CS2312 Lecture 2

Objects and Classes

Objects-Oriented Programming

• Programming using objects
• An object represents an entity in the real world
• Objects have two parts:
  • State:  Properties of an object.
    • Also referred to as field or attributes
  • Behavior: Things the object can do.
    • Also referred to as methods or functions

Why use objects?

• Modularity: Once we define an object, we can reuse it for other applications.
• Encapsulation: Programmers don’t need to know exactly how the object works. Just the interface. (Abstraction)

 

The keyword class tells java that we’re defining a new type of Object.

Classes are a blueprint.

Objects are instances of classes.

Nearly everything in Java (except primitives and arrays) are Objects and have a Class.

The this keyword means this particular object. Objects know themselves

The new keyword creates a new object. new must be followed by a constructor

 

Constructor

Constructors provide objects with the data they need to initialize themselves, like “How to Assemble” instructions.

Constructor name is same as the class name

No return type – never returns anything

Usually initialize fields

We can define constructors to take any number of arguments.

All classes need at least one constructor

 

Example:

Defind a class

public class Baby{
    String name;
    double weighr = 5.0;
    boolean isMale;
    int numPoops = 0;
    Baby[] siblings;

    void sayHi(){...}
    void eat(double foodWeight){...}
}

Using classes

Baby shiloh = new Baby("Shiloh", true);
shiloh.sayHi();
shiloh.eat(1.0)

 

Static types and methods

static

• Applies to fields and methods (Share fields and methods to all instances)
• Means the field/method
  • Is defined for the class declaration,
  • Is not unique for each instance

static field

• A given class will only have one copy of each of its static fields  
  • This will be shared among all the objects.  
• Each static field exists even if no objects
  • of the class have been created.    
• Use the word static to declare a static field.  
• Only one instance of a static field data for the entire class, not one per instance. 

static/class methods

• Static methods are shared by all objects of the class
• One copy for all objects

 But, Have access only to static fields and methods of the class (Cannot access non-static ones)

Example

public class Baby{
    static int numBabiesCreated = 0;
    String name;
    int age;
    ...
    public int getAge(){
        return age;
    }
    public static Baby older(Baby b1, Baby b2){
        if(b1.getAge() > b2.getAge())
            return b1;
        return b2;
    }
}

 

We HAVE TO use the static key word on the main method in the class that starts the program

-No objects exist yet for the main method to operate on