CSC 243 Chapter 4, Fall 2012

Copyright © 2012 Pearson Education, Inc. Chapter 3 Using Classes and Objects Java Software Solutions Foundations of Program Design Seventh Edition John Lewis William Loftus Outline Creating Objects The String Class The Random and Math Classes Formatting Output Copyright © 2012 Pearson Education, Inc. Creating Objects • A variable holds either a primitive value or a reference to an object • A class name can be used as a type to declare an object reference variable String title; • No object is created with this declaration • An object reference variable holds the address of an object • The object itself must be created separately Copyright © 2012 Pearson Education, Inc. Creating Objects • Generally, we use the new operator to create an object • Creating an object is called instantiation • An object is an instance of a particular class title = new String ("Java Software Solutions"); This calls the String constructor, which is a special method that sets up the object Copyright © 2012 Pearson Education, Inc. Invoking Methods • We've seen that once an object has been instantiated, we can use the dot operator to invoke its methods numChars = title.length() • A method may return a value, which can be used in an assignment or expression • A method invocation can be thought of as asking an object to perform a service Copyright © 2012 Pearson Education, Inc. References • Note that a primitive variable contains the value itself, but an object variable contains the address of the object • An object reference can be thought of as a pointer to the location of the object • Rather than dealing with arbitrary addresses, we often depict a reference graphically "Steve Jobs" name1 num1 38 Copyright © 2012 Pearson Education, Inc. Assignment Revisited • The act of assignment takes a copy of a value and stores it in a variable • For primitive types: num1 38 num2 96 Before: num2 = num1; num1 38 num2 38 After: Copyright © 2012 Pearson Education, Inc. Reference Assignment • For object references, assignment copies the address: name2 = name1; name1 name2 Before: "Steve Jobs" "Steve Wozniak" name1 name2 After: "Steve Jobs" Copyright © 2012 Pearson Education, Inc. Aliases • Two or more references that refer to the same object are called aliases of each other • That creates an interesting situation: one object can be accessed using multiple reference variables • Aliases can be useful, but should be managed carefully • Changing an object through one reference changes it for all of its aliases, because there is really only one object Copyright © 2012 Pearson Education, Inc. Garbage Collection • When an object no longer has any valid references to it, it can no longer be accessed by the program • The object is useless, and therefore is called garbage • Java performs automatic garbage collection periodically, returning an object's memory to the system for future use • In other languages, the programmer is responsible for performing garbage collection Copyright © 2012 Pearson Education, Inc. Outline Creating Objects The String Class The Random and Math Classes Formatting Output Enumerated Types Wrapper Classes Components and Containers Images Copyright © 2012 Pearson Education, Inc. The String Class • Because strings are so common, we don't have to use the new operator to create a String object title = "Java Software Solutions"; • This is special syntax that works only for strings • Each string literal (enclosed in double quotes) represents a String object Copyright © 2012 Pearson Education, Inc. String Methods • Once a String object has been created, neither its value nor its length can be changed • Therefore we say that an object of the String class is immutable • However, several methods of the String class return new String objects that are modified versions of the original Copyright © 2012 Pearson Education, Inc. String Indexes • It is occasionally helpful to refer to a particular character within a string • This can be done by specifying the character's numeric index • The indexes begin at zero in each string • In the string "Hello", the character 'H' is at index 0 and the 'o' is at index 4 • See StringMutation.java Copyright © 2012 Pearson Education, Inc. Copyright © 2012 Pearson Education, Inc. //******************************************************************** // StringMutation.java Author: Lewis/Loftus // // Demonstrates the use of the String class and its methods. //******************************************************************** public class StringMutation { //----------------------------------------------------------------- // Prints a string and various mutations of it. //----------------------------------------------------------------- public static void main (String[] args) { String phrase = "Change is inevitable"; String mutation1, mutation2, mutation3, mutation4; System.out.println ("Original string: \"" + phrase + "\""); System.out.println ("Length of string: " + phrase.length()); mutation1 = phrase.concat (", except from