State and Behavior
Recall the blueprint (class) for creating Sphere objects.
public class Sphere {
public double radius;
public double getVolume() {
double vol = 4.0 / 3.0 * 3.14159 * radius * radius * radius;
return vol;
}
}
Each object has state and behavior.
state¶
Data each object has; what the object knows about itself.
behavior¶
Methods we can call on an object; what the object does.
The variable radius is an example of an instance variable or field.
instance variable¶
A variable defined inside a class but outside of any method definition. Each object gets its own copy of the
class's fields.
Every instance of a class gets its own copy of variables (fields).
The code below creates three Sphere objects and sets each object's field to a unique value.
We access an object's instance variable by using its reference variable with the . operator, using the form reference.instanceVariableName.
We call methods using the reference and . operator, but with a method call.
* This way we are telling a certain object to do something.
Drill¶
IntroToObjects/src/drills/SphereWithColor.java
* Add a String field for color.
* Add a method called displaySphere.
* return type: void
* parameters: none
* functionality: Prints the object's radius, color, and volume to the screen.
IntroToObjects/src/drills/SphereWithColorTest.java
* Create several SphereWithColor reference variables and new objects.
* Initialize each object's color and radius fields.
* Call displaySphere for each object.
* Create another SphereWithColor, but do not initialize its fields.
* Call its displaySphere method. What values do you see?