Object Oriented Programming in Java Introduction Object Oriented Programming in Java Introduction to Computer Science II I Java is Object-Oriented I Not pure object oriented Christopher M. Bourke I Differs from other languages in how it achieves and implements OOP [email protected] functionality I OOP style programming requires practice Objects in Java Objects in Java Java is a class-based OOP language Definition Contrast with proto-type based OOP languages A class is a construct that is used as a blueprint to create instances of I Example: JavaScript itself. I No classes, only instances (of objects) I Constructed from nothing (ex-nihilo) I Objects are concepts; classes are Java’s realization of that concept I Inheritance through cloning of original prototype object I Classes are a definition of all objects of a specific type (instances) I Interface is build-able, mutable at runtime I Classes provide an explicit blueprint of its members and their visibility I Instances of a class must be explicitly created Objects in Java Objects in JavaI Paradigm: Singly-Rooted Hierarchy Object methods 1 //ignore: 2 protected Object clone(); 3 protected void finalize(); I All classes are subclasses of Object 4 Class<? extends Object> getClass(); 5 I Object is a java object, not an OOP object 6 //multithreaded applications: I Makes garbage collection easier 7 public void notify(); 8 public void notifyAll(); I All instances have a type and that type can be determined 9 public void wait(); I Provides a common, universal minimum interface for objects 10 public void wait(long timeout); I Avoids complex lattice relations in other OOP languages: no multiple 11 public void wait(long timeout, int nanos); inheritance 12 13 //important ones: 14 public boolean equals(Object obj); 15 public int hashCode(); 16 public String toString(); Objects in JavaII Declaring Classes Object methods Syntax I Classes must be defined in file of the same name ( .java ) I equals returns true/false whether or not obj is equal to this I Class names should be upper-camel case, singular form I Default behavior: uses reference comparison 1 package foo.bar; I hashCode returns an integer hash of this object 2 3 /* imports here */ I Default behavior: typically the memory address of the object 4 I toString provides a human readable representation of the object 5 public class MyClass{ I Default behavior: qualified class name plus JVM memory address 6 7 /* static field declarations */ 8 /* member field declarations */ 9 /* member method declarations */ 10 11 } Object Life CycleI Object Life CycleII Instantiation Persistence I New objects created using the new operator: Student s= new Student(); I Instances persist until they are out of scope, no longer referenced, etc. I All classes have a default constructor if no constructor is defined I Can be serialized (written to disk, database, or transmitted over a I If non-default constructor defined, default constructor is not provided network) I Can have multiple constructors I Serializable interface, using writeObject(java.io.ObjectOutputStream out) I Good practice: constructor variants should call other constructors when appropriate readObject(java.io.ObjectInputStream in) I Example: this(...) I If calling another constructor: must be done first! Object Life Cycle III Inner Classes Destruction I Can set a reference to null: Integer a= 10;a= null; I Java allows you to declare a class within a class I When no reference to an object exists, eligible for garbage collection I Visibility: allowed to be public, private, protected I No guarantee of when it gets destroyed I Static or not I No destructors in Java I Static fields must be final I Cannot rely on finalize() I No static methods allowed unless inner class is static I Cannot rely on System.gc() I Example? I Good practice: Do all cleanup manually in a finally block Anonymous ClassesI Anonymous ClassesII 1 List<Integer> myNumbers= new ArrayList<Integer>(); 2 ... I An anonymous class is a class declared and defined inline 3 Collections.sort(myNumbers, new Comparator<Integer>(){ I Not bound to a (class) name or identifier 4 @Override 5 public int compare(Integer arg0, Integer arg1){ I Useful idiom: allows you to provide a component declared and 6 if(arg0 == null) return-1; defined inline as an argument 7 else if(arg1 == null) return1; I Not just a convenience: prevents reuse of an ad-hoc class 8 else return arg0.compareTo(arg1); 9 }}); I Example: providing a comparator to the collection’s sort method Note: Normally, the sort method cannot handle null values, here we do! Abstract Classes Abstraction I A class can be made abstract I All public methods and fields available to client code I Such a class cannot be instantiated I Static methods/fields available without an instance I Instances must be created from non-abstract subclasses I Interfaces (revisited) I May contain abstract and non-abstract methods I Abstract classes (revisited) I final keyword: I