Learning objec<ves. Classes, Objects, and Methods. Problem Decomposi<on. Problem Decomposi<on (cont d)

Transcription

1 Learning objec<ves Classes, Objects, and Methods CS160 Spring 2012 Decomposi<on of a solu<on into objects and methods Methods Sta<c methods Instance methods Sharing data between methods Local Data Class Data 2 Problem Decomposi<on As discussed in previous slides: Read input Figure out the loop structure Ini<alize variables (loop counters, accumula<ng variables, such as totals) Find an appropriate loop termina<ng condi<on Figure out what happens inside a loop Perhaps nested loops Print output Problem Decomposi<on (cont d) But shouldn t the language help us decompose problems? it does! Decompose a big method into smaller methods We will see two ways of doing this (sta<c and instance methods) Decompose into objects (and classes) We introduce this topic with a simple example, but more details are covered in CS161, CS200, CS314,

2 Methods A method (a.k.a. func<on, procedure, rou<ne) is a piece of code that performs a useful ac<on You defined a rou<ne called main. When you run a Java program, it always begins by running the main method. Methods can also return a value to the program that called them More details in a minute Mysteries Revealed public class Temperature { public static void main(string[] args) { // your code here In the last assignment (P7), you defined a class called Temperature! You also defined a method (think function ) called main that took an array of Strings as its arguments 5 6 Terminology A class is a data type Combines variables with methods A class is like a set (defines the proper<es of the elements) An object is an instance of a class Objects are elements of the set (an object has the proper<es defined in the class) Data inside objects and classes They are of two types They may belong to the class (and will take the same value for all the objects) They may belong to the object (and can take different values for each object) 7 8 2

3 Another mystery: sta<c In order to call a method, you need an instance of the class You must have a String variable called word to call word.length() The length() method can access data in the instance it is called on Such methods are called instance methods Excep<on: sta<c methods Not all methods need to access data specific to objects Sta<c declares that a method will not access instance data Sta<c methods can be called without a class instance Uses the nota<on classname.method() Sta<c methods may access class data, but not instance data public sta<c void main Remember that magic incanta<on at the start of your program? main is the name of your method main is the method called by the OS void says that the main func<on does not return a value What would the OS do with a return value? sta<c promises that main will not access instance variables Because the OS needs to call it without crea<ng a class instance public is des<ned to remain a mystery just a bit longer Sta<c methods main is an example of a sta<c method It can only access class variables (or sta<c variables) Therefore main cannot access instance variables. To use instance variables, we will have main create an instance of its class But first, let s see some sta<c methods First we will see sta<c methods that don t share data Then we will see sta<c methods that can share data 11 Simple example (main method calling snowservice) import java.u<l.scanner; public class SnowRemoval { System.out.println("Enter your address:"); Scanner keyboard = new Scanner(System.in); String address = keyboard.nextline(); int delay = snowservice(address); if(delay==0) System.out.println("My driveway is safe now ); else // assume status is non- nega<ve System.out.println( I have to wait for + delay + hours ); 12 3

4 Simple example (snowservice method) public sta<c int snowservice(string home){ System.out.prink("Clearing driveway of %s...", home); System.out.println( done"); System.out.println("Clearing sidewalk"); return 0; 13 Communica<on between calling and called methods Using arguments and return values Method parameters: Method declares a parameter formal parameter (String home) to state what can be provided by the Caller of the method There can be zero to any number of parameters Method return type and value: Can return void (i.e., nothing) or a type (e.g., int, char, int[], String, etc) If a type is returned, there must be a return statement in the method body Calling method (in this case main): Supplies arguments (actual parameters) that must match the type of the parameters in the method declara<on Uses the return value to do something 14 Communica<on using sta<c variables public class Fun { sta<c int data = 0; public sta<c void main (String[] args) { System.out.println( Storing 5 into store ); store(5); System.out.println( Retrieving + retrieve() + from store ); public sta<c void store(int value) { data = value; public sta<c int retrieve() { return data; 15 Cau<on: Pass by value What do you expect this to print? public class PassByValue { int num = 100; increment(num); System.out.println("Aoer calling increment, num is " + num); public sta<c void increment(int n) { n++; The value of the argument is copied. Any changes to the copy are not reflected in the original argument. 16 4

