5. Strings and Classes

What We Will Cover Announcements Illuminations 5.1: Java Strings 5.1.1: About Strings 5.1.2: Creating Strings 5.1.3: String Methods 5.1.4: String Conversions 5.1.5: StringBuffer Class 5.1.6: Characters and the Character Class 5.1.7: Parsing Strings Exercise 5.1 5.2: Coding Classes to Define Objects 5.2.1: About Classes and Objects 5.2.2: Defining Classes 5.2.3: Instance Variables 5.2.4: Instance Methods 5.2.5: Defining Constructors 5.2.6: Summary Exercise 5.2 5.3: Creating and Using Objects 5.3.1: Instantiating Objects 5.3.2: Two Example Classes 5.3.3: Calling Object Methods 5.3.4: Summary Exercise 5.3 5.4: More Class Features 5.4.1: Objects as Method Arguments 5.4.2: Instance Methods and this 5.4.3: Static Methods and Fields 5.4.4: Garbage Collection and Finalizers 5.4.5: Packages Exercise 5.4 Wrap Up Assignments Due By Next Meeting: A5: Complex Class (10/8/03) Illuminations Questions on Completed Assignments? A4: Using Arrays (10/1/03) About how many comparisons are needed to sort an array of 100 elements using the unmodified bubble sort? About how many comparisons are needed to sort an array of 100 elements using the enhanced bubble sort? How much did our improvements to bubble sort improve its performance? ^ top 5.1: Java Strings Objectives At the end of the lesson the student will be able to: Use Strings in Java programs Convert Strings to primitive types Convert other types to Strings Parse Strings ^ top 5.1.1: About Strings Recall that a string is a sequence of characters String literals are made by enclosing a sequence of characters in double quotes For example: "abc" "b" "3.14159" "$3.95" "My name is Ed" Strings are stored as Unicode characters String objects are used to store string data String is a genuine class and has methods to operate on strings String constant: one or more characters in double quotes Examples: char charVar = `A`; //single quotes String stringVar = "A"; //double quotes String line = "Hello, world!"; More information on Unicode: Unicode Home Page Key Java Classes Key Java classes for string and character processing Class java.lang.String Class java.lang.StringBuffer Class java.lang.Character Class java.util.StringTokenizer ^ top 5.1.2: Creating Strings String assignments are different than for primitive types Actually a reference type because strings are objects in Java For instance: String message; message = new String("Please enter an identification code"); First line declares a reference variable of type String Second line creates a String object in memory as assigns the object location to the reference variable Keyword new is an operator that creates storage for an object Because this allocation occurs while the program is running, known as dynamic memory allocation Could combine the two steps into one line: String message = new String("Please enter an ..."); String literals have a special syntax that does not require the new operator String message = "Please enter an identification code"; Since this form is shorter, it is preferred by most Java programmers String Constructors String class has nine (non-deprecated) constructors Constructors sometimes used to convert from byte or char arrays to strings More information: Class String from the Java API ^ top 5.1.3: String Methods Because strings are class types, most operations on strings use String class methods One exception is assignment, which uses the equals operator Another exception is concatenation, which uses the + sign Most commonly-used methods are: int length(): Returns the length of the string boolean equals(String other): Compares two strings for equality boolean equalsIgnoreCase(String other): Compares two strings for equality, ignoring upper or lower case char charAt(int index): Returns a char at the specified index int compareTo(String other): Compares two strings lexicographically int compareToIgnoreCase(String other): Compares two strings lexicographically, ignoring case String toLowerCase(): Returns a new string with all characters in lowercase String toUpperCase(): Returns a new string with all characters in uppercase String trim(): Returns a new string with all whitespace removed from the beginning and end More methods described in the Class String from the Java API String Concatenation Most string operations are performed using methods rather than operators Because strings are really a Java class However, there is one string operator: + Same sign as used for numerical addition For strings, performs a concatenation operation Joins two strings into one string For example: "Java" + " rules!" Concatenates the two strings into one string: "Java rules!" Note that the + operator has the same precedence for strings and numbers Can sometimes produce surprising results For example: System.out.println("The result is: " + 7 + 2); Produces the output The result is: 72 How can the programmer correct this problem? ^ top 5.1.4: String Conversions Often need to convert String values to numeric types Java provides a very useful set of methods for converting between strings and primitive types These methods are known as wrapper classes Wrap a class structure around a primitive type Each conversion method is a public and static General purpose methods that operate only on their arguments to produce a result Strings to Primitive Values Return Type Conversion Method Example int Integer.parseInt(String) Integer.parseInt("1234") long Long.parseLong(String) Long.parseLong("123456") float