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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 & ArrayList<E>
Driver, Bean, Utility
Reference versus variable
MyGregCalendar
The new keyword
Implicit versus explicit params
Return mutable reference member cloned
Static fields such as Math.PI
passByVal, passByRef, swap
JavaDocs
Control Structures
See edu.uchicago.cs.java.lec02.control
Shorthand
See blackboard
Arrays & ArrayLists
See edu.uchicago.cs.java.lec02.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[] dDatas = 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[] dValues = {2.5, 8.1, 3.79, 0, 6.0};
The above declares and intializes an array of double[]
String[] strNames = {"Harry", "Lary", "Mary",
"Perry"};
The above declares and initializes an array of String[]
Arrays
Use [] to access an element:
double[] dValues = new double[10];
dValues[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 "
+ dValues[2]); //dValues[2] (a double) is promoted to
a
String.
• Get array length as dValues.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[] accountNumbers;
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:
BankAccount[] accounts;
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
multidimensional arrays
• int[][] nNumbers = new int[3][4];
• boolean[][] bExams = {
new boolean[6],
new boolean[9],
new boolean[3],
new boolean[8]
}; //ragged array
Array Lists
• ArrayList class manages a sequence of objects
• Can grow and shrink as needed (it's a Vector)
• ArrayList class supplies methods for many common tasks, such
as inserting and removing elements
• ArrayList is a generic class:
ArrayList<T>
collects objects of type parameter T:
ArrayList<String> strNames = new ArrayList<String>();
strNames.add("Emily");
strNames.add("Bob");
strNames.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 strName = strNames.get(2);
// gets the third element of the array list
• Bounds error if index is out of range
• Most common bounds error:
int n = strNames.size();
strName = strNames.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:
strNames.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:
strNames.remove(1);
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Adding and Removing Elements
strNames.add("Emily");
strNames.add("Bob");
strNames.add("Cindy");
strNames.set(2, "Carolyn");
strNames.add(1, "Ann");
strNames.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.lec02.cast
Promotion & Casting
• Automatic promotion;
operand is of lesser
other and completing
not reduce precision
this occurs when one
precision than the
the operation will
of result.
double dResult = nNumerator / dDenominator;
In the above case, nNumerator is promoted to
a double.
int nResult = (int)(nNumerator /
dDenominator);
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 dBalance = dTotal + dTax; //dBalance == 10.89
int nDollars = (int) dBalance; //nDollars == 10
• Math.round converts a floating-point number to nearest integer:
long lRounded = Math.round(dBalance);
// if dBalance is 13.75, then lRounded 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.parseInt(String str);
Double.parseDouble(String str);
Byte.parseByte(String str);
Boolean.parseBoolean(String str);
etc.
The New Keyword
See edu.uchicago.cs.java.lec02.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 percentOf(double dP, double dA)
{
return (dP / 100) * dA;
}
// More financial methods can be added here.
}
• Call with class name instead of object:
double dTax = Financial.percentOf(dTaxRate, dTotal);
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.getWidth() * rec.getHeight();
}
// 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.
• strName.indexOf(cSpace);
• 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 dY = mthObject.sqrt(); // Error
double dY = 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.
Random rnd = new Random();
rnd.nextInt(10); //correct
Random.nextInt(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
nX?
Answer:
Math mat = new Math();
double dResult = mth.sqrt(nX);
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 BankAccount
{
...
private double dBalance;
private int nAccountNumber;
private static int nLastAssignedNumber = 1000;
}
• If nLastAssignedNumber was not static, each instance of
BankAccount would have its own value of
nLastAssignedNumber
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 BankAccount
{
...
public static final double OVERDRAFT_FEE = 5;
// Refer to it as BankAccount.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;
dPayment = nDollars + nQuarters * QUARTER_VALUE
+ nDimes * DIME_VALUE + nNickels * NICKEL_VALUE
+ nPennies * 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 can 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 charAt(int index)
int compareTo(String anotherString)
boolean endsWith(String suffix)
int indexOf(multiple)
int length()
String substring(int begin, int end)
String trim()
String is Immutable
• 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!
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 strName = “Robert”; we are actually doing this:
String strName = new String(“Robert”);
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley &
Sons. All rights reserved.
Concatenation
• 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, the other is
converted to a string
String strA = "Agent”;
int n = 7;
String strBond = strA + n; // strBond is "Agent7"
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley &
Sons. All rights reserved.
Substrings
• String str2 = strGreeting.substring(7, 12); // str2 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:
• strOne = "\"Quotation\"";
• to print \\
• StrBackSlashes = "\\\\";
• Unicode values
http://oreilly.com/actionscript/excerpts/as3cookbook/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)
\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();
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 nQuantity = scn.nextInt();
• nextDouble reads a double
• nextLine 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.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.
Pass by value and reference
See edu.uchicago.cs.java.lec02.passby
pass by value
pass by reference
Action: Tell my accountant how much
I intend to spend on a new car.
Action: Swipe debit card and enter
pin at the Bently dealership.
Change in bank account: no change.
Change in bank account: -125k.
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; 32bits in 32-bit VM,
and 64bits in a 64-bit VM.
Garbage collected when out-of-scope.
Garbage collected when out-of-scope.
Passed into methods by value
Passed into methods by reference
Objects
See edu.uchicago.cs.java.lec02.console21
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
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 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 Importing a 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
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
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 packageName;
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])
• 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.
Package and 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 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.
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 perSoldiers ArrayList<String>
* @return Person object
*
* <pre>
* Example:
* perSenior = getMostSenior(perVets); //will return a reference to Person
* </pre>
*/
Use the <pre> code here </pre> 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<Person> perDependents;
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; F1
– JDK\src.zip
• To see source code; F3