java - UFRJ

Breaking the Surface
Adriano Cruz
[email protected]
Instituto de Matemática
Departamento de Ciência da Computação
UFRJ
7 de agosto de 2014
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Section Summary
1
Introdução
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Historic
Java Alpha and Beta (1995).
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Historic
Java Alpha and Beta (1995).
Java 1.02 (1996) to Java 1.1 (1997) were just Java.
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Historic
Java Alpha and Beta (1995).
Java 1.02 (1996) to Java 1.1 (1997) were just Java.
Versions 1.2 (1998), 1.3 (2000) and 1.4 (2002) were Java 2.
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Historic
Java Alpha and Beta (1995).
Java 1.02 (1996) to Java 1.1 (1997) were just Java.
Versions 1.2 (1998), 1.3 (2000) and 1.4 (2002) were Java 2.
Versions 1.5 (2004) was called Java 5.0 (Java 5 or Tiger)
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Historic
Java Alpha and Beta (1995).
Java 1.02 (1996) to Java 1.1 (1997) were just Java.
Versions 1.2 (1998), 1.3 (2000) and 1.4 (2002) were Java 2.
Versions 1.5 (2004) was called Java 5.0 (Java 5 or Tiger)
Java 6 (2006), Java 7 (2011), Java 8 (2014)
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The Way Compiled Languages Work
#include<stdio.h>
int main (void) {
printf("Hello world.\n");
return 0;
}
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The Way Java Works
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World.");
}
}
J
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Compiled from "HelloWorld.java"
public class HelloWorld {
public HelloWorld();
Code:
0: aload_0
1: invokespecial #1;
4: return
public static void main(java.lang.String[]);
Code:
0: getstatic
#2;
3: ldc
#3;
5: invokevirtual #4;
8: return
}
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Detailing
Create a source code:
HelloWorld.java
Compile the source code using a Java compiler:
javac HelloWorld.java
The compiler generates a new document coded into Java bytecode.
Runs the compiled code using a Java virtual machine:
java HelloWorld
Byte code is common to all devices.
Each device must have its own Java virtual machine to interpret and
execute it.
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Code structure
Source Code
Class File
Put a class in a source file
Put methods in a class
method 1
Put statements in a method
statement
method 2
statement
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Structure - Class
p u b l i c c l a s s Dog {
}
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Structure - Method
p u b l i c c l a s s Dog {
v o i d bark () {
}
}
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Structure - Statements
p u b l i c c l a s s Dog {
v o i d bark () {
System . out . p r i n t l n( " Bark " ) ;
}
}
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What about the main?
p u b l i c c l a s s HelloWorld {
p u b l i c s t a t i c v o i d main ( String [] args ) {
System . out . p r i n t l n( " Hello World . " ) ;
}
}
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Anatomy of a Class
p u b l i c c l a s s MyFirstApp {
p u b l i c s t a t i c v o i d main ( String [] args ) {
System . out . p r i n t l n( " Hello World !) ;
}
}
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Hello World
Name of the class
public class MyFirstApp {
Name of the method
public static void main (String[] args) {
System.out.println("Hello World!");
}
public so everyone
can access it.
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Print to standard output
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Class with a main
In Java everything goes in a class.
You write a source file (HelloWorld.java) with a .java extension.
You compile the source file into a class file (HelloWorld.class)
with a .class extension.
Running a program means telling the JVM to ”load the class file”, the
start executing its main().
The main() method is where the programs starts running.
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Dumb questions?
Why does everything have to be in a class?
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Dumb questions?
Why does everything have to be in a class?
Java is an OO language....
Adriano Cruz (IM-DCC-UFRJ)
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Dumb questions?
Why does everything have to be in a class?
Java is an OO language....
Do I have to put a main in every class?
Adriano Cruz (IM-DCC-UFRJ)
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Dumb questions?
Why does everything have to be in a class?
Java is an OO language....
Do I have to put a main in every class?
No.
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A Serious Application
p u b l i c c l a s s BeerSong {
p u b l i c s t a t i c v o i d main ( String [] args ) {
i n t beerNum = 3;
String word = " bottles " ;
w h i l e ( beerNum > 0) {
i f ( beerNum == 1) {
word = " bootle " ;
}
System . out . println ( beerNum + " " + word + " of beer on the ←֓
wall " ) ;
System . out . println ( beerNum + " " + word + " of beer . " ) ;
System . out . println ( " Take one down . " ) ;
System . out . println ( " Pass it around . " ) ;
beerNum -= 1;
i f ( beerNum > 0) {
System . out . println ( beerNum + " " + word + " of beer ←֓
on the wall " ) ;
} else {
System . out . println ( " No more bottles of beer on the ←֓
wall . " ) ;
}
}
}
}
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A Problem!
The class BeerSong has little problem. It runs, but the output is
not 100% perfect. See if you can spot the flaw, and fix it.
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Phrase-O-Matic
p u b l i c c l a s s PhraseOMatic {
p u b l i c s t a t i c v o i d main ( String [] args ) {
String [] wordListOne = { " 24/7 " , " multi - Tier " , " 30 ,000 foot " , "B -to - B" ,
" win - win " , " front - end " , " web - based " , " pervasive " , " smart " , "six - sigma " ,
" critical - path " , " dynamic " };
String [] wordListTwo = { " empowered " , " sticky " , " value - added " , " oriented " ,
" centric " , " distributed " , " clustered " , " branded " , " outside -the - box " ,
" positoned " , " networked " , " focused " , " leveraged " , " aligned " ,
" targeted " , " shared " , " cooperative " , " accelerated " };
String [] wordListThree = { " process " , " tipping - point " , " solution " ,
" architecture " , " core competency " , " strategy " , " mindshare " , " portal " ,
" space " , " vision " , " paradigm " , " mission " };
i n t oneLength = wordListOne . length ;
i n t twoLength = wordListTwo . length ;
i n t threeLength = wordListThree . length ;
i n t rand1 = ( i n t ) ( Math . random () * oneLength ) ;
i n t rand2 = ( i n t ) ( Math . random () * twoLength ) ;
i n t rand3 = ( i n t ) ( Math . random () * threeLength ) ;
}
}
String phrase = wordListOne [ rand1 ] + " " + wordListTwo [ rand2 ] + " " +
wordListThree [ rand3 ] + " . " ;
System . out . println ( " What we need is " + phrase ) ;
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The End
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