Float.parseFloat(String) Float.parseFloat("123.45") double Double.parseDouble(String) Double.parseDouble("123.45") Primitive Values to Strings Type to Convert Conversion Method Example int Integer.toString(int) Integer.toString(1234) long Long.toString(long) Long.toString(123456) float Float.toString(float) Float.toString(123.45) double Double.toString(double) Double.toString(123.45) More Information Package java.lang ^ top 5.1.5: StringBuffer Class When a String object is created, its contents cannot change Use a StringBuffer to create and manipulate modifiable string data Like a String object, a StringBuffer represents a sequence of characters However, StringBuffer has mutator methods that can change its sequence of characters For example: StringBuffer str = new StringBuffer("ABCDE"); str.reverse(); Sytem.out.println(str); // displays EDCBA Note that StringBuffer does not return a new String object Instead, it actually modifies the StringBuffer referred to by str More information: Class StringBuffer from the Java API ^ top 5.1.6: Characters and the Character Class A character is a letter, number or special symbol For example: 'a' 'b' 'Z' '3' 'q' '$' '*' Java provides the char data type to represent characters Stores characters as a 16-bit unsigned value using Unicode Large enough to store 65,536 distinct character codes Unicode was designed to provide codes for all the world's alphabets Internally, Java stores a char as an unsigned 16-bit number Range is from 0 to 65535 Only Java type that is unsigned Class Character used to treat primitive char variables as objects One of several type-wrapper classes Provides methods for determining a character's category, e.g. lowercase letter, digit Also has methods for converting characters between uppercase and lowercase More information: Class Character from the Java API Escape Sequences Note that the first 32 character codes are not visible on our monitors Represent control codes Java can access some of the control codes using escape sequences Backslash (\) directly in front of a select character tells the compiler to escape from the normal interpretation Following table has some nonprinting and hard-to-print characters: Escape Character Name Unicode Value   Alert \u0007 \b Backspace \u0008 \f Formfeed \u000C \n Newline \u000A \r Carriage return \u000D \t Horizontal tab \u0009 \\ Backslash \u005C \" Double quote \u0022 \' Single quote \u0027 Alert character causes the bell to ring To hear the bell, print the Unicode value System.out.print('\u0007'); ^ top 5.1.7: Parsing Strings Used to break a String into tokens (substrings) Provide a set of delimiters For example: import java.util.StringTokenizer; class StrToken { public static void main(String[] args) { StringTokenizer st = new StringTokenizer("Hi Mom, Welcome!", " ,!"); while (st.hasMoreTokens()) { System.out.println(st.nextToken()); } } } More information: Class StringTokenizer from the Java API ^ top Exercise 5.1 Take one minute to prepare an answer the following question: Write a method that returns true is a String argument is a palindrome. A palindrome is a string that reads the same forwards and backwards. Ignore case in making the determination. One possible answer ^ top 5.2: Coding Classes to Define Objects Objectives At the end of the lesson the student will be able to: Code instance variables, constructors, and instance methods of classes ^ top 5.2.1: About Classes and Objects In object-oriented programming, objects represent data values Called objects because often used to model objects in the real-world For example, a program modeling the solar system might have one planet object Each object would contain all the data about each planet We create a description of the object by writing a class A class describes the data values that make up an object With class Planet, we might have values such as: String name: name of the planet double diameter: diameter of the planet A class not only describes the data values, but also how to manipulate the data For example, class Planet might have methods: double getDiameter(): returns the diameter of the planet void moveTo(x, y, z): adjust data values to model movement in space Data values of an object are called instance variables Similarly, operations on object data are performed by instance methods Instance variables are said to hold the state of an object Encapsulation and Data Hiding Two important concepts of object-oriented programming: Encapsulation: inclusion of a number of items into a single unit Data hiding: restricting a user from seeing how data is actually stored in a class Publicly available attributes and operations are known as the interface Benefits of Encapsulation and Data Hiding Allows a software engineer to concentrate on what an object is or does Implementation can be handled later or even by someone else Encourages code reuse and extensibility All interactions between objects use the interface Can change the implementation without changing the interface Can also add operations and attributes without changing the existing interface Improves maintainability If an interface is not changed, no other part of the application is affected Can change one class without changing other classes ^ top 5.2.2: Defining Classes Basic Syntax [accessModifier] class ClassName { // Instance variable declaration section variable declarations // Constructor declaration section variable declarations // Instance method