Скачать презентацию Lecture 2 Content Control Structures Branching logic Control

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Lecture 2 Content: Control Structures: Branching logic Control Structures: Looping logic Diagramming Reading Files from Scanner Type Conversions Promotion and Casting Random Numbers Arrays & Array. List Driver, Bean, Utility Reference versus variable My. Greg. Calendar The new keyword Implicit versus explicit params Return mutable reference member cloned Static fields such as Math. PI pass. By. Val, pass. By. Ref, swap Java. Docs

Control Structures See edu. uchicago. cs. java. lec 02. control

Shorthand See blackboard

Arrays & Array. Lists See edu. uchicago. cs. java. lec 02. arrays

Arrays • arrays of prmitivies • arrays of Objects • multidimentional arrays

Arrays • Array: Sequence of values of the same type • Construct array: new double[10] • Store in variable of type double[]: double[] d. Datas = new double[10]; • When array is created, all values are initialized depending on array type: • Numbers: 0 • Boolean: false • Object References: null Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Arrays: declare and init • Array: Sequence of values of the same type double[] d. Values = {2. 5, 8. 1, 3. 79, 0, 6. 0}; The above declares and intializes an array of double[] String[] str. Names = {"Harry", "Lary", "Mary", "Perry"}; The above declares and initializes an array of String[]

Arrays Use [] to access an element: double[] d. Values = new double[10]; d. Values[2] = 29. 95; Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Arrays • Using the value stored: System. out. println("The value of this data item is " + d. Values[2]); //d. Values[2] (a double) is promoted to a String. • Get array length as d. Values. length (Not a method!) • Index values range from 0 to length - 1 • Accessing a nonexistent element results in a bounds error: double[] values = new double[10]; values[10] = 29. 95; // ERROR – out of bounds • Limitation: Arrays have fixed length! Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Declaring. Arrays Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Syntax 7. 1 Arrays Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Make Parallel Arrays into Arrays of Objects // Don't do this int[] account. Numbers; double[] balances; Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Make Parallel Arrays into Arrays of Objects Avoid parallel arrays by changing them into arrays of objects: Bank. Account[] accounts; Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

multidimensional arrays • int[][] n. Numbers = new int[3][4]; • boolean[][] b. Exams = { new boolean[6], new boolean[9], new boolean[3], new boolean[8] }; //ragged array

Array Lists • Array. List class manages a sequence of objects • Can grow and shrink as needed (it's a Vector) • Array. List class supplies methods for many common tasks, such as inserting and removing elements • Array. List is a generic class: Array. List collects objects of type parameter T: Array. List str. Names = new Array. List(); str. Names. add("Emily"); str. Names. add("Bob"); str. Names. add("Cindy"); • size method yields number of elements Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Adding Elements To add an object to the end of the array list, use the add method: names. add("Emily"); names. add("Bob"); names. add("Cindy"); Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Retrieving Array List Elements • To obtain the value an element at an index, use the get method • Index starts at 0 • String str. Name = str. Names. get(2); // gets the third element of the array list • Bounds error if index is out of range • Most common bounds error: int n = str. Names. size(); str. Name = str. Names. get(n); // Error // legal index values are 0. . . n-1 Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Setting Elements • To set an element to a new value, use the set method: str. Names. set(2, "Carolyn"); Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Removing. Elements • To remove an element at an index, use the remove method: str. Names. remove(1); Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Adding and Removing Elements str. Names. add("Emily"); str. Names. add("Bob"); str. Names. add("Cindy"); str. Names. set(2, "Carolyn"); str. Names. add(1, "Ann"); str. Names. remove(1); Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Syntax 7. 2 Array Lists Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Promotion and Casting See edu. uchicago. cs. java. lec 02. cast

Promotion & Casting • Automatic promotion; this occurs when one operand is of lesser precision than the other and completing the operation will not reduce precision of result. double d. Result = n. Numerator / d. Denominator; In the above case, n. Numerator is promoted to a double. int n. Result = (int)(n. Numerator / d. Denominator); you must CAST because you can't assign the return value, intially a double (64 bit flotating precision) to an int (32 bit int precision)

Cast and Round • Cast converts a value to a different type: double d. Balance = d. Total + d. Tax; //d. Balance == 10. 89 int n. Dollars = (int) d. Balance; //n. Dollars == 10 • Math. round converts a floating-point number to nearest integer: long l. Rounded = Math. round(d. Balance); // if d. Balance is 13. 75, then l. Rounded is set to 14 Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Syntax 4. 2 Cast Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Type Conversion • • • Each Wrapper class has some parse functions. Integer. parse. Int(String str); Double. parse. Double(String str); Byte. parse. Byte(String str); Boolean. parse. Boolean(String str); etc.

