Objects and Classes 1: Encapsula3on, Strings and Things CSC 121 Fall 2015 Howard Rosenthal

Transcription

1 Objects and Classes 1: Encapsula3on, Strings and Things CSC 121 Fall 2015 Howard Rosenthal

2 Lesson Goals Understand objects and classes Understand Encapsulation Learn about additional Java classes The Random class The String class The StringBuilder class The File Class The DecimalFormat class 2

3 What is an Object? An object has characteristics or attributes. An object also has behaviors The attributes are the data associated with an object The behaviors are the methods used to manipulate the objects We ve already seen and made a bit of use of some objects including: arrays Strings Object oriented programming is all about manipulating objects to perform tasks this matches the way we really think about things 3

4 Some Examples Objects Shoe A remote control A computer mouse or keyboard Non-objects Blue Fast Rough Interesting Return Digest Many of the non-objects are either attributes or methods of the object 4

5 Some Common Characteris8cs of Objects An object is made of tangible material (the pen is made of plastic, metal, ink). An object holds together as a single whole (the whole pen, not a fog). An object has properties (the color of the pen, where it is, how thick it writes...). An object can do things and can have things done to it. 5

6 Iden8fying Objects An object has identity (each object is a distinct individual). An object has state (it has various properties, which might change). An object has behavior (it can do things and can have things done to it). 6

7 A Simple Example Is the tube of tennis balls an object? Yes. It has identity (my tube of balls is different than yours), it has states (opened, unopened), properties (brand name, location), and behavior (although not much). Is each tennis ball an object? Yes. It is OK for objects to be part of other objects. Although each ball has nearly the same state and behavior as the others, each has its own identity. Could the top two balls be considered a single object? Not ordinarily. Each has its own identity independent of the other. If they were joined together with a stick you might consider them as one object. Is the color of the balls an object? No. It is a property of each ball. Is your understanding of tennis balls an object? Probably not, although it is unclear what it is. Perhaps it is a property of the object called "your brain." 7

8 Encapsula8on Encapsulation is a computer language feature that packages attributes and behaviors into a single unit. The data and the methods comprise a single entity When we create objects from the class Scanner it has attributes i.e. System.in and behaviors the various methods 8

9 Object Oriented Programming In object-oriented programming, the programmer uses a programming language (such as Java) to describe various objects. When the program is run (after being compiled) the objects are created (out of main storage) and they start "doing things" by running their methods. Creating an object allows us to access its methods. The methods must execute in the correct order. For an application, the first method to run is the method named main(). There should be only one method named main() in an application. In a small application, main() might do by itself all the computation that needs to be done. In a larger application, main() will create objects and use their methods. 9

10 Classes A class is a template, or blueprint from which objects are created A programmer may define a class using Java, or may use predefined classes that come in class libraries. - We ve already done that!!! A class is merely a plan for a possible object. It does not by itself create any objects. When a programmer wants to create an object the new operator is used with the name of the class. Creating an object is called instantiation. Up to now we have worked with single classes that have included the main method Individual classes usually don t include a main method, rather the main method instantiates objects from classes, and these objects are then used by the various methods (including the main method) Every time we create an object from a class using the keyword new the object has a reference variable that refers to that type of object If the reference is lost then the object becomes garbage 10

11 An Example The Circle Class Is Instan8ated (1) double radius; class Circle double area() { return πr 2 } double circumference() { return 2πr } radius 5.6; radius 19.2; radius 3.4; three Circle objects double area() double circumference() double area() double circumference() double area() double circumference() 11

12 An Example The Circle Class Is Instan8ated (2) Circle ovalguy; ovalguy = new Circle(5.6); or Circle ovalguy = new Circle(5.6); Remember- the new does the instantiation Until then all you ve created is a reference variable In Chapter 10 we describe how to build the constructors needed to create objects. 12

13 Java Libraries and Packages Packages are large files that contain multiple classes Packages are nothing more than logical groupings of classes You can import a single class or an entire package Importing an entire package does not increase the size of the executable code, which only includes those classes actually used We ve done this already for a few months with the Scanner class: import java.packagename.classname i.e. import java.util.scanner import java.packagename.* i.e. import java.util.* 13

