COP 3330: Object Oriented Programming FALL 2017 STUDY UNION REVIEW CREDIT TO DR. GLINOS AND PROFESSOR WHITING FOR COURSE CONTENT

Transcription

1 COP 3330: Object Oriented Programming FALL 2017 STUDY UNION REVIEW CREDIT TO DR. GLINOS AND PROFESSOR WHITING FOR COURSE CONTENT

2 Object-Oriented Structure

3 Program Structure Main structural elements of Java programming Classes Interfaces Subclasses Methods, Class Variables Objects Instance Variables

4 Objects Objects Instance Variables Software bundle of related state and behavior used within programs Model real-world objects State: variables Behavior: methods Let s use this car as an example object.

5 State In the form of instance variables AKA member variables, fields Specific to object they are representing Allow for encapsulation Can use private instance variables Getters and setters: public String getbrand() { return brand; } What are some instance variables for this object? There are many possible answers, try it!

6 State In the form of instance variables AKA member variables, fields Specific to object they are representing Allow for encapsulation Can use private instance variables Getters and setters: public String getbrand() { return brand; } What are some instance variables for this object? String color = gray; int speed = 100; Person owner = Brigit; Instance variables can be a variety of types!

7 Behavior Tasks are performed through methods Calling a method tells program to perform that specific task or behavior: ObjectName.methodName(); How would you call the setspeed method on the object mycar? Try it!

8 Behavior Tasks are performed through methods Calling a method tells program to perform that specific task or behavior: ObjectName.methodName(); How would you call the setspeed method on the object mycar? mycar.setspeed(100); We will see more about methods later on!

9 Benefits of Objects Modularity: object can be easily used throughout system Information-hiding: interact with object s methods rather than its exact internal representation Code reuse Debugging: only need to change/remove specific object if it causes issues

10 Classes Like blueprint to create individual objects Contain variables and methods Can import methods from other classes through import statements at top of class Related classes may be put into same package Vehicle Car CoolSportsCar

11 Classes Writing new class follows format: public class ClassName{ // Variables // Methods }

12 Objects Vs Classes Classes are broad blueprint Objects are specific instances of that blueprint

13 Methods Method signatures require name, types and names of parameters, and return type (int, String, void, etc.) Valid method signatures: public String getname(car mycar); private static Boolean setspeed(int speed); void adddriver(string name); this keyword refers to object currently running the method Useful when parameter and instance variable have same name

14 Constructor Similar to a method, but name is same as class, no return type Explicit (specified by programmer) or default (Java uses default no argument constructor) Can call one from another by using this() as first line in constructor

15 Constructor Let s code up a class for a House with a few constructors One will call the other

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17 Instantiation In order to create new objects, they need to be instantiated Follow general format: ClassName objectname = new ClassName(); ClassName indicates type of object, which is the class ObjectName indicates name of this specific object/instance of the class new ClassName() calls the constructor for the class and actually sets aside memory for this object

18 Declaration vs Instantiation Declaring an object: Vehicle car; No memory associated with it Instantiating an object: car = new Vehicle(); Object actually exists and has memory

19 Try It! Practice Problem: Create your own class for shoes. It should have instance variables for brand, size, color, and owner. Its methods should include a constructor, a getter for the color, and a setter for the owner. There can be more than one right answer! We ll code it up in a few minutes.

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21 General Java Syntax and Practices

22 Assignment Simple assignment: uses = operator x = 10 * y; Compound assignment: perform operation and assignment x *= 10; y += j; z /= 100; a -= 1;

23 Assignment Conditional assignment: uses? and : First choice assigned if condition is true Example: int x = 100, y = 50; int larger = (x > y)? x : y; What is the value of larger?

