Strings Useful methods of String char charAt(int index) int compareTo(String anotherString) boolean endsWith(String suffix) int indexOf(multiple) int length() String substring(int begin, int end) String trim() Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. When delcaring String, we can treat them as if they are primitives, but they are objects! // String strAnimal= Dog; //these are equivalent // String strAnimal = new String(Dog); //these are equivalent Immutable class: Has no mutator methods (e.g., String ): String strName = "John Q. Public"; String strUppercased = strName.toUpperCase(); // strName is not changed It is safe to give out references to objects of immutable classes; no code can modify the object at an unexpected time. The implicit paramater is immutable! String is Immutable String Pools To optimize performance, the JVM may keep a String in a pool for reuse. Sometimes is does and sometimes it doesn't. Very unpredictable. This means that in some VMs (or even the same VM at different times), two or more object references may be pointing to the same String in memory. However, since you can not rely upon pooling, you MUST assume that each string has its own object reference. Besides, since Strings are immutable, you need not consider the consequences of passing a reference. Using.equals() rather than ==. Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. A string is a sequence of characters Strings are objects of the String class A string literal is a sequence of characters enclosed in double quotation marks: "Hello, World!" String length is the number of characters in the String Example: "Harry".length() is 5 Empty string: "" Although we use the shortcut: String strName = Robert; we are actually doing this: String strName = new String(Robert); The String Class Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Use the + operator: String strName = "Dave"; String strMessage = "Hello, " + strName; // strMessage is "Hello, Dave" If one of the arguments of the + operator is a string, all the others are converted to a string String strA = "Agent; int n = 7; char c = 'b' String strBond = strA + n + c; // strBond is "Agent7c" Concatenation Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. String str2 = strGreeting.substring(7, 12); // str2 is "World" Substring starts at the begin-index and ends at end-index - 1 Substrings Escape Sequences to include the quotes in a string, use the escape sequences: strOne = "\"Quotation\""; to print \\ StrBackSlashes = "\\\\"; Unicode valuescookbook/appendix.html Escape Sequences Escape Sequenc e Character \nnewline \ttab \bbackspace \fform feed \rreturn \"" (double quote) \'' (single quote) \\\ (back slash) \uDDDD character from the Unicode character set (DDDD is four hex digits) Keyboard Input edu.uchicago.cs.java.lec02.scanner Keyboard Input Scanner -- or you can define your own Scanner scn = new Scanner(System.in); Scanner scn = new Scanner(File filSource); throws FileNotFoundException strInput = scn.nextLine(); Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. System.in has minimal set of features it can only read one byte at a time In Java 5.0, Scanner class was added to read keyboard input in a convenient manner Scanner scn = new Scanner(System.in); System.out.print("Enter quantity:"); int nQuantity = scn.nextInt(); nextDouble reads a double nextLine reads a line (until user hits Enter) next reads a word (until any white space) Reading Input Random See edu.uchicago.cs.java.lec02.random Random numbers import java.util.Random; int n = rnd.nextInt(); double d = rnd.nextDouble(); int n = rnd.nextInt(int nNum); rnd.nextInt(20); //0-19 //in this above case 20 is multipled by some real number between 0 and 1 exclusive; then converted to int; it will return 0 to 19. Project || Generate Javadoc || path to the bin of the sdk Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. A static method does not operate on an object double dY = 4; double dY = matObject.sqrt(); // Error double dY = Math.sqrt(9); //correct Static methods are declared inside classes Naming convention: Classes start with an uppercase letter; objects start with a lowercase letter: Math System.out Calling Static Methods Implicit versus Explicit Parameters The implicit parameter is the object reference, whereas the explicit parameter(s) are/is the argument(s) to the method. strName.indexOf(cSpace); Math.pow(2,3); //no implicit param -- static Most often, when we refer to parameters, we mean the explicit parameters. Example: public class Metric { public static double getKilograms(double dPounds){ return dPounds * 0.454; } // More methods can be added here. } Call with class name instead of object: double dKilos = Metric. getKilograms (150); Static Methods Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Syntax 4.3 Static Method Call Suppose Java had no static methods. How would you use the Math.sqrt method for computing the square root of a number x? Answer: Math mat = new Math(); double dResult = mat.sqrt(x); //note that Math methods ARE STATIC Suppose the methods of java.util.Random were static, how would you call them? Answer: int nResult = Random.nextInt(20); //int betweeen 0-19 //note that Random methods ARE NON-STATIC Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. A classs need not be exclusively static or non-static, it can be both. Static Variables Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. A static variable belongs to the class, not to any object of the class: public class BankAccount {... private double balance; private int accountNumber; private static int lastAssignedNumber = 1000; } If lastAssignedNumber was not static, each instance of BankAccount would have its own value of lastAssignedNumber Static Variables Modifier | Class | Package | Subclass | World +++ + public | | | | +++ + protected | | | | +++ + no modifier | | | | +++ + private | | | | Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. public BankAccount() { // Generates next account number to be assigned lastAssignedNumber++; // Updates the static variable accountNumber = lastAssignedNumber; // Sets the instance variable } /this