14 The Random class(1) The Random class is another class that is imported with the import java.util.*; or import java.util.random; It has a many more methods and is therefore more useful than the Math.random() method (even though that basic Math package did not have to be imported Remember how we achieved the ability to read input from the screen: Scanner keyboard = new Scanner(System.in); Then we had a keyboard reader that could take advantage of all the methods in the class: number = keyboard.nextint(); dnumber = keyboard.nextdouble(); 14

15 The Random class(2) So lets create a new object called randy from the class Random Random randy = new Random(); // note that there are no data attributes, also, that randy is a reference variable that now has a value. Then we can use the methods of Random: int number = randy.nextint(n); // returns an integer greater than or equal to zero but less than n (where n is an integer) In general the format is: referencetoobject.method(any necessary parameters) 15

16 The Random class(3) Method & Description boolean nextboolean() This method returns the next pseudorandom, uniformly distributed boolean value from this random number generator's sequence. void nextbytes(byte[] bytes) This method generates random bytes and places them into a user-supplied byte array. double nextdouble() This method returns the next pseudorandom, uniformly distributed double value between 0.0 and 1.0 from this random number generator's sequence. float nextfloat() This method returns the next pseudorandom, uniformly distributed float value between 0.0 and 1.0 from this random number generator's sequence. double nextgaussian() This method returns the next pseudorandom, Gaussian ("normally") distributed double value with mean 0.0 and standard deviation 1.0 from this random number generator's sequence. int nextint() This method returns the next pseudorandom, uniformly distributed int value from this random number generator's sequence. int nextint(int n) This method returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence. long nextlong() This method returns the next pseudorandom, uniformly distributed long value from this random number generator's sequence. void setseed(long seed) This method sets the seed of this random number generator using a single long seed. 16

17 The Random class(4) An example with dice Random die = new Random(); int dicesum = (die.nextint(6)+1) + (die.nextint(6)+1); Those two lines would give us the sum of the two dice rolled You could create a coin object and then flip it. 17

18 The java.lang package This package is imported automatically into every application, so you don t need to use an import statement The java.lang package includes many useful classes Math class (discussed previously and not to be confused with the Random class) String class very useful class within java String operations are handled much better in modern object-oriented languages than in older third generation languages such as Fortran 18

19 The String Class The String class allows you to create String objects a String object is not a primitive data type a String variable is a reference to a String object An example: String mydog = new String( Fido ); Because String objects are so common there is a shortcut used to initialize or assign an reference value to a String object: String mydog = Fido ; Remember this assigns the address of the String literal Fido to the reference mydog 19

20 The String Class (2) There are many methods that are available on a String object int length() is a method that returns the String length: mydog.length() returns 4 An empty String has no characters and length equals 0 char charat(int i) returns the character at position i (remember we start counting from 0): mydog.charat(2) returns d String next() returns a reference to the next input String i.e. keyboard.next() This method skips whitespaces and returns the next group of characters as a String 20

21 String Concatena8on - Review Concatenation is the process of joining or linking Strings together The operator is + We ve already seen concatenation: System.out.println( A boy who is +age + is a man ); When you concatenate you create a new reference address String s = Sam String t = Thomas String w = s + +t = Sam Thomas w.length() = 10 21

22 Strings and Characters - Review A + B = 131 (integer) A + B = AB (String) A + B = AB (String) + A + B = AB (String) A gets cast to String A + B + = 131 (String) = 7 (String) = 34 (String) Key is that without parentheses we are reading left to right 22

23 String Opera8ons String touppercase() converts a String from lower to upper case String s1 = lower String s2 = s1.touppercase(); System.out.println(s1 + +s2); The output is: lower LOWER Note: s1 is not capitalized.; the method returns a new String reference that s2 refers to All java String operations can be found at: There is also a new document called Class String Documentation in the Reference material area 23