Non-abstract methods provide a “default” behavior for subclasses I A final class cannot be subclassed Need to provide a default? Use an abstract class! Don’t or don’t want to? I A final method cannot be overridden in a subclass Use an interface! I A final field cannot be reassigned (does not imply immutable) InterfacesI InterfacesII An interface is an abstract type that defines method signatures that Syntax: must be implemented by a class 1 public interface InterfaceName{ 2 return-type methodName(); I Methods are always public (or default) and abstract 3 ... I May also define ( final static ) constants 4 } I No default implementation can be defined 1 public class Foo implements InterfaceA, InterfaceB{ I A class can implements multiple interfaces 2 ... 3 } I Allows us to simulate multiple inheritance without being locked into a hierarchy Interface Example Advantages of an interfaceI 1 public interface Gradeable{ 2 public double getScore(); 3 public String getLabel(); 4 } Interfaces provide a means to: 5 ... 6 public class Exam implements Gradeable{ I Simulate multiple inheritance 7 ... 8 public double getScore(){ I Provide interface (an is-a relationship) without locking you into a 9 return this.numPoints/ this.totalPoints; hierarchy 10 } 11 public String getLabel(){ 12 return "Exam: "+ this.examNumber; 13 } 14 } Advantages of an interfaceII Other interface items Example: I Collections framework: java.util.List , java.util.Set Person Payable I Note: an interface can extend other interfaces (sub/super interface) I Example: java.util.List extends java.util.Collection Intern Employee Supplier I Though not a class, still provides the is-a relation I You can refer to an implementing class as its interface: Hourly Salaried 1 List l= null; 2 ArrayList al= new ArrayList(); Figure : Inheritance through interfaces 3 l= al; Encapsulation InheritanceI I Encapsulation achieved through declaring member fields, methods I Visibility provides protection I Java supports inheritance through subclassing I Common idiom: Mutators and Accessors (getters and setters) I Syntax: extends keyword public class MySubClass extends MySuperClass{ ...} Modifier Class Package Subclass World I Superclass methods and fields are inherited (provided they are not public Y Y Y Y private) protected Y Y Y N I Subclasses may override superclass methods none (default) Y Y N N private Y N N N I Good practice: use the @Override annotation Table : Java Access Levels InheritanceII Composition I Subclass methods can access super class methods and fields using I Java supports composition through the same mechanism as super encapsulation I Can be a pure substitution: no new methods introduced in a I A Java class can have classes as members subclass, or I What’s responsible for constructing it? I New methods may be declared in a subclass I May be completely internal, may be provided, etc. I Preventing subclassing: final (examples: java.lang.Integer , libraries) 1 public class Foo{ I Recall: all methods in Java are virtual by default 2 private Integer a; 3 private Graph g; I Note: a pure virtual method in java is an abstract method 4 protected List<Integer> myList; 5 } Polymorphism Automatic Type CastingI I Many mechanisms for polymorphism I Many different types of polymorphism supported I Numerical operators support mixed types ( int, double , etc.) I Some types not supported: I Simpler types are up-casted to compatible types and (may be) I Operator overloading not allowed 1 downcasted in the final result I Behavioral polymorphism I A form of coercion I Polymorphic behavior not always apparent 1A form is enforced in that super() must be called first in a constructor, ensuring an is-a relationship before anything else can be done Automatic Type CastingII AutoboxingI 1 inta= 10; 2 doubleb= 20.5,c= 10.5,f; I Autoboxing/unboxing: mixing primitive numeric types with Java 3 intd,e; wrapper classes 4 //d = a + b; <- compile error. 5 // requires explicit cast to int, works in C! I As needed, compiler replaces expressions with value methods 6 f=a+b; I May result in runtime NullPointerException 7 e=(int)(b+c); 8 System.out.println("f = "+f); I Need to be careful when mixing types with comparison operators 9 System.out.println("e = "+e); I Form of coercion AutoboxingII Autoboxing III From the Java documentation: So when should you use autoboxing and unboxing? Use them only when there is an impedance mismatch between reference 1 Integer a= new Integer(10); types and primitives, for example, when you have to put 2 Double pi= new Double(3.14); numerical values into a collection. It is not appropriate to use 3 intb= 20; autoboxing and unboxing for scientific computing, or other 4 doublec=a+b+ pi; 5 //becomes: performance-sensitive numerical code. An Integer is not a 6 //double c = a.doubleValue() + b + pi.doubleValue(); substitute for an int ; autoboxing and unboxing blur the distinction between primitive types and reference types, but they do not eliminate it.