vending machines."); mutation2 = mutation1.toUpperCase(); mutation3 = mutation2.replace ('E', 'X'); mutation4 = mutation3.substring (3, 30); continued Copyright © 2012 Pearson Education, Inc. continued // Print each mutated string System.out.println ("Mutation #1: " + mutation1); System.out.println ("Mutation #2: " + mutation2); System.out.println ("Mutation #3: " + mutation3); System.out.println ("Mutation #4: " + mutation4); System.out.println ("Mutated length: " + mutation4.length()); } } Copyright © 2012 Pearson Education, Inc. continued // Print each mutated string System.out.println ("Mutation #1: " + mutation1); System.out.println ("Mutation #2: " + mutation2); System.out.println ("Mutation #3: " + mutation3); System.out.println ("Mutation #4: " + mutation4); System.out.println ("Mutated length: " + mutation4.length()); } } Output Original string: "Change is inevitable" Length of string: 20 Mutation #1: Change is inevitable, except from vending machines. Mutation #2: CHANGE IS INEVITABLE, EXCEPT FROM VENDING MACHINES. Mutation #3: CHANGX IS INXVITABLX, XXCXPT FROM VXNDING MACHINXS. Mutation #4: NGX IS INXVITABLX, XXCXPT F Mutated length: 27 Quick Check Copyright © 2012 Pearson Education, Inc. What output is produced by the following? String str = "Space, the final frontier."; System.out.println (str.length()); System.out.println (str.substring(7)); System.out.println (str.toUpperCase()); System.out.println (str.length()); Quick Check Copyright © 2012 Pearson Education, Inc. What output is produced by the following? String str = "Space, the final frontier."; System.out.println (str.length()); System.out.println (str.substring(7)); System.out.println (str.toUpperCase()); System.out.println (str.length()); 26 the final frontier. SPACE, THE FINAL FRONTIER. 26 Outline Creating Objects The String Class The Random and Math Classes Formatting Output Enumerated Types Wrapper Classes Components and Containers Images Copyright © 2012 Pearson Education, Inc. Class Libraries • A class library is a collection of classes that we can use when developing programs • The Java standard class library is part of any Java development environment • Its classes are not part of the Java language per se, but we rely on them heavily • Various classes we've already used (System , Scanner, String) are part of the Java standard class library Copyright © 2012 Pearson Education, Inc. The Java API • The Java class library is sometimes referred to as the Java API • API stands for Application Programming Interface • Clusters of related classes are sometimes referred to as specific APIs: – The Swing API – The Database API Copyright © 2012 Pearson Education, Inc. The Java API • Get comfortable navigating the online Java API documentation Copyright © 2012 Pearson Education, Inc. Packages • For purposes of accessing them, classes in the Java API are organized into packages • These often overlap with specific APIs • Examples: Package java.lang java.applet java.awt javax.swing java.net java.util javax.xml.parsers Purpose General support Creating applets for the web Graphics and graphical user interfaces Additional graphics capabilities Network communication Utilities XML document processing Copyright © 2012 Pearson Education, Inc. The import Declaration • When you want to use a class from a package, you could use its fully qualified name java.util.Scanner • Or you can import the class, and then use just the class name import java.util.Scanner; • To import all classes in a particular package, you can use the * wildcard character import java.util.*; Copyright © 2012 Pearson Education, Inc. The import Declaration • All classes of the java.lang package are imported automatically into all programs • It's as if all programs contain the following line: import java.lang.*; • That's why we didn't have to import the System or String classes explicitly in earlier programs • The Scanner class, on the other hand, is part of the java.util package, and therefore must be imported Copyright © 2012 Pearson Education, Inc. The Random Class • The Random class is part of the java.util package • It provides methods that generate pseudorandom numbers • A Random object performs complicated calculations based on a seed value to produce a stream of seemingly random values • See RandomNumbers.java Copyright © 2012 Pearson Education, Inc. Copyright © 2012 Pearson Education, Inc. //******************************************************************** // RandomNumbers.java Author: Lewis/Loftus // // Demonstrates the creation of pseudo-random numbers using the // Random class. //******************************************************************** import java.util.Random; public class RandomNumbers { //----------------------------------------------------------------- // Generates random numbers in various ranges. //----------------------------------------------------------------- public static void main (String[] args) { Random generator = new Random(); int num1; float num2; num1 = generator.nextInt(); System.out.println ("A random integer: " + num1); num1 = generator.nextInt(10); System.out.println ("From 0 to 9: " + num1); continued Copyright © 2012 Pearson Education, Inc. continued num1 = generator.nextInt(10) + 1; System.out.println ("From 1 to 10: " + num1); num1 = generator.nextInt(15) + 20; System.out.println ("From 20 to 34: " + num1); num1 = generator.nextInt(20) - 10; System.out.println ("From -10 to 9: " + num1); num2 = generator.nextFloat(); System.out.println ("A random float (between 0-1): " + num2); num2 = generator.nextFloat() * 6; // 0.0 to 5.999999 num1 = (int)num2 + 1; System.out.println ("From 1 to 6: " + num1); } } Copyright © 2012 Pearson Education, Inc. continued num1 = generator.nextInt(10) + 1; System.out.println ("From 1 to 10: " + num1); num1 = generator.nextInt(15) + 20; System.out.println ("From 20 to 34: " + num1); num1 = generator.nextInt(20) - 10; System.out.println ("From -10 to 9: " + num1); num2 = generator.nextFloat(); System.out.println ("A random float (between 0-1): " + num2); num2 = generator.nextFloat() * 6; // 0.0 to 5.999999 num1 = (int)num2 + 1; System.out.println ("From 1 to 6: " + num1); } } Sample Run A random integer: 672981683 From 0 to 9: 0 From 1 to 10: 3 From 20 to 34: 30 From -10 to 9: -4 A random float (between 0-1): 0.18538326 From 1 to 6: 3 Quick Check Copyright © 2012 Pearson Education, Inc. Given a Random object named gen, what range of values are produced by the following expressions? gen.nextInt(25) gen.nextInt(6) + 1 gen.nextInt(100) + 10 gen.nextInt(50) + 100 gen.nextInt(10) – 5 gen.nextInt(22) + 12 Quick Check Copyright © 2012 Pearson Education, Inc. Given a Random object named gen, what range of values are produced by the following expressions? gen.nextInt(25) gen.nextInt(6) + 1 gen.nextInt(100) + 10 gen.nextInt(50) + 100 gen.nextInt(10) – 5 gen.nextInt(22) + 12 Range 0 to 24 1 to 6 10 to 109 100 to 149 -5 to 4 12 to 33 Quick Check Copyright © 2012 Pearson Education, Inc. Write an expression that produces a random integer in the following ranges: Range 0 to 12 1 to 20 15 to 20 -10 to 0 Quick Check Copyright © 2012 Pearson Education, Inc. Write an expression that produces a random integer in the following ranges: gen.nextInt(13) gen.nextInt(20) + 1 gen.nextInt(6) + 15 gen.nextInt(11) – 10 Range 0 to 12 1 to 20 15 to 20 -10 to 0 The Math Class • The Math class is part of the java.lang package • The Math class contains methods that perform various mathematical functions • These include: – absolute value – square root – exponentiation – trigonometric functions Copyright © 2012 Pearson Education, Inc. The Math Class • The methods of the Math class are static methods (also called class methods) • Static methods are invoked through the class name – no object of the Math class is needed value = Math.cos(90) + Math.sqrt(delta); • We discuss static methods further in Chapter 7 • See Quadratic.java Copyright © 2012 Pearson Education, Inc. Copyright © 2012 Pearson Education, Inc. //******************************************************************** // Quadratic.java Author: Lewis/Loftus // // Demonstrates the use of the Math class to perform a calculation // based on user input. //******************************************************************** import java.util.Scanner; public class Quadratic { //----------------------------------------------------------------- // Determines the roots of a quadratic equation. //----------------------------------------------------------------- public static void main (String[] args) { int a, b, c; // ax^2 + bx + c double discriminant, root1, root2; Scanner scan = new Scanner (System.in); System.out.print ("Enter the coefficient of x squared: "); a = scan.nextInt(); continued Copyright © 2012 Pearson Education, Inc. continued System.out.print ("Enter the coefficient of x: "); b = scan.nextInt(); System.out.print ("Enter the constant: "); c = scan.nextInt(); // Use the quadratic formula to compute the roots. // Assumes a positive discriminant. discriminant = Math.pow(b, 2) - (4 * a * c); root1 = ((-1 * b) + Math.sqrt(discriminant)) / (2 * a); root2 = ((-1 * b) - Math.sqrt(discriminant)) / (2 * a); System.out.println ("Root #1: " + root1); System.out.println ("Root #2: " + root2); } } Copyright © 2012 Pearson Education, Inc. continued System.out.print ("Enter the coefficient of x: "); b = scan.nextInt(); System.out.print ("Enter the constant: "); c = scan.nextInt(); // Use the quadratic formula to compute the roots. // Assumes a positive discriminant. discriminant = Math.pow(b, 2) - (4 * a * c); root1 = ((-1 * b) + Math.sqrt(discriminant)) / (2 * a); root2 = ((-1 * b) - Math.sqrt(discriminant)) / (2 * a); System.out.println ("Root #1: " + root1); System.out.println ("Root #2: " + root2); } } Sample Run Enter the coefficient of x squared: 3 Enter the coefficient of x: 8 Enter the constant: 4 Root #1: -0.6666666666666666 Root #2: -2.0 Outline Creating Objects The String Class The Random and Math Classes Formatting Output Copyright © 2012 Pearson Education, Inc. Formatting