no-args constructor create a serial number the bank account Static Variables Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. A Static Variable and Instance Variables Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Exception: Static constants, which may be either private or public: public class BankAccount {... public static final double OVERDRAFT_FEE = 5; // Refer to it as BankAccount.OVERDRAFT_FEE } //in the above case, OVERDRAFT_FEE is a constant Static Variables Class Objects Harry tells you that he has found a great way to avoid those pesky objects: Put all code into a single class and declare all methods and variables static. Then main can call the other static methods, and all of them can access the static variables. Will Harrys plan work? Is it a good idea? Answer: Yes, it works. Static methods can access static variables of the same class. But it is a terrible idea. As your programming tasks get more complex, you will want to use objects and classes to organize your programs. Use static appropriately and parsimoniously. Self Check 8.15 Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Constants Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Syntax 4.1 Constant Definition Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. A final variable is a constant Once its value has been set, it cannot be changed Named constants make programs easier to read and maintain Convention: Use all-uppercase names for constants final double QUARTER_VALUE = 0.25; final double DIME_VALUE = 0.1; final double NICKEL_VALUE = 0.05; final double PENNY_VALUE = 0.01; payment = dollars + quarters * QUARTER_VALUE + dimes * DIME_VALUE + nickels * NICKEL_VALUE + pennies * PENNY_VALUE; Constants: final Packages Packages help organize your code Packages disambiguate Packages avoid naming conflicts Using the fully qualified name of an object, you need not import it (though it's best to import). Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. The API Documentation of the Standard Java Library Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Package: a collection of classes with a related purpose Import library classes by specifying the package and class name: import java.awt.Rectangle; You dont need to import classes in the java.lang package such as String and System Packages Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Syntax 2.4 Importing a Class from a Package Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. PackagePurposeSample Class java.lang Language support Math java.util Utilities Random java.io Input and output PrintStream java.awt Abstract Windowing Toolkit Color java.applet Applets Applet java.net Networking Socket java.sql Database Access ResultSet javax.swing Swing user interface JButton omg.w3c.dom Document Object Model for XML documents Document Packages Package: Set of related classes Important packages in the Java library: Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. To put classes in a package, you must place a line package packageName ; as the first instruction in the source file containing the classes Package name consists of one or more identifiers separated by periods Organizing Related Classes into Packages Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Can always use class without importing: java.util.Scanner in = new java.util.Scanner(System.in); Tedious to use fully qualified name Import lets you use shorter class name: import java.util.Scanner;... Scanner in = new Scanner(System.in) Can import all classes in a package: import java.util.*; Never need to import java.lang You dont need to import other classes in the same package Importing Packages Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Use packages to avoid name clashes java.util.Timer vs. javax.swing.Timer Package names should be unambiguous Recommendation: start with reversed domain name: com.horstmann.bigjava edu.sjsu.cs.walters : for Britney Walters classes ( ) Path name should match package name: com/horstmann/bigjava/Financial.java Package Names Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Base directory: holds your program's Files Path name, relative to base directory, must match package name: com/horstmann/bigjava/Financial.java Package and Source Files Which of the following are packages? a. java b. java.lang c. java.util d. java.lang.Math Answer: a.No b.Yes c.Yes d.No Self Check 8.18 Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Is a Java program without import statements limited to using the default and java.lang packages? Answer: No you simply use fully qualified names for all other classes, such as java.util.Random and java.awt.Rectangle. Self Check 8.19 Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Gregorian Calendar Extra Credit Asciify Example Polymorphism: assingment to references An object (instantiated in memory) may be assigned to the following reference types: 1/ a reference type of the same type: Double dubVal = new Double(91.5); 2/ any superclass reference, including abstract classes. i.e; any class in the upline of that objects hierarchy. Number numNum = new Double(9.15); Object objNum = new Double(91.5); Is-a; or to be or not to be....that is the question. To check whether an instantiated object may be stored in reference of another type, it must affirmatively answer this question: "Is object a referenceType?" Upcasting/Downcasting Polymorphism: assingment to references 3/ Any Interface that the object implementsComparable comNum = new Double(91.5); Interfaces A class implements an interface rather than extends it. Any class that implements the interface must override all the interface methods with it's own methods. Interface names often end with "able" to imply that they add to the capabilty of the class. An interface is a contract; it defines the methods Inheritence A class that extends a superclass inherets all the fields and methods of its superclass AND those of its entire class hierarchy. A superclass is more generic and contains less data. A subclass is more specific and contains more data. Access to members through get and set. Interfaces A class implements an interface rather