Classes vs. objects

There are no classes and there are no objects.

However, there are classes of objects and instances of objects.

What exactly is a class?

• class is a blueprint of an object:
  

What exactly is an object?

• a real thing, actual instance of the class:
  

Using Java code to describe a class:

public class Person {
    // Attributes
    private String name;

    private String address;

    // Java methods
    public String getName() {
        return name;
    }

    public void setName( String name_ ) {
        name = name_;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress( String address_ ) {
        address = address_;
    }
}

Using UML to model a class diagram:

Size of an object is determined by

• number of data items (aka data variables)

• calculated as the total number of bytes in each data item.

Note: number of Java methods or C++ functions does not measure the object size.

• Although each method can be measured in the number of bytes of the actual code, there is not necessarily a physical copy of one method for each object. Rather, each object points to the same physical code.

Keep objects small:

• minimum required data

• minimum methods required by functionality.

Each object should be designed to have specific tasks.

A system should contain many small interacting objects rather than a few large objects.