Overview of Java (continue)

Announcements You should have access to your repositories and HW0 If you have problems getting HW0, let me know If you’ve added this class on or after Jan 26, you need to contact me so that I set up your repository Always check the Announcements page! 2 Spring 15 CSCI 2600, A Milanova

Outline Overview of Java, continued Last time we covered Model for variables Type safety Compilation vs. interpretation This lecture A bit more on type safety Reflection Subtype polymorphism; classes and interfaces Spring 15 CSCI 2600, A Milanova 3

A Bit More on Type Safety Type safety: no operation is ever applied on an object that does not support the operation E.g., a+b where a and b are 2DPoint s will be rejected as + cannot be applied on 2DPoint s E.g., a.substring(0,10) cannot be applied if a is an int Type safety prevents runtime errors --- if an operation is applied on an object of the wrong type, this almost certainly leads to errors! Spring 15 CSCI 2600, A Milanova 4

Type Safety Java ensures type safety with a combination of compile-time (static) and runtime checks Compiler checks and rejects a large number of programs. E.g., String s = 1 is rejected, String s = new Integer(1) is rejected Some checks are left for runtime. Can you think of a reason why? E.g., at B b = (B) x; the Java runtime checks if x refers to a B, and throws an Exception if it doesn’t Spring 15 CSCI 2600, A Milanova 5

Type Safety and Exceptions Exceptions are good! They prevent application of operations on the wrong type Object x = new A(); B q = (B) x; // ClassCastException // because A is not a B int case2 = q.foo(1); The exception prevents execution from reaching q.foo(1) and applying foo(1) on an object (A) that does not have a foo(int) 6

Type Safety Java is type safe while C++ is type unsafe Java forbids a larger set of errors E.g., in Java, a[i] will throw an Exception if i is out of the bounds of array a. In C++, a[i] can access arbitrary memory, which can lead to insidious errors! Thus, it is easier to write correct programs in Java, because the language itself prevent so many errors 7

Reflection Roughly, the ability of the program to execute code synthesized at runtime. Also, the ability to query metadata about classes E.g., in JavaScript: eval(string) executes string. string can be arbitrary code synthesized at runtime Java supports reflection. C++ does not 8

Reflective Object Creation Standard way to instantiate an object: new Another way: reflective object creation: // First, translate the String argument (the first command-line // argument in this case) into a Class object: Class cl = Class.forName(args[0]); // Next, instantiate the class: Set s = (Set) cl.newInstance(); The class of the object created here is not known until runtime! We can specify a java.util.HashSet or a java.util.TreeSet or other at the command line 9

Reflection Reflection is a very powerful technique Sometimes, we do need to use classes unknown at compile time Important design patterns use reflection. More on this later in our class Disadvantages Awkward code Slows program Prevents optimizations Spring 15 CSCI 2600, A Milanova 10

Reflection Avoid using reflection If you have to use, use only object creation, and access the object through an interface, as shown here: // Translate the string into a Class object: Class cl = Class.forName(str); // Instantiate the class: Set s = (Set) cl.newInstance(); // Access the Set through its interface methods s.add(something) // Set is an Interface defined in the Java library 11

Classes and Inheritance Base class and derived class E.g., in C++: class queue : public list queue is the derived class, list is the base class In Java: class queue extends list In Java we say superclass and subclass Other terms are parent class and child class By deriving new classes programmer forms class hierarchies 12

13 Subtype Polymorphism Subtyping and subtype polymorphism – the ability to use a subclass where a superclass is expected Thus, dynamic method binding Advantages? Disadvantages? In C++ static binding is default, dynamic binding is specified with keyword virtual In Java dynamic binding is default, static binding is specified with final

14 Example: Application Draws Shapes on Screen abstract class Shape { public void draw(); } class Circle extends Shape { … } class Square extends Shape { … } void DrawAll(Shape[] list) { for (int i=0; i < list.length; i++) { Shape s = list[i]; s.draw(); } Dynamic dispatch: Call s.draw() resolves at runtime. It can resolve to Circle.draw() or to Square.draw() depending on what object s refers to. In the static context, we only know of Shape.