The New Keyword See edu. uchicago. cs. java. lec 02. newkeyword

Using the new Keyword • Unless you use the ‘new’ keyword, nothing has been instantiated and the object does NOT exist in memory. • (the exception is static) • Driver/Bean/Utility

Static Methods • Example: public class Financial { public static double percent. Of(double d. P, double d. A) { return (d. P / 100) * d. A; } // More financial methods can be added here. } • Call with class name instead of object: double d. Tax = Financial. percent. Of(d. Tax. Rate, d. Total); Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Static Methods • Here is another example: • public class Geometry { public static double area(Rectangle rec) { return rec. get. Width() * rec. get. Height(); } // More geometry methods can be added here. } Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Implicit versus Explicit Parameters • The implicit parameter is the object reference, whereas the explicit parameter(s) are/is the argument(s) to the method. • str. Name. index. Of(c. Space); • Math. pow(2, 3); //no implicit param -- static • Most often, when we refer to parameters, we mean the explicit parameters.

Calling Static Methods • A static method does not operate on an object double d. Y = mth. Object. sqrt(); // Error double d. Y = Math. sqrt(9); //correct • Naming convention: Classes start with an uppercase letter; objects start with a lowercase letter: • Sometimes you will find nonstatic implementations in Java that would seem like perfect candidates for static, such as Random rnd = new Random(); rnd. next. Int(10); //correct Random. next. Int(10); //incorrect Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 8. 12 Suppose Java had no static methods. How would you use the Math. sqrt method for computing the square root of a number n. X? Answer: Math mat = new Math(); double d. Result = mth. sqrt(n. X); Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Syntax 4. 3 Static Method Call Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Static Variables • A static variable belongs to the class, not to any object of the class: public class Bank. Account { . . . private double d. Balance; private int n. Account. Number; private static int n. Last. Assigned. Number = 1000; } • If n. Last. Assigned. Number was not static, each instance of Bank. Account would have its own value of n. Last. Assigned. Number Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Static Variables • Static constants, which may be either private or public: public class Bank. Account { . . . public static final double OVERDRAFT_FEE = 5; // Refer to it as Bank. Account. OVERDRAFT_FEE } //in the above case, OVERDRAFT_FEE is a constant 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.

Constants: final • 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; d. Payment = n. Dollars + n. Quarters * QUARTER_VALUE + n. Dimes * DIME_VALUE + n. Nickels * NICKEL_VALUE + n. Pennies * PENNY_VALUE; Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 8. 15 - funny 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 call the other static methods, and all of them can access the static variables. Will Harry’s 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. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Strings

Useful methods of String • • char. At(int index) int compare. To(String another. String) boolean ends. With(String suffix) int index. Of(multiple) int length() String substring(int begin, int end) String trim()

String is Immutable • Immutable class: Has no mutator methods (e. g. , String): String str. Name = "John Q. Public"; String str. Uppercased = str. Name. to. Upper. Case(); // str. Name 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! Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

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.

The String Class • 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 str. Name = “Robert”; we are actually doing this: String str. Name = new String(“Robert”); Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Concatenation • Use the + operator: String str. Name = "Dave"; String str. Message = "Hello, " + str. Name; // str. Message is "Hello, Dave" • If one of the arguments of the + operator is a string, the other is converted to a string String str. A = "Agent”; int n = 7; String str. Bond = str. A + n; // str. Bond is "Agent 7" Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Substrings • String str 2 = str. Greeting. substring(7, 12); // str 2 is "World" • Substring starts at the begin-index and ends at end-index - 1 Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Escape Sequences • to include the quotes in a string, use the escape sequences: • str. One = ""Quotation""; • to print \ • Str. Back. Slashes = "\\"; • Unicode values http: //oreilly. com/actionscript/excerpts/as 3 cookbook/appendix. html

Escape Sequences Escape Sequenc e Character n newline t tab b backspace f form feed r return " " (double quote) ' ' (single quote) \ (back slash) u. DDDD character from the Unicode character set (DDDD is four hex digits)

Keyboard Input edu. uchicago. cs. java. lec 02. scanner

Keyboard Input • Scanner -- or you can define your own • Scanner scn = new Scanner(System. in); • Scanner scn = new Scanner(File fil. Source); throws File. Not. Found. Exception • str. Input = scn. next. Line();

Reading. Input • 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 n. Quantity = scn. next. Int(); • next. Double reads a double • next. Line reads a line (until user hits Enter) • next reads a word (until any white space) Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Random See edu. uchicago. cs. java. lec 02. random

Random numbers • import java. util. Random; • int n = rnd. next. Int(); • double d = rnd. next. Double(); • int n = rnd. next. Int(int n. Num); rnd. next. Int(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.

