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Curs 2 Storage types Tablouri Clase Java –Membri, Constructori,... Membri statici – Singleton Pattern
Curs 2
Storage types Tablouri Clase Java –Membri, Constructori,...
Membri statici – Singleton Pattern
Where storage lives
Registers: (fastest storage) are place different inside the processor The number of registers is severely limited You don’t have direct control
The stack: lives in the general RAM area has direct support from the processor via its stack pointer The stack pointer is moved down to create new memory and
moved up to release that memory extremely fast and efficient way to allocate storage object references and primitive values
The heap: a general-purpose pool of memory (also in the RAM area) important flexibility in using storage on the heap it takes more time to allocate heap storage than it does for the stack Java objects are placed on the heap
Static storage: “in a fixed location” (it’s also in RAM). static data are available for the entire time a program is running
Constant storage: constants are often placed directly in the program code
Non-RAM storage: streamed objects, persistent objects
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Memory Segments
The Von Neumann architecture is defined by the fact that both the machine
instructions and data are stored in the same memory space.
This fact necessitates dividing memory into
text segment, and
data segment.
In modern operating systems, there are more than just these two segments.
Most systems use a single text segment, but actually use up to three data
segments, depending on the storage class of the data being stored there.
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Code & Data segments
The code segment, also known as text segment contains the machine
instructions of the program.
The data segment contains the static data of the program, i.e. the variables
that exist throughout program execution.
Global variables in a C or C++ program are static, as are variables
declared as static in C, C++, or Java.
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Stack DrawSquare()
{
[...]
DrawLine ();
[...]
}
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Size of the segments
The heap segment is a pool of memory used for dynamically allocated memory, such as with malloc() in C or new in C++ and Java.
The size of the text and data segments are known as soon as compilation or assembly is completed.
The stack and heap segments, on the other hand, grow and shrink during program execution.
For this reason, they tend to be configured such that they grow toward each other.
This way, they each have a fixed starting point (one adjacent to the text and data segments, and the other at one end of the entire memory space), and the boundary between them is flexible.
Both can grow until all available memory is used.
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Reprezentare
+---------------+
| |
| text | (fixed size)
| |
+----------------+
| |
| data | (fixed size)
| |
+----------------+
| stack | | growth
+----------------+ V
| |
| free |
| |
+----------------+ ^
| heap | | growth
+----------------+ http://www.cs.uwm.edu/classes/cs315/Baco
n/Lecture/HTML/ch10s04.html
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JAVA
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Tipuri de date referinta
Tipurile Java neprimitive sunt obiectele si tablourile
( tablourile sunt de fapt obiecte).
Tipurile Java neprimitive sunt numite in general “tipuri referinta" pentru ca
se utilizeaza prin “ referinta”
(adresa obiectului sau tabloului este stocata intr-o variabila, transmisa
metodelor, etc.).
Tipurile primitive sunt “utilizate prin valoare“
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Nu sunt pointeri in Java!
Referentierea si dereferentierea obiectelor se face automat.
Java nu permite manipularea pointerilor sau a adreselor de memorie:
NU este permisa transformarea prin cast a unei referinte la obiect sau la
tablou la intregi si vice-versa.
NU este permisa aritmetica cu adrese.
NU este permisa calcularea marimii reprezentarii in octeti pentru
nici un tip primitiv sau obiect.
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null
Valoarea implicita pentru variabilele de tip referinta este null.
null este un cuvant rezervat si corespunde unei valori care indica faptul
ca variabila nu refera nici un obiect.
(in C NULL este doar o constanta egala cu 0)
null nu poate fi transformat prin cast la nici un tip primitiv. Nu trebuie sa
fie considerat a fi egal cu zero desi ar putea fi implementat in acest fel.
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Exemplu
Complex p, q;
p = new Complex();
// p refers to a Complex object.
q = p;
// q refers to the same object of type Complex.
p.setReal(10.2);
// A change to the object through p...
double r = q.getReal();
// ...is also visible through q. // r now contains 10.2
int i = 3;
// i contains the value 3.
int j = i;
// j contains a copy of the value in i.
i = 2;
// Changing i doesn't change j.
// Now, i == 2 and j == 3.
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Exemplu –swap - eronat!
public void swap( Object a, Object b)
{
Object temp = a;
a = b;
b = temp;
}
Parametrii se transmit prin valoare!
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Copierea Obiectelor
Complex a = new Complex(1,1);
Complex b = new Complex();
a = b;
variabila a contine o referinta la obiectul pe care b il refera. Obiectul pe care a il referea este pierdut.
