Objects Classes Strings Review CSC 123 Fall 2018 Howard Rosenthal

Transcription

1 Objects Classes Strings Review CSC 123 Fall 2018 Howard Rosenthal

2 Lesson Goals Review objects and classes Review Encapsulation Review the Random class Review the String class Review arrays of String objects Learn practical applications including encryption 2

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4 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 String Scanner Object oriented programming is all about manipulating objects to perform tasks this matches the way we really think about things 4

5 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 5

6 Some Common Characteristics 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. 6

7 Identifying 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). 7

8 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." 8

9 Encapsulation 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 9

10 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) by constructors that are a part of the object 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. 10

11 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 As we will see, garbage can be collected 11

12 An Example The Circle Class Is Instantiated (1) double radius; class Circle double area() { return π*radius 2 } double circumference() { return 2*π*radius } three Circle objects radius 5.6; double area() double circumference() radius 19.2; double area() double circumference() radius 3.4; double area() double circumference() 12

13 An Example The Circle Class Is Instantiated (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 Lesson 8 we describe how to build the constructors needed to write classes and create objects. 13

14 A Little More About Classes Each class actually has three parts Description of the instance variables. These can be constants or initialized during object creation They can also be changed by the methods Description of the constructors. A constructor accepts the input parameters sent when creating the object with the new operator An object can have several different constructors Description of the methods. Each object can have multiple methods (think of the Scanner object) Creating separate class objects without a main program and using constructors will be a key part of CS

15 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.*; 15

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17 The Random class (1) The Random class is another class that is imported with import java.util.random; or with import java.util.*; The Random class has many different random methods, making it more useful than the Math.random() method Remember Math.random() is a method within the Math class, while Random is a separate class with its own methods 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: int number = keyboard.nextint(); double decnumber = keyboard.nextdouble(); 17

18 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 in this instantiation When no seed is provided the seed defaults to the 64 bit integer (type long) representation of time of day randy is a reference variable that now has a value (a memory location) You can also create a Random object with a fixed seed which is a long integer (or therefore byte, short or int due to casting up); You then get the same sequence of numbers each time you run the program Random randy1 = new Random(seed);//seed is any type of integer including long This can also be done using setseed(seed): Random randy = new Random(); randy.setseed(seed); In either case we can then 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: referencevariabletoobject.method(any necessary parameters) 18

19 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. 19

20 The Random class (4) An example with dice Random die = new Random(); int dicesum = (die.nextint(6)+1) + (die.nextint(6)+1); Modify DiceRollSim.java (from Lesson 4 use the Random class instead of the Math class; Then try it by using a fixed seed that you read in. You can create the Random class directly with this seed or use setseed. Programs are: DiceRollSimRandomClass.java DiceRollSimRandomClassWithSeed.java DiceRollSimRandomClassSetSeed.java Those two lines would give us the sum of the two dice rolled You could create a coin object and then flip it. 20

21 Putting It All Together Passing Arrays and Using Random 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 formatted to two decimal places. Hint: You ll need to create a Random object and use the method nextboolean() in the booleanfull method See RandomArrayBoolean.java 21

22 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 22

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24 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 24

25 The String Class (2) Visualizing A String Object mydog Fido length() method2() method3()

26 The String Class (3) There are many methods that are available for use with a String object int length() is a method that returns the String length: mydog.length() returns 4 Fido has four characters An empty String has no characters and length equals 0 char charat(int i) returns the character at position i We start counting from 0 as the index of the first character in the String the leftmost character has index 0 mydog.charat(2) returns d String next() returns a reference to the next input String next() is a Scanner method i.e. keyboard.next() This method skips whitespaces and returns the next group of characters as a String 26

27 String Concatenation Review (1) Concatenation is the process of joining or linking String objects together The operator is + A String is a special object that has this one special operator, but it is still not primitive!! When you concatenate you create a new object at a new reference address the old objects remain String s = Sam ; String t = Thomas ; String w = s + +t; //w then has the value Sam Thomas w.length() has the value 10 27

28 String Concatenation Review (2) Visualizing String Concatenation s Sam length() method2() method3() t Thomas length() method2() method3() w Sam Thomas length() method2() method3()

29 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 29

30 String Operations Are Implemented Via Methods String touppercase() converts a String from lower to upper case String s1 = lower ; String s2 = s1.touppercase(); System.out.printf( %s %s\n, 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 Do Programming Exercises 1 here UpperCaseConversion.java 30