24 Common String Methods (1) Method Explanation Example char charat(int index) int compareto(string t) int comparetolgnorecase(string t) String concat(string t) s.charat(i) returns the character at index i. All Strings are indexed from 0. compares two Strings, character by character, using the ASCII values of the characters.s.compareto(t) returns a negative integer/ 0/positive integer if the String s lexicographically precedes/equals/ follows the String t. similar to compareto(...) but ignores differences in case. s.concat(t) returns s with t appended. boolean endswith(string suffix) s.endswith(t) returns true if t is a suffix of s. boolean startswith(string prefix) s.startswith(t) returns true if t is a pre x of s. boolean equals(object t) (The strange parameter will make sense later. For now, think of the parameter as String.) boolean equalslgnorecase(string t) s.equals(t) returns true if s and t are identical. s.equalsignorecase(t) returns true if s and t are identical, ignoring case. String s "Titanic"; s.charat(3) returns 'a' String s Shrek ; String t Star Wars ; String u Shrek ; s.compareto(t) returns a negative number. s.compareto(u) returns 0. t.compareto(s) returns a positive number. String s E.T. ; String t e.t. ; s.comparetoignorecase(t) returns 0. String s Spider ; s.concat( -Man ) returns Spider-Man. String s Forrest Gump ; s.endswith( ump ) returns true String s Jurassic Park ; s.startswith( Jur ) returns true s.startswith( jur ) returns false String s FINDING NEMO ; String t Finding Nemo ; s.equals(t) returns false s.equals( FINDING NEMO ) returns true String s FINDING NEMO ; String t Finding Nemo ; s.equalsignorecase(t) returns true 24

25 Common String Methods (2) Method Explanation Example int indexof(string t) int indexof(string t, int from) s.indexof(t) returns the index in s of the first occurrence of t and returns 1 if t is not a substring of s. s.indexof(t, from) returns the index in s of the first occurrence of t beginning at index from; an unsuccessful search returns -1. String s The Lord Of The Rings ; s.indexof( The ) returns 0; s.indexof( Bilbo ) returns 1. String s The Lord Of The Rings ; s.indexof( The, 6) returns 12; int length() s.length() returns the number of characters in s. String s Jaws ; s.length() returns 4 String replace(char oldchar, char newchar) String tolowercase() String touppercase() String trim() s.replace(oldch, newch) returns a String obtained by replacing every occurrence of oldch with newch. s.tolowercase() returns a String formed from s by replacing all uppercase characters with lowercase characters. s.touppercase() returns a String formed from s by replacing all lowercase characters with uppercase characters. s.trim() returns the String with all leading and trailing white space removed. String s Harry Potter ; s.replace ( r, m ) returns "Hammy Pottem" String s The Lion King ; s.tolowercase() returns "the lion king" String s The Lion King ; s. touppercase() returns "THE LION KING" String s Attack of the Killer Tomatoes ; s.trim() returns "Attack of the Killer Tomatoes" 25

26 String Opera8ons substring (1) There are two versions of substring String substring( int start) String substring( int start, int to ) Create a new String object that contains the characters of the method's String from the index start to the end of the String. Create a new String object that contains the characters of the method's String from the index start and continuing to index to-1. 26

27 String Opera8ons substring (2) Line of code: String line = "buttercup" ; New String buttercup String a = line.substring( 0 ); String b = line.substring( 6 ); String c = line.substring( 9 ); String d = line.substring( line.length() ); String e = line.substring( line.length()+1 ); String f = line.substring( -1 ); buttercup (a new String is created, containing all the characters of the original) cup (an empty String is created) (an empty String is created) EXCEPTION EXCEPTION 27

28 String Opera8ons substring (3) Line of code: String line = "buttercup" ; New String "buttercup" String a = line.substring( 0, 6 ); "butter" String b = line.substring( 6, 9 ); String c = line.substring( 0, 1 ); "cup" "b" String d = line.substring( 0, 0 ); "" String e = line.substring( 5, 5 ); "" String f = line.substring( 4, line.length()+1 ); EXCEPTION 28

29 More on Strings and String Methods - References String encrypt(string msg, int shift) really is accepting a String reference, not the String itself Strings are immutable You can change the value in the reference pointer, but not the String itself String s = Big ; s = s.tolowercase(); // A new String is created s Big big 29