Pass by value and reference See edu. uchicago. cs. java. lec 02. passby

pass by value pass by reference Action: Tell my accountant how much I intend to spend on a new car. Change in bank account: no change. Action: Swipe debit card and enter pin at the Bently dealership. Change in bank account: -125 k.

Primitives versus Objects memory Primitives Objects Variables store values and are allocated memory depending on their type. How much? . . . refer to Java Primitives slide. References store memory addresses. The size of the allocation for the object reference is VM specific, but is usually the native pointer size; 32 bits in 32 -bit VM, and 64 bits in a 64 -bit VM. Garbage collected when out-of-scope. Passed into methods by value Passed into methods by reference

Objects See edu. uchicago. cs. java. lec 02. console 21

Class Object • A blueprint is to a house as a class is to an object • Class = Blueprint

Class Objects

Class Object • A blueprint is to a car as a class is to an object • Class = Blueprint

Class Objects

Spot the “class” here

Constructors • Has the same name and as the class (very important - otherwise not a constructor) • The ONLY method that is allowed to be capitalized; and must be capitalized. • If you don't define a constructor, a default noarg constructor is implied that will set fields to zero, or null. • If you define any constructor, then the default constructor is not available to you.

Using anonymous objects • Anonymous objects are useful for "hardcoding" objects and passing them into Constructors. • The enclosing class will contain a reference to this object, so we can find it later.

Review of Eclipse

Important shortcuts in Eclispe Ctrl-space – materialize template code “. ” - when used on a class or object reference, this pulls up code-complete. Alt-shift-r – used to rename variable-names globally throughout the application Ctrl-shift-f – Format Refactor || Extract method() / Convert Local Variable to Field Source || Generate Window || Preferences || Java || Code Style || Formatter || Edit || On/Off Tags //@formatter: off //############ PSEUDOCODE ############# \${cursor} //############################## //@formatter: on Window || Preferences || Java || Code Style || Fields || Edit List: b, y, s, c, n, l, d, f, str

Console 21

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 intial 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 • • 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).

Packages • Package: a collection of classes with a related purpose • Import library classes by specifying the package and class name: import java. awt. Rectangle; • You don’t need to import classes in the java. lang package such as String and System Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Syntax 2. 4 Importinga Class from a Package Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Packages • Package: Set of related classes • Important packages in the Java library: Package Purpose Sample Class java. lang Language support Math java. util Utilities Random java. io Input and output Print. Stream java. awt Abstract Windowing Toolkit Color java. applet Applets Applet java. net Networking Socket java. sql Database Access Result. Set javax. swing Swing user interface JButton omg. w 3 c. dom Document Object Model for XML documents Big Java by Cay Horstmann Document Copyright © 2009 by John Wiley & Sons. All rights reserved.

Organizing Related Classes into Packages • To put classes in a package, you must place a line package. Name; as the first instruction in the source file containing the classes • Package name consists of one or more identifiers separated by periods Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Organizing Related Classes into Packages • 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 Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Importing. Packages • 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 don’t need to import other classes in the same package Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Package. Names • 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 ([email protected] sjsu. edu) • Path name should match package name: com/horstmann/bigjava/Financial. java Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Packageand Source Files • Base directory: holds your program's Files • Path name, relative to base directory, must match package name: com/horstmann/bigjava/Financial. java Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Self Check 8. 18 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 Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Java Doc • Generate default Java. Docs. • Using decorations/tags. • Project || Generate Java. Doc. – In Eclipse; navigate to JDKbinjavadoc. exe to configure. • Scope; public to private methods • Use F 1 to pull up your javadocs.

Self Check 8. 19 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. Big Java by Cay Horstmann Copyright © 2009 by John Wiley & Sons. All rights reserved.

Gregorian Calendar Example

Asciify Example (if we have time)

Adding examples code to javadocs /** * * @param per. Soldiers Array. List * @return Person object * *

` * Example: * per. Senior = get. Most. Senior(per. Vets); //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 str. First. Name; private String str. Last. Name; private byte y. Age; //-128 to 127 private boolean b. Veteran; private String str. Social. Security. Num; private Array. List per. Dependents;

Imports and the Java API • Determine the version of Java you’re using. From the cmd line> java –version • http: //download. oracle. com/javase/6/docs/api/ • java. 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; F 1 – JDKsrc. zip • To see source code; F 3