More Encapsulation Overloading

1. More EncapsulationOverloading COMP 401, Spring 2014 Lecture 6 1/28/2013

2. The Value of A Reference Variable • A variable for a reference type holds a reference to an object in memory. • Also known as a pointer • The value of this reference is a location in memory. • If you set a reference variable equal to the value of another reference variable, you now have two variables the point to the same object. • More importantly, if you use one variable to change the underlying object, the other variable “sees” that change. • Really because there is only one object with two variables that point to it. • lec6.ex1

3. null references • The value null is valid for any reference type variable • Really means no value at all. • Or in other words, this variable doesn’t point to anything. • Any attempt to access instance fields or methods will result in NullPointerException • If a variable could legitimately be null, then your code needs to check for null before trying to dereference it. • lec6.ex2

4. The Merits of Immutability • An immutable object is one whose fields (i.e., state) are set upon construction and do not change. • Implication: no setters, just getters • Why immutability? • Can be shared as a part of a plurality of other objects without danger. • Automatically “thread-safe”

5. lec06.ex3 • If points are immutable, then triangle class does not have to worry about points changing. • Related to the principle of encapsulation. • Suppose we wanted an immutable triangle class that worked with possibly mutable points.

6. Arrays Are Mutable • Be aware of passing arrays to/from methods. • Even though individual elements of an array may be immutable, the array itself is not. • Element may be changed to be something new or different. • Permanently affects the array which may not be what you intend. • lec6.ex4

7. Polymorphism • Poly = many, morph = forms • General principle of providing access to an abstraction or method in many forms • Idea is that different forms “fit” different contexts • Note: underlying functionality is the same. • In OO programming, principle is evident in a number of different places. • Constructor overloading • Method overloading

8. Constructors • What happens when you don’t define a constructor. • Default constructor with no arguments. • Creates new object with all fields set to default value • Numeric fields set to 0 • Boolean fields set to false • String, Array, and any other sort of reference value field set to null. • lec6.ex5.v1

9. Constructor Overloading • Can define multiple versions of the constructor. • Distinguished from each other by type and number of parameters • Must be some difference otherwise the compiler won’t be able to tell them apart. • When you use the constructor, the right one will be chosen based on the parameters provided. • Note that if you still want a default no-argument constructor, you have to provide it explicitly. • lec6.ex5.v2

10. Constructor Chaining • Common pattern is to “chain” one constructor off of another. • First line of code in the constructor must be the this keyword as a function with parameters • Matching constructor is called first and allowed to execute. • Then remaining code in original constructor called. • Can chain multiple constructors one on to another • lec6.ex5.v3

11. Method Overloading • Regular methods can also be overloaded • Same method name defined more than once. • Return type may not be the same. • But usually is. • Method type must be the same. • Instance method or static class method • Parameter list must somehow be different • Again, this is how the compiler knows which one is meant. • Either different in number or type (or both) • One version can call another • No restrictions on when • No special syntax • lec5.ex5.v4, lec6.ex5.v5

12. Why Overload? • Provides access to constructor / method in a more context specific way. • Limitations of overloading • Does not handle the case when you have two different situations that aren’t distinguished by the number or type of parameters being passed.