30 More on Strings and String Methods - equals Be careful with == and Strings. you are comparing references, not Strings themselves use boolean equals(str1) to compare 2 Strings strx.equals(str1) In the example: String s = new String( ABC ); String t = new String( ABC ); s ==t is not true (s and t refer to different objects s.equals(t) is true- they have the same String value 30

31 More on Strings and String Methods compareto (3) Being able to compare Strings allows us to alphabetize (sort) them, which in turn allows us to search them use int s.compareto(t) or int s.comparetoignorecase(t) String s Shrek ; String t Star Wars ; String u Shrek ; s.compareto(t) returns a negative number. s.compareto(u) returns 0. t.compareto(s) returns a positive number. In other words, if the String that is invoking the method occurs earlier in an alphabetical list than the parameter the number that is returned is negative 31

32 The String Class Some Special Characteris8cs We typically create Strings without the use of new String day6 = Friday ; This creates a new object named day6 However if I now execute the statement String dayf = Friday ; a new object is not created, dayf just refers to the same object as day6 But if I now execute the following statement: String friday = new String( Friday ); a new object is created See String_Test_Example.java 32

33 Visualizing the Friday Example day6 Friday dayf friday Friday constructors constructors methods methods 33

34 String Opera8ons An example Write a program that encrypts a message using the Caesar Cypher. The program accepts one line of text and a character shift value. Eliminate all nonalphanumeric characters and blanks and print the result as all capitals. Enter a message on one line Hello Jim XRAY Enter an integer in the range of The encrypted message is OLSSVQPTEYHF Strategy Capitalize Eliminate non-characters Shift there are 26 characters so we wrap around if character values go beyond 90, the ASCII value of Z See CaesarCipher.java 34

35 The Null Value and Strings null is a special value that means "no object." Your program should set a reference variable to null when it is not referring to any object. Programs often set variables to null when they are declared: String a = null; class NullDemo1 { public static void main (String[] arg) { String a = "Random Jottings"; String b = null; String c = ""; if ( a!= null ) System.out.println( a is " +a ); if ( b!= null ) System.out.println("b is " +b ); if ( c!= null ) System.out.println( c is " +c ); } } 35

36 The nextline() Method (1) nextline() advances the Scanner to the beginning of the next line and returns a reference to a String object comprised of the the characters that were read (or skipped if you wish) in the process, including whitespace, but not including the newline character System.out.print( Enter message on one line: ); String message = keyboard.nextline(); System.out.print( Enter an integer in the range 0-25 ); int shift = keyboard.nextint(); If the inputs are Cato 8 then message points to Cato and shift gets the value 8. 36

37 The nextline() Method (2) However, if you reverse the process: System.out.print( Enter an integer in the range 0-25 ); int shift = keyboard.nextint(); System.out.print( Enter message on one line: ); String message = keyboard.nextline(); The results are different: In the second case once you read the 8 the Scanner still hasn t moved to the next line The nextline() then see nothing prior to the line return and returns an empty String In this case using next instead of nextline fixes the problem 37

38 The StringBuilder Class (1) StringBuilder is a different class than String, even though they do many of the same things With the StringBuilder Class you do not create a new object with each alteration of the String The StringBuilder class may improve performance for concatenation and modification The String class is preferable and more efficient for read-only Strings While there are some similar methods, remember that the type of object determines which method will be executed and that some overloaded method may execute differently A complete list of StringBuilder Methods can be found at: Also uploaded into references 38

39 The StringBuilder Class (2) To create a StringBuilder object: StringBuilder sb = newstringbuilder(); //initial capacity is 16 characters StringBuilder sb = newstringbuilder(50); //initial capacity is 50 characters StringBuilder sb = newstringbuilder( Hello ); //initializes sb to Hello The capacity of a StringBuilder object expands automatically 39