Output • It is often necessary to format output values in certain ways so that they can be presented properly • The Java standard class library contains classes that provide formatting capabilities • The NumberFormat class allows you to format values as currency or percentages • The DecimalFormat class allows you to format values based on a pattern • Both are part of the java.text package Copyright © 2012 Pearson Education, Inc. Formatting Output • The NumberFormat class has static methods that return a formatter object getCurrencyInstance() getPercentInstance() • Each formatter object has a method called format that returns a string with the specified information in the appropriate format • See Purchase.java Copyright © 2012 Pearson Education, Inc. Copyright © 2012 Pearson Education, Inc. //******************************************************************** // Purchase.java Author: Lewis/Loftus // // Demonstrates the use of the NumberFormat class to format output. //******************************************************************** import java.util.Scanner; import java.text.NumberFormat; public class Purchase { //----------------------------------------------------------------- // Calculates the final price of a purchased item using values // entered by the user. //----------------------------------------------------------------- public static void main (String[] args) { final double TAX_RATE = 0.06; // 6% sales tax int quantity; double subtotal, tax, totalCost, unitPrice; Scanner scan = new Scanner (System.in); continued Copyright © 2012 Pearson Education, Inc. continued NumberFormat fmt1 = NumberFormat.getCurrencyInstance(); NumberFormat fmt2 = NumberFormat.getPercentInstance(); System.out.print ("Enter the quantity: "); quantity = scan.nextInt(); System.out.print ("Enter the unit price: "); unitPrice = scan.nextDouble(); subtotal = quantity * unitPrice; tax = subtotal * TAX_RATE; totalCost = subtotal + tax; // Print output with appropriate formatting System.out.println ("Subtotal: " + fmt1.format(subtotal)); System.out.println ("Tax: " + fmt1.format(tax) + " at " + fmt2.format(TAX_RATE)); System.out.println ("Total: " + fmt1.format(totalCost)); } } Copyright © 2012 Pearson Education, Inc. continued NumberFormat fmt1 = NumberFormat.getCurrencyInstance(); NumberFormat fmt2 = NumberFormat.getPercentInstance(); System.out.print ("Enter the quantity: "); quantity = scan.nextInt(); System.out.print ("Enter the unit price: "); unitPrice = scan.nextDouble(); subtotal = quantity * unitPrice; tax = subtotal * TAX_RATE; totalCost = subtotal + tax; // Print output with appropriate formatting System.out.println ("Subtotal: " + fmt1.format(subtotal)); System.out.println ("Tax: " + fmt1.format(tax) + " at " + fmt2.format(TAX_RATE)); System.out.println ("Total: " + fmt1.format(totalCost)); } } Sample Run Enter the quantity: 5 Enter the unit price: 3.87 Subtotal: $19.35 Tax: $1.16 at 6% Total: $20.51 Formatting Output • The DecimalFormat class can be used to format a floating point value in various ways • For example, you can specify that the number should be truncated to three decimal places • The constructor of the DecimalFormat class takes a string that represents a pattern for the formatted number • See CircleStats.java Copyright © 2012 Pearson Education, Inc. Copyright © 2012 Pearson Education, Inc. //******************************************************************** // CircleStats.java Author: Lewis/Loftus // // Demonstrates the formatting of decimal values using the // DecimalFormat class. //******************************************************************** import java.util.Scanner; import java.text.DecimalFormat; public class CircleStats { //----------------------------------------------------------------- // Calculates the area and circumference of a circle given its // radius. //----------------------------------------------------------------- public static void main (String[] args) { int radius; double area, circumference; Scanner scan = new Scanner (System.in); continued Copyright © 2012 Pearson Education, Inc. continued System.out.print ("Enter the circle's radius: "); radius = scan.nextInt(); area = Math.PI * Math.pow(radius, 2); circumference = 2 * Math.PI * radius; // Round the output to three decimal places DecimalFormat fmt = new DecimalFormat ("0.###"); System.out.println ("The circle's area: " + fmt.format(area)); System.out.println ("The circle's circumference: " + fmt.format(circumference)); } } Copyright © 2012 Pearson Education, Inc. continued System.out.print ("Enter the circle's radius: "); radius = scan.nextInt(); area = Math.PI * Math.pow(radius, 2); circumference = 2 * Math.PI * radius; // Round the output to three decimal places DecimalFormat fmt = new DecimalFormat ("0.###"); System.out.println ("The circle's area: " + fmt.format(area)); System.out.println ("The circle's circumference: " + fmt.format(circumference)); } } Sample Run Enter the circle's radius: 5 The circle's area: 78.54 The circle's circumference: 31.416