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Java Class Structure

Code style in the exam

Don't expect best practices
Really bad code there
The International Obfuscated C Code Contest: http://www.ioccc.org/
- http://www.ioccc.org/2015/dogon/prog.c

#define So long
#define R rand()
#include <math.h>
#include <X11/Xlib.h>
#define T(i,F) ((So long)(i)<<F)
#define O(c,L,i) c*sin(i)+L*cos(i)
#define y(n,L) for(n=0; n<L 3; n++)     
#define P(v,L) d=0; y(l,)d+=T(L*l[v],l*20);
#define X(q) q>>10&63|q>>24&4032|q>>38&258048       
char J[1<<18]; int G[W*p],_,k,I=W/4+1,w=p/4+1; float C,B,e;            

unsigned So long A,n,d, t,h,x, f,o,r,a,l,L,F,i,s,H=1<<18,b=250,D[1<<14],z[W*p],q
=0x820008202625a0;main(){Display *j=XOpenDisplay(0);Window u=XCreateSimpleWindow
(j,RootWindow(j,0),0,0,W,p,1,0,0);XImage *Y=XCreateImage(j,DefaultVisual(j,0),24
,2,0,(char*)G,W,p,32,0); XEvent M; for(XMapWindow(j,u); XSelectInput( j,u,1)&&a-
65307; ){ if(!H){ if(XCheckWindowEvent(j,u,1,&M)){ a=XLookupKeysym(&M.xkey,0);*(
a&1?&C:&                                                                B)-=(.05
-a/2% 2*                                                                .1)*!(a-
1& 4092^                                                                3920);a+
2&0xfe0^                                                                0xfc0||(
s=a+2&31                                                                ); }else
{ y(k,p+        ){ F=k%w* 4|k/w;               float a[6],S=(F-p        /2.) /p;
y(_,W+){        i=_%I*4|_/I; if(               F<p&i<W){ o=1; L=        i+F*W;if
(l=i&3);        else{ l=F&3; o=W               ; } h=z[L-o*l]; f        =z[L+(4-
l)*o]; t        =F-p/2||i-W/2; r               =h^f; if(!l| !t|(        int)r|(!
(h- 3&3)        &&258063&r>>38))               { float V=(i-W/2.        )/p,U=O(
S,1,B),m        =32768,Q=m; a[4]               =O(-1,S,B); a[3]=        O(U,V,C)
; a[5]=O        (-V,U,C); P((a+3               ),s*42); t||(A=d)        ;f=0;y(n
,){float        N=a[n+3], E=1024               /fabs(N); b= N<0;        float K=
(((q>>20        *n)^~-b)+!b&1023               )/1024.; y(d,)a[d        ]=a[d+3]
*E; a[n]        =round(a[n]); P(               a,K); i=q+d; P(a,        1); e=E*
K; for(;                e<m; i+=d){ l=X(i); t=r=l^(l^l-(                1<<6*n))
&63<<6*n                ; if(b){ r=l; l=t; } if(J[r])l=r                ; if(t=J
[l]){x=(                n-1)?(i|i>>40)&1023|i>>8&4190208                |4194304
:i&1023|                i>>28&4190208|(b^l==r)<<23; if(h                =D[(x>>6
&0xf|x>>                14&0x3f0)+t*768]){ o=h; f=n|l*4|                x<<32; m
=e; } if                (t==8&e<Q)Q=e; } e+=E; } }b=(255                -((f&3)+
2)%3*51)                *(1-m/32768); o=o*b>>8; G[L]=o>>                32<<8|o&
16711935                ; z[L]=3*(Q<=m)|f|b<<56; } else{                d=l*(f<<
8>>40)+(                4-l)*(h<<8>>40)>>2&16774143; o=D                [(d>> 6&
15|d>>14                &1008)+J[(int)h/4]*768]*(b=h>>56                )>>8; G[
L]=o>>32                <<8 | o&                16711935                ; z[L]=(
int) h|d                <<32|b<<                56; } }}                } q +=A;
XPutImage               (j, u+0,               DefaultGC                (j,J[X(q
)]=0),Y,                0,0,0,0,                W,p); }}                else{ L=
--H/768;                J[H] =R%                16*(R%4<                abs((H>>
6&63)-32                                                                )+abs((H
>>12&63)                                                                -32)-3); 
i=H &15;                                                                F=H %768
>>4; if(                                                                L<16){if
(L-1|!(R                                                                %3))b=R%
96^255; l=i*i*3+i*81/4&3; a=L>3?L-8?L-5?9858122:12365733-488848*((i+F/4*4)%8&&F%
4):R%2*5298487:3352537*L*L-14202379*L+19205553; if(L==4)if(F<l+18)a=6990400;else
if(l>F-19)b*=0.7; if(L==3){ if((i-1&15)<14&(F-1&15)<14&!(F&16)){ a=12359778; _=7
-i; k=7-F%16; _^=_>>31; k^=k>>31; b=196-R%32+(k>_?k:_)%3*42;} else{ b*=1+R%2*(.5
-(i&1)); } } D[H]=(a&16711935|(a&65280)<<24)*(b>>(F>>5))>>8&0xff00ff00ff; } } }}