Pentru a copia un obiect se foloseste metoda clone():
Vector b = new Vector();
Vector c = b.clone();
Pentru a se putea folosi metoda clone() clasa respectiva trebuie sa implementeze interfata Cloneable.
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Verificarea egalitatii
Operatorul == testeaza daca doua variabile refera acelasi obiect, si nu daca doua obiecte au
aceeasi stare.
Metoda equals() se poate folosi pentru a testa egalitatea.
Daca o clasa are atribute de tip referinta, metoda equals() ar trebui suprascrisa!
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Tablouri
Tablourile se utilizeaza folosind referinte.
Sunt create dinamic (new); ca si obiectele: -> clasa Array
Garbage collecter-ul elibereaza memoria corespunzatoare lor atunci cand nu mai sunt referite.
byte octet_buffer[] = new byte[1024];
Complex tc[] = new Complex[10];
Elementele unui tablou se initializeaza cu valoarea implicita a tipului lui.
(deci pentru obiecte = null)
=>Creare unui tablou cu elemente de tip obiect nu implica si crearea obiectelor constitutive.
for (int i=0; i<10; i++) tc[i] = new Complex(i, i);
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Tablouri(2)
creare si initializare tablou
int lookup_table[] = {1, 2, 4, 8, 16, 32, 64, 128};
creare anonima -> parametrii
Menu m = createMenu("File",
new String[] {"Open...", "Save", "Quit" });
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Tablouri multidimensionale
se implementeaza ca tablouri de tablouri ca si in C
byte TwoDimArray[][] = new byte[256][16];
int threeD[][][] = new int[10][][];
double temperature_data[][][] = new double[100][][10]; // illegal
String param_info[][] = { {"foreground", "Color", "foreground color"}, {"background", "Color", "background color"}, {"message", "String", "the banner to display"}};
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Tablouri multidimensionale nerectangulare
short triangle[][] = new short[10][];
// Tablou uni- dimensional cu elemente de tip tablou
for(int i = 0; i < triangle.length; i++) {
// pentru fiecare element al tabloului
triangle[i] = new short[i+1];
// se aloca un nou tablou
for(int j=0; j < i+1; j++)
// fiecare element al noului tablou
triangle[i][j] = (short) i + j;
// se initializeaza cu o valoare
static int[][] twodim =
{{1, 2}, {3, 4, 5}, {5, 6, 7, 8}};
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Exemple public class ex_tablou {
public static void main(String[] args) {
int[][] multi = new int[][]{
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
};
int note[][] = new int[3][5];
for (int i=0; i<3;i++)
for (int j=0; j<5;j++)
note[i][j]=10;
for (int[] e: note ){
for (int ee: e )
System.out.print(ee+" ");
System.out.println();
}
}
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Variabile si parametrii de tip tablou
C vs. Java
void reverse(char strbuf[], int buffer_size) { char buffer[500];
...}
void reverse(char[] strbuf, int buffer_size) { char[] buffer = new char[500];
...}
lungimea unui tablou nu face parte din tip ca in C
String[] strings;
// this variable can refer to any String array
strings = new String[10];
// one that contains 10 Strings
strings = new String[20];
// or one that contains 20.
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Clase
- atribute/campuri
- metode
Modificatori de acces:
- private
- public
- protected
- “friendly” (fara specificator)
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Constructori
Metode speciale
rol esential in crearea obiectelor!
cel putin un constructor la fiecare clasa!
acelasi nume cu clasa!
nu returneaza nimic!
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UML
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24
UML limbaj (standard) folosit pentru
specificare
vizualizare
constructie
documentare
sistemelor software
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25
UML
se pot specifica entitatile/conceptele care intervin intr-o aplicatie orientata-
obiect, precum si relatiile dintre ele.
reprezentare grafica
private “-”
public “+”
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S
-n: int
-e: E[]
+S( )
+card( ): int
+add( x: E)
+belong( x: E): int
+getElem(i:int):E
Exemplu: multime de elemente de tip E
Varianta 1 (nu e riguros corecta – dar acceptata)
Java dependenta
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S
-n: Integer
-e : E[*]
+S ( ) <<constructor>>
+card ( ): Integer
+add ( x: E)
+belong (x: E): Integer
+ getElem(i: Integer):E
Variant 2 (corecta)
(tipuri UML )
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Tipuri UML
Integer
Real
Character
String
Boolean
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Diagrama de clasa
Strings
Sunt obiecte, instante ale clasei java.lang.String or java.lang.StringBuffer
Obiectele String sunt imutabile (nemodificabile)
Obiectele StringBuffer sunt mutabile (modificabile)
Cateva metode length charAt equals compareTo indexOf lastIndexOf substring
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Exemplu
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clasa Circle
Clasa cu constructor implicit
public class Circle {
public double x, y;
// The coordinates of the center
public double r;
// The radius
// Methods that return the // circumference and area of the circle
public double circumference() {
return 2 * 3.14159 * r;
}
public double area() {
return 3.14159 * r*r;
}
public static void main(String args[]){
Circle c = new Circle();
}
}
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Constructor
class Circle{
// . . .