24 Assignment Conditional assignment: uses? and : int x = 100, y = 50; int larger = (x > y)? x : y; What is the value of larger? Evaluates condition (x > y) If condition is met (x is larger than y), then first choice (x) is assigned Otherwise, second choice (y) is assigned Assigns x to larger since x = 100 > y = 50 larger = x = 100

25 Order of Precedence Operators: Special symbols that perform specific actions on one, two, or three operands Evaluated in order of precedence Parentheses first * and / and % next + and next + used with both addition and string concatenation = is evaluated last

26 Order Example What would be the output from the example below? Try it! j = 10; j++; x = j; System.out.println(j); System.out.println(x); System.out.println(x++); System.out.println(++x); System.out.println(x); System.out.println( x+j );

27 Order Example What would be the output from the example below? Try it! j = 10; j++; x = j; System.out.println(j); System.out.println(x); System.out.println(x++); System.out.println(++x); System.out.println(x); System.out.println( x+j ); Output: x+j

28 Mathematic Methods Built in methods to perform math functions easily Explore the Math class API Math.pow(x, y), Math.sqt(x), Math.min(x, y), Math.max(x, y) But what if want to find the minimum or maximum of more than two values? Try it! Use the Math.max() method to find the maximum of three numbers.

29 Max or Min of More than Two Numbers Math.max( Math.max(x, y), z); First compares x and y Then compares greater of x and y with z Use the method more than once

30 Expressions Expressions consist of variables, operators, and method invocations Constructed following appropriate Java syntax Evaluate to a single value Doesn t need to be a complete statement/working line of code Examples: result = Value1 == value 2

31 Expression Statement Complete unit of execution with a terminating semicolon myvalue = 800; Question: Evaluate the line: x + y z = 100; Is it an expression or expression statement? Why?

32 Control Flow Statements Control Flow Statements: regulate order in which statements get executed Examples: if while if-else do-while if-else if-else break switch continue for return

33 if statement Decision making statement Only executes certain section of code if particular condition true if(condition) { // code executed if condition met }

34 if else statement Different from if statement since provides specific alternate code to execute if condition is not met Else always associated with immediately preceding if Use curly braces! Once a condition is satisfied, remaining conditions are not evaluated

35 Example What is the output of this code? int x = 50; if(x > 5) System.out.println( Hello! ); else if(x > 40) System.out.println( Goodybe! );

36 Example What is the output of this code? int x = 50; if(x > 5) System.out.println( Hello! ); else if(x > 40) System.out.println( Goodybe! ); Output: Hello!

37 Switch Statement Decision making statement Break statements are necessary, or else fall through All statements after matching case label are executed until a break is encountered Tests expressions only on single value while if-else can test expressions or ranges of values

38 Example What is the output of this code? int x = 10; switch(x){ case 10: System.out.println( Hello! ); case 5: System.out.println( Goodybe! ); default: System.out.println( Again! ); break; }

39 Example What is the output of this code? int x = 10; switch(x){ case 10: System.out.println( Hello! ); case 5: System.out.println( Goodybe! ); default: System.out.println( Again! ); break; } Output: Hello! Goodbye! Again!

40 For Loops Two different forms Traditional: for(int i = 0; i < 100; i++) for(; ; ;) is also an acceptable form For each: for(string temp : names)

41 While Loops While loops execute while a condition is true while(x < 10) x++; May never execute if condition is not initially met Do-While loops also execute while a condition is true do { x++; }while(x < 10); Will execute at least once since condition is evaluated after body of loop

42 Break, Continue Break: Exits out of closest loop Use labels if wish to break out earlier first: while(.. ) while(.) if(.) break first; Continue: jumps to end of current loop iteration for(i = 0; i < 10; i++) if(i % 2 == 0) continue; j += 2;

43 Chars vs Strings Chars and Strings are more distinct in Java than in C Character: single letter, primitive type Assigned using single quotes char a = a ; Compared with == a == b

44 Chars vs Strings String: sequence of characters Reference type, not primitive Defaults to null Immutable (can t be changed) once created Enclosed in double quotes: String name = Brigit ;

45 Strings Convert to string: Integer.toString(100); Convert to integer: Integer.parseInt( 100 ); Useful methods in String API s.length(); s.charat(); Concatenation with + Use Method.equals() rather than == to compare Strings

46 Strings: Self-Test! You re given the following variables: int x = 100; String name = Hello ; Write the Java code to create the String Hello100 from the two variables x and name Is a String one character in length equivalent to a character? Is null string the same as empty string?