Classes vs. Objects

Class: building blocks. Template to create objects
Objects: runtime instances of classes
State: all objects of all our classes

Class members

Fields: class-level variables
Methods: functions, procedures
Method signature:

access modifier return type methodName(parameter type parameter name, ...)

Valid class

public class Person {
	
}

...

class House {
	Person owner;
	int rooms;
	void cleanRoom(int roomNumber) {}
}

Java building blocks

Classes
Interfaces
Enums (OCP)

😱 Spot errors

public class Main {
    public static void main(String[] args) {
        new House().rooms();
    }
}

class House {
    String ownerName;
    int rooms;
    void cleanRoom(int roomNumber) {
        roomNumber = rooms;
    }
    int Rooms;
    int rooms() {}
}

😱 Missing return statement

Easier with an IDE & compiler, right? :-D

Comments

// one line Comments

/*
Multiline comment
*/

/*
 * Fancy Multiline comment
 *
 */
 
/**
 *
 * Java Doc comment (NOT IN THE EXAM)
 */

😱 Spot errors

// hello // world

/* // hello world */

/*
/*
// Hello
// World
 */
 */

Rules .java source files

one public class in every file, at most
if there's one public class -> file name same as class
as much non-public classes as we want

😱 Spot errors

// Wolf.java

class Wolf {
}

class wolf {
    
}

😱 Spot errors

// House.java

public class House {
}

class Person {

}

class Monkey {

}

Main method

Entry point to our program
Java Runtime injects parameters

public static void main(String[] args) {

}

Main method II

All valid

public static void main(String[] main) {
	// write your code here
}

public static void main(String... main) {
	// write your code here
}

public static void main(String m[]) {
	// write your code here
}

😈 Watch out

public void main(String m[]) {
	// write your code here
        System.out.println("Hello");
}

Error: el método principal no es static en la clase Main, 
defina el método principal del siguiente modo:
   public static void main(String[] args)

😱 Spot errors

public class Main {
    public static void main(String args) {
        new House().rooms();
    }
}

class House {
    String ownerName;
    int rooms;
    void cleanRoom(int roomNumber) {
        roomNumber = rooms;
    }
    int Rooms;
    int rooms() { return rooms; }
}

😱 Spot errors

public class Main {
    public static void main(String args) { // array of Strings!
        new House().rooms();
    }
}

class House {
    String ownerName;
    int rooms;
    void cleanRoom(int roomNumber) {
        roomNumber = rooms;
    }
    int Rooms;
    int rooms() { return rooms; }
}

Exercise: print all parameters from command line

if its a number?

JRE vs JDK

JRE: to run Java programs
JDK: to compile Java programs
JDK contains a JRE in order to run (test) programs
- inside the JDK folder there's a JRE with java command
- javac & other tools, only in JDK

Compiling from command line

$ javac -version
$ java Main

Compile & run classes by hand. Read page 14

Imports

PIC: Package, Imports, Classes
import java.lang.*; by default
import one class or all classes in that package (wildcard)
non recursive
package name: lowercase legal identifiers separated by .

😈 Watch out

Redundant

import java.lang.Exception;
import java.lang.*;

Error

import java.lang..;

Packages as namespaces

avoid conflicts with two classes, same name
FQCN: Fully Qualified Class Name
default package == no package

😈 Watch out

Error!

import java.sql.Date;	// try with *
import java.util.Date;

public class Main {
    public static void main(String m[]) {
        Date d;
    }
}

Exercise

create our own packages, compile from command line
launch from command line java package.ClassWithMainMethod

Constructors

methods to initialize objects after creation
creation in three phases
Stack (references), Heap (Objects, References)

😈 Watch out

public class A {
    String a = "Wow";
    
    public A() {
        a = "much A's";
    }
    
    public void A() {
        
    }

    public static void main(String m[]) {
        A a = new A();
    }
}

Using object fields (get & set values)

public class A {
    String a = "Wow";
    String s = a + " much String!";    // reading field

    public static void main(String m[]) {
        A a = new A();
        
        a.a = "Hello";                  // setting field

        System.out.println(a.a);        // reading field
    }
}

Instance initializer blocks

public class Doge {
    String meme = "Wow!";