31 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 prefix 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 31

32 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" 32

33 String Operations substring (1) The substring method is overloaded String substring( int start) String substring( int start, int end ) 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 end-1. 33

34 String Operations 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 34

35 String Operations 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 35

36 More on Strings and String Methods - References String encrypt(string msg, int shift) really is accepting a String reference, not the String itself This is the same thing as passing array references 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 36

37 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) The comparison is done character by character based on the Unicode value 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 37

38 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 Remember object.method(parameter list) invokes a method 38

39 The String Class Some Special Characteristics 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 StringTestExample.java 39

40 Visualizing the Friday Example day6 Friday dayf friday Friday method1 method2 o o o method1 method2 o o o 40

41 A Few Special String Methods String names = String.join(,, Peter, Paul, Mary ); //first comma indicates the delimiter, the other commas separates the String(s) // sets names to Peter, Paul, Mary Or if String [] arraynames = { Peter, Paul, Mary }; Then String names = String.join(,, arraynames);//note the space after the first comma // sets names to Peter, Paul, Mary You can also split the names apart into an array: String names = Peter, Paul, Mary ; String [] result = names.split(, ); // result[0] = Peter, result [1] = Paul, result[2] = Mary 41

42 String Methods 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 non- alpha 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-alpha characters Shift there are 26 characters so we wrap around if character values go beyond 90, the ASCII value of Z See CaesarCipher.java 42

43 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 unless you plan on using it shortly thereafter. Until a reference variable refers to an actual object you cannot use the methods associate with the object Programs often set variables to null when they are declared: String a = null;// null is a value here, not a String class NullDemo1 { public static void main (String[] arg) { String a = "Random Jottings"; String b = null; String c = ""; if ( a!= null ) System.out.printf( a is %s\n, a ); if ( b!= null ) System.out.printf("b is %s\n, b ); if ( c!= null ) System.out.printf( c is %s\n, c ); } } 43

44 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.printf( Enter message on one line: ); String message = keyboard.nextline(); System.out.printf( Enter an integer in the range 0-25: ); int shift = keyboard.nextint(); If the inputs are: Cato 8 then message refers to Cato and shift gets the value 8. 44

45 The nextline() Method (2) However, if you reverse the process: System.out.printf( Enter an integer in the range 0-25: ); int shift = keyboard.nextint(); System.out.printf( 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 nextint() doesn t read one int and move 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 because next looks for a non-blank character See NextLineTester.java Then do Programming Exercises 2 CountingWords 45

46 Putting It all Together String Arrays It is possible to have arrays of objects We have already seen that 2-dimensional arrays are arrays of arrays Arrays of String objects can be used to organize, alphabetize, etc., an array of Strings When you create an array of Strings you may do so as follows: String [] names = new String [5]; When you create this String each element of the array is initialized to null. You can then populate individual cells as follows: for (int i=0; i < names.length; i++) { System.out.printf( Please enter a String\n ); names[i] = keyboard.nextline(); } At this point names[i] has the value of a String You can access any method for the String i.e. names[i].charat(j) Do Programming Exercises 3 RandomStrings; V2 uses an array of String objects 46

47 Example: Declaring A String Reference and Instantiating The Elements String [] names = new String[4]; names[0] = new String( Karen ); names[1] = new String ( Jose ); names[2] = new String ( Victor ); names[3] = new String ( Morgan ); Remember We are declaring an Array of String Objects 47

48 Picturing Instantiation Of An Array of Objects Karen names names[0] names[1] names[2] names[3] method1 method2 o o o Victor method1 method2 o o o Jose method1 method2 o o o Morgan method1 method2 o o o names[0] through names[3] are reference variables, each of which refers to a String but remember when you concatenate or print a String variable it prints the actual instance value. 48

49 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.printf("Hello %s\n, args[0]); System.out.printf("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 49

50 Programming Exercises 1 UpperCaseConversion 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 &&####!!!! 50

51 Programming Exercises 2 CountingWords 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. 51

52 Programming Exercises 3 RandomStrings Write a program that prints 25 random String objects of length 4 such that each String is composed of uppercase alphabetical characters Do it by storing the String objects in an array String [] randstr = new String [25]; which you then print out all at once line by line. 52