47 Strings: Self-Test! You re given the following variables: int x = 100; String name = Hello ; String hundred = Integer.toString(100); String result = name + hundred; Is a one character String equivalent to a character? No; char is primitive and String is reference type Is null string the same as empty string? No; strings default to null but are specifically set to empty Empty string: String name = ;

48 Arrays Must hold variables/objects all same data type Index starts at 0 Access specific index: a[0] String[] myarray and String myarray[] both acceptable myarray.length defined for every array All index values already initialized 0 for primitive types, null for reference types Can t be resized

49 ArrayList ArrayList extends AbstractList and implements List interface Size increases and decreases as appropriate Cannot hold primitive types (sorry char, int, double) Need to be initialized like other objects ArrayList<String> hi = new Arraylist<String>();

50 ArrayList ArrayList API lists many useful methods:.get().add(e).add(i, e).remove(i).remove(o).indexof().size().contains().isempty()

51 Wrapper Classes But what if I want to store an int in an ArrayList? Wrapper Classes: Allow primitives to be used as reference variables Integer for int i.e. int x = 100; Integer myx = new Integer(x); Double for double Character for char

52 Input Scanner class: text scanner can parse primitive types and strings Input broken into string tokens Whitespace is default delimiter between tokens Create scanner object: Scanner myscan = new Scanner(inputStream);

53 Input and Scanning myscan.next(); myscan.nextint(); myscan.nextdouble(); myscan.nextline(); myscan.hasnext();

54 Output System.out.print() System.out.println() System.out.printf() Most like C Requires arguments if there are format specifiers Format specifiers: %d, %s, %c

55 Methods, Variables, and Class Specifics

56 Variables Class Variables: use static keyword Value shared by all instances of the class i.e. for Car class, a class variable would be numwheels All cars will have numwheels = 4 Instance variables: non-static Each object has its own i.e. for Car class, an instance variable would be owner Each car will have a distinct owner unique to that car

57 Class and Instance Variables Example Let s code up an example of class and instance variables using a class for Laptop

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59 Constants Declared with static final Static makes it consistent across all objects Final prevents its value from being changed later All caps by naming convention public static final WHEELS = 4;

60 Instance vs Class Methods Instance Method: depends in some way on instance variables Use dot operator with object name: myinstance.method(); Class Method: static keyword used Behavior not dependent on instance variables Dot operator with class name: Class.method();

61 Overloading For methods and constructors All have same name Differ in order, number, or types of input parameters Different method signatures Useful if users may have different parameters at different points when calling the method

62 Overloading Try it! 1. Overload the following method two different ways: public void editcar(); 2. Are the following method signatures valid overloading? public void addname(string name, int age); public void addname(string lastname, int id);

63 Overloading 1. Overload the following method two different ways public void editcar(int number); public void editcar(string name); 2. Are the following method signatures valid overloading? No! Their parameters are of the same type and in the same order.

64 Overloading Try it! Add to the Laptop class to overload at least one method.

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66 Overriding Declaring a field or method in a subclass that s already in the superclass Must use same signature and access modifier Or else would be overloading Can t override final method

67 Overriding Let s try it with our Laptop class! We re going to make the Computer class

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70 Levels of Access Different access modifiers for classes, methods, and variables Package-private is default access for classes, methods, variables Public: accessible anywhere Private: only same class, not subclasses; better than protected Protected: accessible by same class, subclasses, classes in same package

71 Inheritance Create new class using existing class Superclass: class from which other classes inherit fields and methods Subclass: inherits all fields and methods from superclass Can modify inherited methods through overriding Can add own fields and methods

72 Inheritance Use keyword extends public class Person extends Human Every class extends a superclass Object class inherently Can extend only one superclass One superclass can have many subclasses

