ------------------ getLength ( ) getWidth( ) box2 ------------------ getLength ( ) getWidth( ) box3 ------------------ getLength ( ) getWidth( )