5 Cau<on: Pass by value Another example public class PassByValueString { String word = new String("Good morning"); changegree<ng(word); System.out.println("Aoer calling changegree<ng, word is " + word); public sta<c void changegree<ng(string w) { w = new String("Good night"); Gree<ng remains unchanged 17 Incorrect Swapping public class Swapper { String s1 = "Mar<n"; String s2 = "Scorcese"; swap(s1, s2); System.out.println( main: Aoer swap, s1=" +s1+ " and s2=" +s2); public sta<c void swap(string x, String y) { System.out.println( swap: Before swap, x= +x+ and y= +y); String temp = x; x = y; y = temp; System.out.println( swap: Aoer swap, x=" +x+ " and y=" +y); Nothing gets swapped! 18 Simplifying Temperature s main method Original code with one long main method (Temperature0.java) Refer to sample program version 1 (called Temperature1.java) Pseudo- code Check the number of command- line args Print error message and exit if there are no arguments Otherwise, loop through each argument and call processcity on each argument 19 Decomposing processcity Refer to sample program version 2 (called Temperature2.java) In version 1, processcity was doing three logical tasks: Ini<alizing the counters Reading the file to set the counters Prin<ng the histogram Version 2 makes two new methods, one for reading the file and the other for prin<ng the histogram 20 5

6 Further simplifica<on For prin<ng the histogram, prin<ng stars can be taken out as a separate method (see Temperature3.java) There can be many different ways of decomposing a solu<on Reuse can be taken into account Ooen, smaller methods can be reused in other programs Top- down thinking Break up a big program into smaller pieces The main method calls the other pieces. The pieces can themselves call other pieces Use methods for subtasks The general rule is: Break subtasks into tasks un<l tasks are trivial Every subtask is a method Some methods (subtasks) may call others Objects An object in Java is A set of methods (think: func<ons) A set of members (think: variables) Fancy CS buzzwords: Objects encapsulate data and func3onality Objects encapsulate behavior and state Objects: Concept The idea is that a program manipulates data via methods (func<ons) In P7, the temperature files were an important collec<on of data So, too, was the names of the text files submised by the user on the command line You wrote a method (called main)

7 Object Example: String You have been using objects all along String is an example of an object in Java The characters are the data in the object Methods include: length() : how long is the string? charat(int): what character is at a given posi<on? Syntax: You call an object s method using. and args () E.g.: word.charat(5); word.length() You access an object s data using just. Another example: Scanner Scanner is a more complex object Its data is a stream of characters May come from a file May come from the terminal (a stream) May come from a string Its ac<ons are to parse and interpret the characters next() returns the next valid string nextint() returns the next valid integer nextdouble() returns the next valid double and there are many more (see on- line Java reference) Yet another example: arrays Arrays are the simplest objects Their data is a set of variables of the type provided Their data includes a variable that stores the length of the array Note that array lengths are stored in variables, not computed by func<ons This is why there are no () s aoer args, if args is an array. Classes as data types Classes are data types (just like primi<ves): int counter; String word; MyClass example; By conven<on, class names are capitalized Variables with object types s<ll need names E.g. counter, word, and example above Variables cannot be used un<l they are assigned values True for both primi<ve and object types

8 Object Instances The value assigned to a variable of an object type is an object instance Just like 9 is an instance of int (except that int is not a class, it s a primi<ve type) For example: String word = new String( the ); word is a variable of type String. String( the ) creates an instance of String All 0bject instances are created using the keyword new. 29 More Mysteries Revealed Scanner terminal = new Scanner(System.in); Declares Scanner a is variable an object called class terminal that parses of character type Scanner streams so that they can be easily read as strings, ints or other data types System Initializes is terminal an object to instance be a that specific holds Scanner information that reads about the from system System.in you are on. The data in within System describes the input device your terminal. 30 Methods inside a class Order of wri<ng methods is arbitrary Generally constructors are wrisen first Shared data problem: what if two methods need to share data? One subtask reads input and creates an array of words Another subtask checks each word in the array and does something with it Solu<on #1 Method1 for subtask 1 returns a value,v Method2 for subtask 2 uses the value,v Example: public sta+c void main(string[] args) { String[] wordlist = readinput(); processwords(wordlist);

9 Solu<on #2 Use instance variables Define String[] wordlist; as an instance variable Any method of a class can access its variables readinput() can create & write the array processwords() can access it Data Variables in Classes How does a method access data in a class? Every method can access the class instance it is called on Think of word.length(); it can access the data in the string word Think of the class instance as a hidden argument to the method Class variables look like any other variables in the code of a method They do not need to be re- declared Simple example public class Course { private String department, number private String[] sec<ons = new String[2]; public Course(String dept, String num) { department = dept; number = num; public String getfullname(){ return new String(department + " " + number); Course c1 = new Course("CS", "160"); System.out.println(c1.getFullName()); 10/8/2012 CS 160, Fall Semester Temperature example using instance methods Refer to the example posted under sample programs. This is version 4 (called Temperature4.java). Shows one way to define a constructor and instance methods that operate on the object s data. 36 9