Dynamic Method Binding Dynamic method binding (dynamic dispatch) is the ability to invoke a new, refined method, in a context, where an earlier version is expected Example on previous slide: s.draw() binds dynamically (i.e., at runtime) to one of Circle.draw() or Square.draw() Spring 15 CSCI 2600, A Milanova 15

Subtype Polymorphism Polymorphism is an important concept in programming languages and software Polymorphism means “many forms” Example on slide 14. DrawAll is polymorphic because it works with many concrete types, Circle, Square, Triangle It’s subtype polymorphism because these concrete types are related: they are all “subtypes” of Shape 16

Benefits of Subtype Polymorphism Enables extensibility and reuse! In our example, we can extend the Shape hierarchy without modifying DrawAll We can reuse Shape and DrawAll Subtype polymorphism enables the Open/closed principle (credited to Bertrand Meyer) Software entities (classes, modules) should be open for extension but closed for modification In other words, it’s easy to extend the code with additional functionality 17 Spring 15 CSCI 2600, A Milanova

18 Benefits of Subtype Polymorphism abstract class Shape { public void draw(); } class Circle extends Shape { … } class Square extends Shape { … } class Triangle extends Shape { … } Makes it very easy to extend with a Triangle! The Java code requires no changes in DrawAll! It would have been very hard to extend analogous C code! Spring 15 CSCI 2600, A Milanova

“Science” of software design teaches Design Patterns. A lot on Design patterns later in our class Design patterns promote design for extensibility and reuse. Maintainable software Nearly all design patterns make use of subtype polymorphism! Spring 15 CSCI 2600, A Milanova 19 Benefits of Subtype Polymorphism

20 Some Differences C++ vs. Java Access control modifiers – public, private, and others What portion of the class is visible to users ? Public, protected or private visibility Java: Has package is default; protected is slightly different from C++ C++: Has friend classes and functions Visibility of superclass members C++: a subclass can restrict visibility of superclass members Java: a subclass can neither increase nor restrict visibility of superclass members Inheritance C++ allows for multiple inheritance Java allows single class inheritance & multiple interface “inheritance” E.g., class B extends A implements I, J, K… Spring 15 CSCI 2600, A Milanova

Subclassing in Java Subtype polymorphism is achieved by subclassing There is a subtle difference between subtyping and subclassing. More on this later! Typical subclassing in good Java code: Subclass an abstract class In our example, Shape is abstract, Circle and Square are concrete subclasses Implement an interface Shape is an interface and Circle and Square implement the interface (next slide) 21

22 An Interface interface Shape { public void draw(); } class Circle implements Shape { // must implement void draw() public void draw() { … } … // other methods } class Square implements Shape { public void draw() { … } … // other methods } Spring 15 CSCI 2600, A Milanova

Subclassing in Java Rarely we subclass a concrete class. Subtle issues arise In homework 0, we had the choice to inherit BallContainer (reuse through inheritance), or to include BallContainer (reuse through composition) Composition is almost always the better choice! Spring 15 CSCI 2600, A Milanova 23

Abstract Classes vs. Interfaces in Java Two mechanisms to define abstract supertypes Abstract classes and Interfaces Abstract class Can contain implementation for some methods Has at least one empty method A good choice when the concrete subclasses share implementation 24 Spring 15 CSCI 2600, A Milanova

Abstract Classes vs. Interfaces in Java Interface Cannot contain any implementation, just signatures! E.g., MapEntry from the Java library: interface MapEntry { boolean equals(Object o); K getKey(); V getValue(); int hashCode(); V setValue(V value); } 25 Spring 15 CSCI 2600, A Milanova This is a generic interface, K and V are type parameters. (Like C++ templates) There is no implementation, all methods are just signatures.

Abstract Classes vs. Interfaces public abstract class AbstractMapEntry implements Map.Entry { public abstract K getKey(); public abstract V getValue(); // Implements public boolean equals(Object o) { if (o == this) // much more here… } … 26

On Tuesday Reasoning about code and Hoare Logic Spring 15 CSCI 2600, A Milanova 27