implementation section method definitions } Contains three general sections: Variables Constructors Methods public class Date { // variable declaration section private int month; private int day; private int year; // constructor definition section public Date() { month = 12; day = 25; year = 2002; } public Date(int newMonth, int newDay, int newYear) { month = newMonth; day = newDay; year = newYear; } // method definition section public int getMonth() { return month; } public int getDay() { return day; } public int getYear() { return year; } public void setDate(int mm, int dd, int yyyy) { day = dd; month = mm; year = yyyy; } public void showDate() { System.out.println(month + "/" + day + "/" + year % 100); } } Access Modifiers and Data Hiding Note the keywords private and public Referred to as access modifiers or visibility modifiers private: can only be accessed by member methods of this class public: can be accessed by member methods of any class Setting private accessibility on all data members is a good design practice Enforces security by requiring access only through public member methods Allows us to change how data may be stored at some future time Without affecting code outside the class Restricting a user from seeing how data is actually stored is known as data hiding Programming Style: Class Naming Conventions Use nouns to name classes -- they represent objects Start class names with a capital letter Capital letter differentiates classes from methods and variables ^ top 5.2.3: Instance Variables Our class Date declares 3 instance variables: month, day, year Each object initializes the variables to an initial value Default initialization value of instance variables is: Integer (byte, short, int, long): 0 Floating-point (float, double): 0.0 Character (char): '\u0000' Boolean: false Objects: null Keyword private means that the variables cannot be read outside of the class Rule of thumb: make all data members private Instance variables represent the state of the object Two objects can have the same state if their variables have the same value private int month; private int day; private int year; ^ top 5.2.4: Instance Methods Instance methods are the operations that can occur on the object Objects use methods to send information to each other Syntax for declaring instance methods: [accessModifier] returnType methodName(parameter list) { // statements of the method } In general, you should declare most class methods public However, to prevent other classes from using a method, declare it private Name of the method combined with the parameter list form its signature Can have many methods with the same name in a class However, each must have a different signature Programming Style: Method Naming Conventions As mentioned before, use verbs for method names Appropriate since they perform an action Also start method names with a lower case letter Not required, but is a convention that professional programmers follow Get Methods When variables are declared private, may still need access to them Use public methods to access the data Called get methods (a.k.a. accessor methods) Use the name of the variable with the word get prepended For example: public int getMonth() { return month; } Set Methods May also need to modify private variables Use public methods to modify the data Called set methods (a.k.a. mutator methods) Use the name of the variable with the word set prepended Allows the programmer to verify the value before setting it For example: public void setDate(int mm, int dd, int yyyy) { day = dd; month = dd; year = yyyy; } ^ top 5.2.5: Defining Constructors Constructors are called whenever an object is created from a class Primary purpose is to initialize the instance variables Syntax for declaring constructors: [accessModifier] ClassName(parameter list) { // statements to initialize instance variables // other initializing statements as needed } For now, use the public access modifier for all class constructors Other objects call the constructor when creating new objects Constructors must use the same name and capitalization as the class name Constructors can have zero or more parameters, just like methods Name of the constructor combined with the parameter list form its signature Can have many constructors in a class, but each must have a different signature Constructor Header Constructors are just like member methods with the following restrictions: Only accessibility modifiers permitted (e.g. public, private) Must use the same name as the class Cannot return a value -- thus no return type specified Can only be called using the new operator public Date() { month = 12; day = 25; year = 2002; } Default Constructor Constructor with no parameters If no constructor defined, then compiler supplies one for the class Default constructor supplied by compiler is equivalent to: <ClassName>() {} As you can see, the compiler-supplied constructor does nothing If a class defines one or more constructors, it cannot rely on the compiler-suppled constructor Programming tip: include your own default constructor Overloaded Constructors Like methods, constructors can be overloaded In our example, we had two constructors public Date(int newMonth, int newDay, int newYear) { month = newMonth; day = newDay; year = newYear; } Overloading allows appropriate initialization of objects under different conditions ^ top 5.2.6: Summary Classes contains three general sections: Instance variables Constructor