40 The StringBuilder Class (3) How the equals method works for StringBuilder vs. String String s = new String( Monkey Business ); String t = new String( Monkey Business ); System.out.println(s.equals(t)); In the case above true is printed out -for String objects the equals method compares values, not references StringBuilder s = new StringBuilder( Monkey Business ); StringBuilder t = new StringBuilder( Monkey Business ); System.out.println(s.equals(t)); In the case above false is printed out for StringBuilder objects the equals method compares references, not values 40

41 The Mysterious String [] args String [] args is simply an array of Strings i.e. reference pointer to objects It allows you to send inputs to the main method via the command line public class TestMain { public static void main(string [] args) { System.out.println("Hello " + args[0]); System.out.println("Hello " + args[1]); } If you execute: Java TestMain Jerry Newman The output is: Hello Jerry Hello Newman What happens is that Java implicitly creates an array for you of a length equal to the number of input parameters i.e. in the above program what happens is: String [] args = { Jerry, Newman }; You have an array args of type String and args.length is equal to 2. See the TestMain application 41

42 Files A File is a collection of data saved under a single name Files can be structured or free text Java creates File objects Once a File is instantiated you can read or write from the file This is very convenient when testing programs, since you can use the same data over and over again 42

43 Input and Output There are many data sources and destinations that can be used 43

44 Processing Streams A processing stream operates on the data supplied by another stream. Often a processing stream acts as a buffer for the data coming from another stream. Character streams are intended exclusively for character data. Byte streams are intended for general purpose input and output. A buffer is a block of main memory used as a work area. For example, disks usually deliver data in blocks of 512 bytes, no matter how few bytes a program has asked for. Usually the blocks of data are buffered and delivered from the buffer to the program in the amount the program asked for. 44

45 Using Files in java Programs (1) You must include the following statement before the class statement import java.io.*; or the following 3 specific classes in the java.io package import java.io.file; import java.io.printwriter; import java.io.ioexception; You must also include the statement throws IOException after the parentheses of any method that uses File class methods: public static void main (String [] args) throws IOException You must also create a file in the program using the new parameter File inputfile = new File( filename ); If the filename isn t in the same directory as the class file you must use the full path name You can create a String variable with the filename first allows greater flexibility i.e. String inputfilename = input.txt ; Finally create a Scanner for the new input file Scanner inputreader = new Scanner( filename ); or Scanner inputreader = new Scanner(inputfilename); Once this is done you can use the same input functions as you have used previously 45

46 Using Files in java Programs (2) You need to use the File method.exists() i.e. inputfile.exists(); This return true if an input file exists, false otherwise if (!inputfile.exists()) { System.out.println("Error: "+ inputfilename + " not found"); System.exit(0); //This will cause you to exit the program if there is no input file to read from } You want to use the Scanner method hasnext() i.e. inputreader.hasnext(); This method returns a false when you reach the end of an input file You need to close the file that the Scanner is reading at the end Use the method.close(); i.e. inputreader.close(); See Test_File_Input.java. And Test_File_Input_v2.java 46

47 Using Files in java Programs (3) Using an output file involves almost the same process File outputfile = new File( filename ); If the filename isn t in the same directory as the class file you must use the full path name You can create a String variable with the filename first allows greater flexibility i.e. String outputfilename = output.txt ; File outputfile = new File(outputfilename); PrintWriter outputwriter = new PrintWriter(outputFile); outputfile is a reference variable for a new object of type File outputwriter is a reference variable for a new object of type PrintWriter Now you can use outputwriter to write to the file i.e. outputwriter.println((message +intvar + is whatever ); You need to close the file that the PrintWriter is writing to at the end Use the method.close(); i.e. outputwriter.close(); 47

48 Examples: Reversal.java and Reversal2.java Read in lines one by one from an input file, reverse all the characters from the input file, and write the results to an output file. Perform the reversal in a separate method called reverser Strategy Create an input text file In the code create your File, Scanner and PrintWriter Objects Read in text a line at a time from the file until file is completely read Call reverser to reverse the text reverser reads in the characters from back to front to create the new String which it passes back to main Alternatively read the String from front to back and append each new character to front of the new String Write the String to the output file 48