73 Abstract Classes Blueprint for similar subclasses Declared with format: public abstract MyClass{ Can t be instantiated i.e. can t instantiate broad Vehicle superclass All vehicles either car, truck,

74 Abstract Classes Includes concrete methods With implementation (method body) Also includes abstract methods Without implementation (no method body, just signature) Subclass must implement all abstract methods Subclass is abstract otherwise

75 Interfaces Specifies set of methods a class must implement Class won t compile if doesn t implement them all Class becomes more formal about behavior promises to provide Declared as: public interface MyInterface{

76 Interfaces Different than superclass since all methods (other than static or default) are abstract All methods are public and abstract Don t have to include keywords No instance variables

77 Abstract Classes and Interfaces Let s create abstract class Friend BestFriend vs Frenemy

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81 Multiple Inheritance of Type Class can implement more than one interface Extend just one superclass Object can have multiple types Its own class All the interfaces its class implements i.e. if class Car implements Vehicle, then both instantiations below are valid: Car mycar = new Car(); Vehicle mycar = new Car();

82 Polymorphism Subclasses share functionality of their parent class but also define their own unique behaviors Treating objects of different types the same based on their common characteristics Hierarchy, implement same interfaces Instance of class can be of type its supertype List mylist = new ArrayList();

83 Organizing Information

84 List Ordered collection of objects All elements must be same type or common superclass Allows for duplicates and null elements Type in angled brackets <> i.e. List<String>

85 Iterator Java class to traverse a collection Iterator<String> myiterator = mylist.iterator(); while(iter.hasnext()) String temp = iter.next();

86 Set Unordered collection of objects Elements must be same type or superclass No duplicates, only 1 null element Type within angle brackets <> Need to save in SortedSet or other structure (like an array or List) if want ordered elements

87 Map Maps keys to values No duplicate keys Each key mapped to at most 1 value Can have null key or value Keys and values must be reference types Map<Integer, String> = new HashMap();

88 Generics Perform same actions on different kinds of objects Use uppercase letter (typically T) to indicate use of generics Generic method: declared with one or more type parameters Types will be input parameters and/or return types Example Method: what if want to print an array, but don t know what type of data will be held in the array? Public static <T> void printarray(t[] inputarray) {. }

89 Generic Classes Class is generic with one or more generic type parameters Use angled brackets with uppercase letter after class name Try it! Declare a generic class called MyClass There are several possible answers

90 Generic Classes Try it: Declare a generic class called MyClass public class MyClass<T>{ } public class MyClass<T, V>{ }

91 Stacks, Queues, Linked Lists Stack: LIFO ($ is life!) Push, pop Queue: FIFO Enqueue, dequeue Linked list: dynamic list of nodes, alternative to arrays Java API for built-in functionality for all three classes

92 Raw Type Raw Type: If don t specify type when instantiating a generic Stack mystack = new Stack(); Stack is a generic, and it wasn t declared with a type in angled brackets Introduce diamond operator when want to restrict what type of objects generic can hold Stack<String> mystack = new Stack<String>();

93 User Interfaces, Exceptions, and Threads

94 Java GUI Graphical User Interface Visual, user-friendly way to interact with application AWT (Abstract Window Toolkit): basic GUI Swing: platform independent, most widely used Java FX: more multimedia, still in development Focus on Swing

95 Swing Start with top-level container (JFrame, JDialog, JApplet) Every UI component must be part of a containment hierarchy Tree of components with a top-level container as its root At least one containment hierarchy will have JFrame at its root Each UI component can only be contained once

96 Event Handling Event Source: physical action (mouse click, keyboard press) Event Object: encapsulates info about the event Event Listener: instance of a class that listens for certain events (i.e. certain keyboard click) JVM notifies listener when specific event occurs JVM passes event object to listener Listener handles event in a method, the event is processed

97 Action Listener Declare an event handler class and specify that the class either implements an ActionListener interface or extends a class that implements ActionListener public class MyClass implements ActionListener { Register an instance of the event handler class as a listener on one or more components somecomponent.addactionlistener(instanceofmyclass); Include code that implements the methods in listener interface public void actionperformed(actionevent e) {...//code that reacts to the action... }