public Circle(double x, double y, double r) {
this.x = x;
this.y = y;
this.r = r;
}
public static void main(String args[]){
Circle c = new Circle(10.0,10.0,20);
// Circle c=new Circle(); incorect
}
}
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Mai multi constructori
public class Circle {
public double x, y, r;
public Circle(double x, double y, double r) {
this.x = x; this.y = y; this.r = r; }
public Circle(double r) {
x = 0.0; y = 0.0; this.r = r; }
public Circle(Circle c) {
x = c.x; y = c.y; r = c.r; }
public Circle() {
// x = 0.0; y = 0.0; r = 1.0; }
public double circumference() {
return 2 * 3.14159 * r; }
public double area() {
return 3.14159 * r*r; }
}
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Apelul unui constructor in alt constructor
public class Circle {
public double x, y, r;
public Circle(double x, double y, double r) {
this.x = x; this.y = y; this.r = r;
}
public Circle(double r) {
this(0.0,0.0,r);
}
public Circle(Circle c) {
this(c.x, c.y, c.r);
}
public Circle() {
// x = 0.0; y = 0.0; r = 1.0;
}
public double circumference() {
return 2 * 3.14159 * r; }
public double area() { return 3.14159 * r*r; }
}
Regula: apelul altui constructor = prima instructiune
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Variabile de tip clasa (statice)
public class Circle {
public double x, y, r;
public static int num_obj;
public Circle(double x, double y, double r) {
this.x = x; this.y = y; this.r = r;
num_obj++;
}
// ...
// cate obiecte au fost create!!!
public static void main(String args[]){
System.out.println(Circle.num_obj);
Circle c = new Circle(10.0,10.0,20);
System.out.println(Circle.num_obj);
}
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Observatii:
1. Datele membru statice nu se memoreaza ca stare a obiectului.
2. Unei date membru statice i se aloca memorie o singura data
indiferent de numarul obiectelor de tipul clasei respective existente
in memorie.
3. Referirea datelor membre statice se face astfel:
NumeClasa.numeDataMembru sau
numeObiect.numeDataMembru.
4. Daca o data membru statica nu este initializata explicit, atunci ea va fi
initializata cu valoarea implicita corespunzatoare tipului sau.
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Initializator static
public class Circle {
static private double sines[] = new double[1000];
static private double cosines[] = new double[1000];
// A static initializer "method" that fills them in.
// Notice the lack of any method declaration!
static {
double x, delta_x; int i;
delta_x = (Math.PI/2)/(1000-1);
for(i = 0, x = 0.0; i < 1000; i++, x += delta_x) {
sines[i] = Math.sin(x);
cosines[i] = Math.cos(x);
}
} // . . .
}
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Metode de tip clasa (statice)
public class Circle {
public double x, y, r;
// An instance method.
public Circle bigger(Circle c) {
if (c.r > r) return c;
else return this;
}
// A class method.
public static Circle bigger(Circle a, Circle b) {
if (a.r > b.r) return a;
else return b;
}
// . . .
public static void main(String args[]){
Circle c1 = new Circle(10.0,10.0,20);
Circle c2=new Circle();
if ( c1.bigger(c2)==c1 )
System.out.println(“c1 is bigger than c2”);
if ( Circle.bigger(c1, c2)==c1 )
System.out.println(“c1 is bigger than c2”);
}
}
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Observatii:
1. Metodele de tip clasa (statice) se declara folosind atributul static in fata.
2. O metoda statica nu poate folosi direct membrii nestatici ai clasei (atribute sau
metode)!
3. Metodele statice pot apela doar alte metode statice si pot referi doar atributele
statice.
4. In implementarea unei metode statice nu se poate folosi cuvantul this.
(... Ce inseamna this?)
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Singleton pattern
se permite instantierea unui singur obiect pentru o clasa
implementarea se bazeaza pe folosirea
membrilor statici si
constructor privat
class Sunic{
private static Sunic instance;
private Sunic(){}
public static Sunic getInstance(){
if (instance==null) instance = new Sunic();
return instance;
}
}
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