49 Example EncryptFile.java Write a program that encrypts a file of text one-line at a time using an interactively supplied code word and after capitalizing the text outputs that text to an output file. Assume that the text line contains only alpha characters and no white spaces. Use the code word, which has no spaces, as follows: Convert the code word to capitalized code word Use the code word one character at a time to shift corresponding character in message by the position of the code word i.e. if Code word is Dram and word to be coded is Hungrier shift as follows: H 3, U 17 N 0, G 12, R 3, I 17, E 0, R 12; Strategy: Create necessary objects for input and output Figure out shift Shift character i based on the position in the alphabet of the character in the code word, going through the code word characters in a round robin method Add character to message line When complete print line to the output file 49

50 The DecimalFormat Class (1) Allows you to easily format decimal number by pattern pattern looks like: 0.### The 0 s and # s tells you how many places to display With a 0 the number (even a 0) is always displayed Need to either create a separate DecimalFormat object for each pattern or reset the instance variable for the object Use import java.text.* or import java.text.decimalformat to use the DecimalFormat class 50

51 The DecimalFormat Class (2) DecimalFormat myformat = new DecimalFormat( 0o.### ); double x = ; double y =.9876 System.out.println(myformat.format(x) + + myformat.format (y)); myformat.applypattern( ##.## ); // changes the pattern System.out.println(myformat.format (x) + + myformat.format (y)); output: Note: DecimalFormat always rounds (up or down) 51

52 The DecimalFormat Class (3) value of double format pattern Output String ###.###" " " ###.#" "123.5" ###.##" "123.4" #,###.## 1, ###" "123" ###" " " " " " " "0.00" "89.01" "0." "89." " " " " "000.0" "-123.5" "0.00" "-89.01" "0." "-89." -1.1 " " " " 0.0" "- 89.0" 52

53 The DecimalFormat Class (4) Summary DecimalFormat is a class We create objects with specific formatting The String format(string pattern) is the method So far we have shown the method with one argument However, you can have two arguments when you create a DecimalFormat object: DecimalFormat numform = new DecimalFormat(" 00.00;-00.00"); System.out.println( "Pos = " + numform.format(12.6) ); System.out.println( "Neg = " + numform.format(-12.6) ); In this case we retain right justification

54 PuPng it all together RandomArray.java Write a program that does the following: The main program requests from the user the dimensions of a two dimensional array. The main program creates a two-dimensional array. The main program calls a method booleanfull that randomly fills the two-dimensional array with either true or false values in each cell. The main program then calls a second method called calcpercent that returns the percentage of elements in the two dimensional array that are true. The main program then prints out the array and on the next line prints out the percent of cells in the array that have the value true true formatted to two decimal places. Hint: You ll need to create a Random object and use the method nextboolean() in the booleanfull method 54

55 Programming Exercises Class (1) Write a program called AddBlanks that reads in lines from a file one at a time and adds a blank after every fourth character. Write to an output file. 55

56 Programming Exercises Class (2) Exercise 2. Uppercase Conversion Write a program that accepts a String and displays another String composed of the characters of the first String but with all lowercase letters capitalized. Any non-alphabetical letters, such as punctuation, should be left unchanged. For example, the String When Homer blew up the nuclear plant, he yelled #!#!#!& DOH &&####!!!! should become WHEN HOMER BLEW UP THE NUCLEAR PLANT, HE YELLED #!#!#!& DOH &&####!!!! 56

57 Programming Exercises Class (3) Exercise 8. Counting Words Write a method that accepts a String and returns the number of words in the String. For example, the String This sentence has too many words in it. has 8 words. 57

58 Programming Exercises Lab (1) Exercise 3. Random Strings Write a program that prints 25 random String of length 4 such that each String is composed of uppercase alphabetical characters. First do this without storing in an array, and then after storing in an array String [] randstr = new String [25]; Which you then print out all at once line by line. 58

59 Programming Exercises Lab (2) Exercise 12. College Transcript A text file stores the courses you have taken along with the corresponding grade (A,B,C, D, F) that you received in each course. The file might look like: Intro to Sociology A Physics Experimental Psych o o o B C Write a program that uses such a file to calculate a GPA. Print the GPA with two decimal places. 59