CS 211 Classes, Objects

1. CS 211Classes, Objects

2. Today’s lecture • Review chapter 5 • go over exercises

3. Classes and Objects • What is a class? (What do we place in a class?) • What is an object?(where do we create them?) • How do we create a class? • How do we create an object? • How do we use an object?

4. Object Uniqueness • Multiple objects of one type can be created that are distinct. • Each occupies a separate spot in memory. // create many objects Knapsack knap1 = new Knapsack(); Knapsack knap2 = new Knapsack(); knap1.size = 5; knap2.size = 8; System.out.println(knap1.size);System.out.println(knap2.size);

5. Objects are values Objects are just values of a particular type (the given class type). "Every expression has a type": class-definitions are types too! Object-yielding expressions (yield a value of a reference type): • constructor calls • methods with a return type that is a class Nothing Special… Arrays of a reference type work just like arrays of primitive types. Expressions involving reference types are still just expressions. Variables can store reference types.

6. Default Constructors Default Constructor: Java provides a default constructor definition when none is present in a class: all instance variables get default values: primitive types get 0, 0.0, false, or ' '; reference types (class types) get the null value.null is a special value: it represents a value of any reference type but has no actual reference value (no instance variables, no methods). Attempting to use null like an actual object is a very common run-time error.

7. Let’s go over the exercises

8. Questions?