Java Building blocks

p. 38

• Primitive Data Types

Creating Objects

instanciate a class:
Park p = new Park(); p stores the refe to object
constructor:
- the name of the constructor matches the name of the class. - NO return type. purpose of a constructor is to initialize fields.

initialize on line
or
initialize in constructor

public classs Chick {
    public Chick(){
        // do something..
    }
}

public classs Chick {
    public void Chick(){} // NOT A CONSTRUCTOR (return type...)
}

Volgorde init telt

• Order of appearance!
• constructor runs after all fields and instance init blocks have run!

Werkt niet

public class ObjectsInits {
    public static void main(String[] args) {
        Name n = new Name();
        System.out.println(n.full);
    }
}

class Name {
    String full = first + " " + last; // first en last zijn er nog niet... Illegal forward reference
    String first = "hj";
    String last = "hubeek";

}

Werkt wel

class Name {
    String first = "hj";
    String last = "hubeek";
    String full = first + " " + last; // nu wel ...
}

class Name {

    String first = "hj";
    String last = "hubeek";
    String full = first + " " + last; // speel met hier of hierboven plaatsen... spoiler hierboven gaat t mis ivm illegal forward referrence
}
Name n = new Name();
System.out.println(n.full);

hj hubeek

blocks in classes

gaan op volgorde van de code
dan constructor!!!

Niet correcte open en closing braces => wont compile
balanced parentheses problem

count blocks => tel de {}
count instance initializers NIET IN METHOD!!!

class BlockesInMyClass{
    String name;
    {System.out.println("vanuit block boven constructor...");}
    BlockesInMyClass() {
        System.out.println("vanuit constructor...");
        this.name = "hjh";
    }
    {System.out.println("vanuit block onder constructor...");}
    public void func(){
        // do something...
        {System.out.println("block in func...");}

    }
}

BlockesInMyClass b = new BlockesInMyClass();
System.out.println(b.name);
b.func();

vanuit block boven constructor...
vanuit block onder constructor...
vanuit constructor...
hjh
block in func...

2 Types: Primitive & Reference Types

8 primitive types
a primitive is just a single value in memory
String is NOT a primitive maar object

.MAX_VALUE float en double hebben ook .POSITIVE_INFINITY and NaN

alle numeric types zijn SIGNED dus 1 bit voor negative value

range kennen van byte -128 tot 127

char is UNSIGNED

char dus geen negative waarden

char kan dus (2x) grotere waarden bevatten dan short

short d = 'd';
char mammal = (short) 83;
int i = 0b10;

System.out.println(d);
System.out.println(mammal);
System.out.println(i);

100
S
2

literals

when a numbers is represented in code dan heet t n literal.

long max = 3123123123123; does not compile want is int(!)

long max = 3123123123123L; nu wel want is long

long max = 3123123123123l; nu wel (Met kleine letter l!!!!!)NIET DOEN!!! LEEST ALS een 1 ws... GRGRG!

Octal

zet er een 0 voor 017

Hex

0x of 0X ervoor 0xde

Binairy

0b od 0B ervoor 0b10

underscore

Niet als eerste of laatste character

Niet voor of na de komma in t numme

double notAtStart = _1000.00; // DOES NOT COMPILE

double notAtEnd = 1000.00_; // DOES NOT COMPILE

double notByDecimal = 1000_.00; // DOES NOT COMPILE

double notByDecimal = 1000._00; // DOES NOT COMPILE

double magWel = 10_0_0.0_0; // WERKT WEL MAAR LELIJK

double magWel2 = 1________2; // WERKT WEL MAAR LELIJK

Reference Types

reference types refers to an object.

a ref points to an object by storing its memory address. pointer.

a value is assigned :
- to another obj of the same or compatible type
- to a new object using the new keyword.

Reference type can be null; NULL
Primitive Types NEVER can be NULL!!!!

int v = null; //DOES NOT COMPILE

String s = null;// wel!

Types beginnen standaard met null;

Will je null of een helper method voor bool, byte, short, int, char,double, float, long use wrapper classes: Boolean, Byte, Short, Integer, Character, Double, Float, Long

long l = null;

|   long l = null;

incompatible types: <nulltype> cannot be converted to long

Declaring vars

initializing a var

Identifiers

naming

snake_case
cameCase

variablelen beginnen met:

letter
$ Dollar sign
_ Underscore

geen number als start
_ alleen als naam mag niet meer....
geen reserved keyword...

MAG NIET:
int 3DjqkoewarClass; NIET MET NUMBER STARTEN
byte holly@vine; GEEN @
String *$coffee; GEEN *
boudle public; RESERVED keyword
shirt _; geen underscore..... is geen swift

Declaring multiple vars

int i1, i2, i3 =0; 3 declared 1 initialized.

int num, String value; DOES NOT COMPILE DIFFERENT TYPE in same statement...

double d1, double d2; MAG OOK NIET

Local vars

public int notValid() { int y= 10; int x; int reply = x+ y; // DOES NOT COMPILE return reply; }

public int isValid() { int y= 10; int x; x = 3 int reply = x+ y; // DOES COMPILE return reply; }

Passing Constructor and Method Params

moeten declared and init zijn

Var keyword

voor leesbaarheid...

assign en define on same line!!!

var is reserved TYPEname not a reserved word!!!! Dus kan als class, interface of enum...niet doen dus...

public class VarKeyWord{ var tricky = "this is a string";// DOEs NOT COMPILE }

public void twoTypes() { int a, var b = 3; /// DNC var n = null; ///DNC }

var o = (String)null;//Doet t wel...

Can be used: Local variable declaration inside methods or initializers. Local variables in for-loops.

Cannot be used: Class or instance variable declaration. Method parameters. Method return types. Constructor parameters. Multiple var declaration

Var scope

scope binnen
constructor
method
initializer block

public void eat(int pieces){ int bites = 1; } // heeft dus 2 vars

public void eatIfHungry(boolean hungry){
if(hungry) {
int cheese = 1;
}
System.out.println(cheese); // DNC !!!
}

public int bla(var a, var b) {} // DNC!!! alleen BINNEN constructor Method init blockkk!!!!!

public classs Var {
public void var() {
var var = "var";
}
public void Var() {
Var var = new Var();
}

}
COMILES!!!!! Var is niet een system keyword....

public class var { public var(){} } ?? DNC !!!!!!!!

DUS var NIET VOOR CLASS INTERFACE EN ENUM!!!!!!

Garbage Collection

System.gc();

not garanteed to do anything.

• the object no longer has any ref to it.
• all refs to obj are out of scope.

finalize() is deprecated in java 9. can run zero or 1 times. never twice.

var Rules

1. var is used as a local variable in a constructor, method or initializer block
2. var cannot be used in params of constructor, method, instance vars, class variables...
3. var is always initialized on the same line where it is declared. declared + initiialized.
4. value var can change, type NEVER
5. var cannot be init with null
6. var niet in multiple-variable declaration
7. var is reserved type name not reserved word. can be used as an identifier for class, interface or enum name...

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