Instance methods Instance variables store the attributes (data) of an object Constructors are called whenever an object is created from a class Instance methods are the operations that can occur on the object Access modifiers control access to variables, constructors and methods private: can only be accessed by member methods of this class public: can be accessed by member methods of any class Use private for all variables and public for most methods Use "get" methods to return values of private variables Use "set" methods to control setting of new values to private variables ^ top Exercise 5.2 Take one minute to prepare an answer the following question: Given a class named Timer, which of the following is a valid definition of a constructor: void Timer() {} Timer Timer() {} private final Timer() {} Timer(Timer t) {} public static void Timer(String args[]) {} ^ top 5.3: Creating and Using Objects Objectives At the end of the lesson the student will be able to: Code a class that creates objects from a user-defined class Use the methods of objects to accomplish the required tasks ^ top 5.3.1: Instantiating Objects Two steps to Instantiating an object from a class First step is to create a reference variable Must use the class name as the type Date d; Second step is to create the object Use the new operator followed by the class name Returns a reference to a new instance of the class Reference then assigned to the variable d = new Date(); Each object has a unique identity from the reference Each object has its own copy of the variables declared in the class definition Can also declare and Instantiate in one line Date d = new Date(); Can add a method main to test the Date class Common practice among Java programmers // not required -- used to test the class Date public static void main(String[] args) { Date d1 = new Date(); Date d2 = new Date(); d2.setDate(7, 4, 2002); d1.showDate(); d2.showDate(); } Creates two different objects of type Date Each objects has its own set of data variables Each object is referenced through its reference variable: d1 or d2 ^ top 5.3.2: Two Example Classes Use an example to demonstrate how to define classes Will discuss its features in the following sections public class Counter { private int value; // instance variable private int size; // instance variable public Counter() { // default constructor size = 100; } public Counter(int maxSize) { // constructor size = maxSize; } public int get() { // accessor method return value; } public void reset() { // mutator method value = 0; } public void click() { value = (value + 1) % size; } } To Instantiate and test our class, we use another class public class CounterTest { public static void main(String[] args) { Counter c1 = new Counter(); //create a Counter Counter c2 = new Counter(100); //create a second one c1.click(); // increment Counter c1 c2.click(); // increment Counter c2 c2.click(); // increment Counter c2 again System.out.println("Counter1 value is " + c1.get()); System.out.println("Counter2 value is " + c2.get()); c1.reset(); System.out.println("Counter1 value is " + c1.get()); } } ^ top 5.3.5: Accessing Private Fields One important note about private fields Any object of a particular class can access the private fields of another object of that class For example we could add a constructor to class Counter Could use another Counter object as an argument to the constructor public Counter(Counter c) { size = c.size; } Note that c.size accesses the private field of c ^ top 5.3.3: Calling Object Methods To call methods of an object, code the object name followed by a period, the method name and the arguments: objectName.methodName(argumentList); For example, to send no arguments and return no value: c1.click(); Is a message sent? ^ top 5.3.4: Summary To create an object from a class, use the new operator objectName = new ClassName(argumentList); To call methods of an object, use the following syntax: objectName.methodName(argumentList); Objects can be part of the data for other classes ^ top Exercise 5.3 Take one minute to prepare an answer the following question: An object is... what classes are instantiated from an instance of a class a plan for creating objects a reference to an attribute a variable ^ top 5.4: More Class Features Objectives At the end of the lesson the student will be able to: Use the this reference Use static members in programs ^ top 5.4.1: Objects as Method Arguments Java passes the value stored in a variable the same way for both primitive types and reference types When a method is called, value of each argument is copied to its corresponding parameter Thus an argument's value can never be changed within a method Passing a reference value has implications not present when passing primitive values Called method gets access to the exact same object passed by the calling method Situation is shown in the following diagram: For Example Following program demonstrates passing a reference value (e.g. String) Note that the method call and parameter declaration are similar to primitive types public class PassMessage { public static void main(String[] args) { String s1 = "Original message"; displayMsg(s1); } public static void displayMsg(String msg) { System.out.println("In displayMsg: " + msg); } } Since the original message displays in the displayMsg method, we successfully passed a reference type ^ top 5.4.2: Instance Methods and this Instance methods belong to every object of the