    {
        meme += "Much instance initializer blocks ";
    }

    public Doge() {
        meme += " so exciting";
    }

    public static void main(String[] args) {
        Doge d = new Doge();

        System.out.println(d.meme);
    }

    {
        meme += " very confusing!";

    }
}

Instance initializer blocks

[fit] WAT

Instance initializer blocks

only for people who doesn't know to chain constructors
to make code confusing
to build objects without using constructors

Java Reserved words

abstract	boolean	     break	    byte	     case	    catch
char	    class	     const	    continue	 default	do
double	    else	     extends	final	     finally	float
for	        goto	     if	        implements	 import	    instanceof
int	        interface    long	    native	     new	    package
private	    protected    public	    return	     short	    static
strictfp	super	     switch	    synchronized this	    throw
throws	    transient	 try	    void	     volatile	while
assert	    enum

Misterios

Transient: no se serializa esa propiedad (y así no se graba a disco / envia por la red)
Volatile: puede ser accedida por múltiples hilos: está sincronizada
Strictfp: compatibilidad operaciones floating point ¹
Goto: reservado ²

Tipos primitivos vs. Clases Wrapper

boolean Boolean byte Byte char Character double Double float Float int Integer long Long short Short

Identificadores

Legales: los que compilan
Convenciones de código de Sun
Estándares de nombrado de JavaBeans

Convenciones de nombres Sun

Clases: 1ª mayúscula + CamelCase
Interfaces: 1ª mayúscula + CamelCase
atributos: 1ª minúscula + CamelCase
métodos: 1ª minúscula + CamelCase
constantes: CONST_NAME

JavaBeans naming standars

JavaBean:
- constructor público sin argumentos
- propiedades privadas
- get / set / is (boolean)
- public void setProp(PropTipo val)
- public PropTipo getProp();
- Listeners:
  - add / remove Listener

Identificadores: reglas

empiezan por letra [A-Za-z], $, _
NO empiezan por número
desp. del 1er. carácter, cualquier combinac de letras, números y $,_
case sensitive
no se pueden usar las palabras clave de Java

Identificadores (test)

int $$;	
int $___10;	
int 1niesta;	
int :hola;	
int hola:;	
int otro_identificador$;	
int e$te_e$ta_mal;	
int pero-Este-Bien;

Reference types

reference: pointer
references: variables en la pila, apuntan a objetos en el heap
Stack:
- variables locales
- parámetros
Heap:
- variables de instancia (iVars)
- objetos

String name = null;

Declarando variables

declarar variables tipos simples
- no podemos usar null con tipos simples
declarar variables referencias

Declarando variables

int i = null;

String s1, s2, s3;

s1 = new String("Hello");
s2 = "World!";

MiClase c = new MiClase();

instancia un nuevo objeto en el montón
lo inicializa
lo liga a la referencia c (que es una var. local, luego está en la pila)

Variable initialization (rules)

local vars
fields
- iVars: se establecen a un valor por defecto
- primitivos
- referencias a objetos
- arrays
class variables

Blocks of code & variable scope

class
method
if, while, switch, ...

int i = 10;

{
    int i = 11;     // error: redefining var
    int j = 10;     // only visible inside block
}

alcance de variables

static
iVars
local / automatic
block

Order of elements

PIC: Package, Imports, Classes
methods, fields: everywhere

Destroying objects

Garbage collector
finalize()
- not guaranteed to be called
jconsole

Benefits of Java

OO
Encapsulation
Platform independent
Robust
Simple
Secure

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Java Class Structure

Code style in the exam

Classes vs. Objects

Class members

Valid class

Java building blocks

😱 Spot errors

😱 Missing return statement

Easier with an IDE & compiler, right? :-D

Comments

😱 Spot errors

Rules .java source files

😱 Spot errors

😱 Spot errors

Main method

Main method II

All valid

😈 Watch out

😱 Spot errors

😱 Spot errors

Exercise: print all parameters from command line

JRE vs JDK

Compiling from command line

Imports

😈 Watch out

Packages as namespaces

😈 Watch out

Exercise

Constructors

😈 Watch out

Using object fields (get & set values)

Instance initializer blocks

Instance initializer blocks

[fit] WAT

Instance initializer blocks

Java Reserved words

Misterios

Tipos primitivos vs. Clases Wrapper

Identificadores

Convenciones de nombres Sun

JavaBeans naming standars

Identificadores: reglas

Identificadores (test)

Reference types

Declarando variables

Declarando variables

Variable initialization (rules)

Blocks of code & variable scope

alcance de variables

Order of elements

Destroying objects

Benefits of Java

Examples of Encapsulation & Coupling

Footnotes