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Strings
Useful methods of String
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•
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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
• 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!
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.
• Using .equals() rather than ==.
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, 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"
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.
Project || Generate Javadoc || path to the bin of the
sdk
Calling Static Methods
• 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
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.
Static Methods
• 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);
Syntax 4.3 Static Method Call
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley &
Sons. All rights reserved.
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.
Static Variables
• A classs need not be exclusively static or non-static, it can be both.
Modifier | Class | Package | Subclass | World
————————————+———————+—————————+——————————
+———————
public | ✔ | ✔ | ✔ | ✔
————————————+———————+—————————+——————————
+———————
protected | ✔ | ✔ | ✔ | ✘
————————————+———————+—————————+——————————
+———————
no modifier | ✔ | ✔ | ✘ | ✘
————————————+———————+—————————+——————————
+———————
private | ✔ | ✘ | ✘ | ✘
Static Variables
• 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
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.
Static Variables
• 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
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley &
Sons. All rights reserved.
Class
Objects
Self Check 8.15
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.
Use static appropriately and parsimoniously.
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.
Constants
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
final
final
final
double
double
double
double
QUARTER_VALUE = 0.25;
DIME_VALUE = 0.1;
NICKEL_VALUE = 0.05;
PENNY_VALUE = 0.01;
payment = dollars + quarters * QUARTER_VALUE
+ dimes * DIME_VALUE + nickels * NICKEL_VALUE
+ pennies * PENNY_VALUE;
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley &
Sons. All rights reserved.
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).
The API Documentation of the Standard Java Library
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley &
Sons. All rights reserved.
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
Document
Big Java by Cay Horstmann
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.
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.
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 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
object’s 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
implements Comparable 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).
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
Big Java
by Cay
Horstmann
JButton
Copyright © 2009 by John Wiley &
Sons. All rights reserved.
Document Object Model for XML
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
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
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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