class Can only be invoked on objects of the class Body of an instance method can access all members (including static member) How can this work? All methods are passed an implicit parameter named this For instance: get() Is called as if it had the parameter this get(this) this refers to the object that calls the method Even though passed implicitly, can still use the this parameter Used with the dot '.' operator like any other object reference this.size Use of this reference is optional unless needed to clarify variable scope For instance, we could have defined the overloaded constructor of Counter public Counter(int size) { // constructor this.size = size; } Need this to differentiate between instance variable size and parameter size ^ top 5.4.3: Static Methods and Fields A static method does not operate on an object Receives all its data as arguments If declared public, static methods are referred to as general purpose methods Implies that the method performs a general-purpose task Often used to construct dialog boxes and in Math methods Classes may also have static fields Implies there is just one value for any object in a class Example Using static Members Classic example: keep track of how many objects of a class are created Cannot use an instance variable in the class definition Which instance variable do you use? Could use another class with an instance variable However, a more elegant solution is to use a static variable Such a variable belongs to the class and not to any object Let's add an object count to the Counter class First need to define a static variable Declared and initialized just like instance variable but with the keyword static private static int numObjs = 0; Need to increment the variable whenever a new object is instantiated Can add an increment statement to the object constructors public Counter2() { // default constructor size = 110; numObjs++; } public Counter2(int maxSize) { // constructor size = maxSize; numObjs++; } Also need a method to retrieve the count public static int numObjs() { return numObjs; } Note that a static method does not have a calling object reference (i.e. this) Thus cannot refer to a (non-static) instance variable of the class Likewise, a static method cannot call a non-static method of the class Putting everything together we get the Counter2 class public class Counter2 { private int value; // instance variable private int size; // instance variable private static int numObjs = 0; public Counter2() { // default constructor size = 100; numObjs++; } public Counter2(int maxSize) { // constructor size = maxSize; numObjs++; } public int get() { // accessor method return value; } public void reset() { // mutator method value = 0; } public void click() { value = (value + 1) % size; } public static int numObjs() { return numObjs; } } Must also update the CounterTest class to use the new features public class Counter2Test { public static void main(String[] args) { Counter2 c1 = new Counter2(); //create a Counter Counter2 c2 = new Counter2(100); //create a second one c1.click(); // increment Counter c1 c2.click(); // increment Counter c2 c2.click(); // increment Counter c2 again System.out.println("Counter1 value is " + c1.get()); System.out.println("Counter2 value is " + c2.get()); c1.reset(); System.out.println("Counter1 value is " + c1.get()); System.out.println(Counter2.numObjs()); } } Further Information Designing with Static Members ^ top 5.4.4: Garbage Collection and Finalizers Garbage Collection Garbage collection returns memory to system Java performs garbage collection automatically An object is marked for garbage collection if no references to object Can manually choose to run the garbage collector System.gc(); Finalizing Objects Before garbage collection, Java gives an object a chance to clean up Known as finalization and runs the finalize method Defined in java.lang.Object Receives no parameters Returns void ^ top 5.4.5: Packages A way of grouping and naming a collection of related classes Can serve as a library of classes Do not have to be in the same directory as your program Packages organized in a hierarchy -- similar to file directories Dot '.' operator used to uniquely identify package members Fully-qualified package name describes the location of the files Using Packages One way to use packages is to use the fully-qualified type name For example: java.util.StringTokenizer st = new java.util.StringTokenizer(s); Quickly becomes tedious so use the import statement instead Syntax: import <fully-qualified type name>; To use classes from a package, put an import statement at the start of the file For example: import java.util.StringTokenizer; Can also use "wildcard" notation to select all packages import java.util.*; Can now use the simple type name (i.e. class name) StringTokenizer st = new StringTokenizer(s); Further Information Creating and Using Packages: from the Java Tutorial ^ top Exercise 5.4 Take one minute to prepare an answer the following questions. What would be the results of attempting to compile and run the following program? class MyClass { static MyClass ref; String[] args; public static void main(String[] args) { ref = new MyClass(); ref.func(args); } public void func(String[] args) { ref.args = args; } } ^ top Wrap Up Due Next Class: A5: Complex Class (10/8/03) ^ top Home | WebCT | Announcements | Schedule | Expectations | Syllabus Help | FAQ's | HowTo's | Links Last Updated: October 01 2003 @17:21:38