than extends it. Any class that implements the interface must override all the interface methods with it's own methods. Interface names often end with "able" to imply that they add to the capabilty of the class. SprintRace Example Poster Example Console21 One player and one dealer play against one another in a game of 21. A player starts with some money (100 dollars) and bets 10 dollars each hand. The game is played with a 52-card chute. The game ends when the player decides to quit. Game-play starts with the player introducing himself by name, and the dealer dealing himself and the player two cards each. After the initial deal If the dealer and the player both have blackjack, then push, ask to play again. If the dealer has blackjack then the player loses his 10 dollars, ask to play again. If the player has blackjack, then the player wins 15 dollars, ask to play again If neither dealer nor player have blackjack, then the player has the option to either hit (so long as the total value of his hand is less than or equal to 21) or stick. If the player busts when hitting, then the player loses his 10 dollars, and is asked to play again. If the player does not bust, then the dealer is obliged to hit while his total is less than 17. If the dealer busts, then the player wins $10, and is asked to play again. If neither dealer nor player busts, then a tie will push. If the player has the highest hand, then the player wins $10, and is asked to play again. If the dealer has the highest hand, then the player loses $10, and is asked to play again. Once the hand is over, both players return their cards, and those cards are put-back onto the top of the chute. The chute is shuffled once 52 cards have been dealt. Packages Packages help organize your code Packages disambiguate Packages avoid naming conflicts Using the fully qualified name of an object, you need not import it (though it's best to import). Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Package: a collection of classes with a related purpose Import library classes by specifying the package and class name: import java.awt.Rectangle; You dont need to import classes in the java.lang package such as String and System Packages Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Syntax 2.4 Importing a Class from a Package Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. PackagePurposeSample Class java.lang Language support Math java.util Utilities Random java.io Input and output PrintStream java.awt Abstract Windowing Toolkit Color java.applet Applets Applet java.net Networking Socket java.sql Database Access ResultSet javax.swing Swing user interface JButton omg.w3c.dom Document Object Model for XML documents Document Packages Package: Set of related classes Important packages in the Java library: Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. To put classes in a package, you must place a line package packageName ; as the first instruction in the source file containing the classes Package name consists of one or more identifiers separated by periods Organizing Related Classes into Packages Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. For example, to put the Financial class introduced into a package named com.horstmann.bigjava, the Financial.java file must start as follows: package com.horstmann.bigjava; public class Financial {... } Default package has no name, no package statement Organizing Related Classes into Packages Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Can always use class without importing: java.util.Scanner in = new java.util.Scanner(System.in); Tedious to use fully qualified name Import lets you use shorter class name: import java.util.Scanner;... Scanner in = new Scanner(System.in) Can import all classes in a package: import java.util.*; Never need to import java.lang You dont need to import other classes in the same package Importing Packages Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Use packages to avoid name clashes java.util.Timer vs. javax.swing.Timer Package names should be unambiguous Recommendation: start with reversed domain name: com.horstmann.bigjava edu.sjsu.cs.walters : for Britney Walters classes ( ) Path name should match package name: com/horstmann/bigjava/Financial.java Package Names Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Base directory: holds your program's Files Path name, relative to base directory, must match package name: com/horstmann/bigjava/Financial.java Package and Source Files Which of the following are packages? a. java b. java.lang c. java.util d. java.lang.Math Answer: a.No b.Yes c.Yes d.No Self Check 8.18 Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Java Doc Generate default JavaDocs. Using decorations/tags. Project || Generate JavaDoc. In Eclipse; navigate to JDK\bin\javadoc.exe to configure. Scope; public to private methods Use F1 to pull up your javadocs. Is a Java program without import statements limited to using the default and java.lang packages? Answer: No you simply use fully qualified names for all other classes, such as java.util.Random and java.awt.Rectangle. Self Check 8.19 Big Java by Cay Horstmann Copyright 2009 by John Wiley & Sons. All rights reserved. Gregorian Calendar Example Adding examples code to javadocs /** * perSoldiers ArrayList Person object * * Example: * perSenior = getMostSenior(perVets); //will return a reference to Person * */ Use the code here tages in javadocs before the methods Good idea! Object Composition Objects are composed of instance fields Instance fields can be primitives or other objects //fields of this class private String strFirstName; private String strLastName; private byte yAge; //-128 to 127 private boolean bVeteran; private String strSocialSecurityNum; private ArrayList perDependents; Imports and the Java API Determine the version of Java youre using. From the cmd line> java versionjava.lang.* is automatically imported. This default behavior is know as convention over configuration. So Eclipse is very good about importing packages and catching compile-time errors. To use the javadoc for the core Java API; F1 JDK\src.zip To see source code; F3