98 Exception Handling Exception: runtime program error (not syntax error) Exception is short for exceptional event Disrupts normal flow of instructions Program may or may not terminate

99 Exception Handling Handle exception in try-catch block or throw it User error, programmer error, or failure of physical resources Examples: NullPointerException, ArrayIndexOutOfBoundsException, FileNotFoundException Exception classes all subtypes of java.lang.exception class Exception class is subclass of Throwable class Error is also subclass of Throwable

100 Checked Exception Occurs at compile time, AKA compile time exceptions Can t be ignored at time of compilation Programmer needs to fix

101 Unchecked Exception Occurs at time of execution, AKA runtime exception Programming bugs, logic errors, incorrect API usage Ignored at time of compilation

102 Errors Not exceptions Problems beyond user or programmer Typically ignored in code and at compilation time, rarely can do anything about it Example: stack overflow causes error Normally programs can t recover from them

103 Exception Handling: Try, Catch, Finally Try block: first step in exception handler Encloses code that may throw exception Must be present with either catch or finally clause Catch block: must immediately follow brace of try block Can have more than one Handles exception of type specified by argument Finally: always executed after exiting try block, unless JVM exits Helps clean up resources

104 Exception Handling try { // Try to open file } catch (FileNotFoundException e) { // File wasn t found // Print error to user, exit program }

105 Throws If method doesn t handle checked exception, method must declare it with throws keyword at end of method signature throws postpones handling of a checked exception throw invokes an exception explicitly Can define your own exceptions in Java All will be child of Throwable

106 Throws public static void main(string[] args) throws FileNotFoundException { } // Try to open and use file // If file not found, exception will be thrown and code won t execute

107 Concurrency and Multithreading Concurrency supported by allowing programs to have multiple threads of execution Allow lengthy child process to run while rest of the program still runs i.e. can still use browser when downloading an app online Each thread has own call stack and program counter Threads share app-wide resources (memory, files) Run like separate programs but still tied together

108 Thread States New: thread first created, not running Runnable: starts running, acquires resources Waiting: must wait for another thread to finish Timed waiting Blocked: threat is waiting for another to release a resource Terminated: threat completes or crashes

109 Threads Priority: inherit priority from thread that spawned it High priority ones usually preempt lower ones Starve: constantly gets preempted, never finishes Deadlock: two threads hold onto resource while waiting for other to release its resource Monitor: lock held by one thread at a time, blocks other threads needing lock until this thread releases it (synchronized keyword)

110 Final Practice Problems

111 Final Practice Problem 1 Write a class called Student that extends the superclass Person Include instance variables for name, age, and height Include class variable school that is set to UCF Include two different constructors Include a getter and setter method for each instance variable

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113 Final Practice Problem 2 Create a Car class Instance variables for speed, name, list of driver names Constructor that takes input parameter for car name Setter for speed Getter for car name

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115 Final Practice Problem 2 Write code to perform the following actions: Create a new car object Set the speed of the car to 20 Add a new String to the list of drivers Print the name of the car you have chosen

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117 Final Practice Problem 3: Find the Errors public class ErrorProne { private static int x = 10; private ArrayList<String> mylist; private int y = 100; } private int ErrorProne() { mylist = new ArrayList<Integer>(); } public void setx(int new) { x = new; }

118 Final Practice Problem 3: Find the Errors public class ErrorProne { private static int x = 10; private ArrayList<String> mylist; private int y = 100; } private int ErrorProne() { mylist = new ArrayList<Integer>(); } public void setx(int new) { x = new; }

119 Practice Problem Create an Alien class Make an ArrayList of Alien objects in a separate class Manipulate our ArrayList in our separate class

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122 Summary Object Oriented Structure Data manipulation and arithmetic Blocks, statements, expressions Variables, methods, and classes Generics User Interfaces Exceptions Threads

123 Any Questions?