java

---

Features

• Erstrelease 1995

• angelehnt an C/C++

• objektorientiert

• interpretiert

• statisch typisiert

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Interpretiert?

Compilation bei Bedarf

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---

Details

def sum(a, b):
    return a + b

  2           0 LOAD_FAST                0 (a)
              2 LOAD_FAST                1 (b)
              4 BINARY_ADD
              6 RETURN_VALUE

111000100110100011010001101101111110110000001001011011000011011011 
010011001000010001011000011011000011100011101101010110101011000101 
100100000101101000100111100000101101101111011011000000101001111100 

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Statisch typisiert

byte b = 127;
short countries = 195;
int austrians = 8902600;
long population = 7_793_010_144L;
float average = 2.573F;
double gravity = 9.80665;
boolean typed = true;
char character = 'a';
String name = "Java";

---

Typ	Byte	Min	Max
byte	1	-128	127
short	2	-32 768	32 767
int	4	-2 147 483 648	2 147 483 647
long	8	-2⁶³	2⁶³-1

Typ	Byte	Stellen exakt
float	4	~7
double	8	~16

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Two's Complement

 5 = 0101

-5?
~5 -> 1010
     +   1
      ----
      1011 = -5

---

Overflow

int max = 2_147_483_647;
System.out.println(max + 1);

-2147483648

double max = 1.7976931348623157E308;
System.out.println(2 * max);

Infinity

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Syntax

• Anweisungen werden mit ; abgeschlossen

  
  doThis();
      

• Blöcke werden durch { } gekennzeichnet

  
  public static void main(String[] args) {
      System.out.println("Hello World");
  }
      

---

Hello World

myFile.py
if __name__ == '__main__':
    print('Hello World')

python myFile.py
Hello World

public class HelloWorld {

    public static void main(String[] args) {
        System.out.println("Hello World");
    }
} 

javac HelloWorld.java
java HelloWorld
Hello World

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Variablen

🐍
number_of_students = 25
number_of_students = 24

Typ nameOfTheVariable;

int numberOfStudents = 25;
int numberOfStudents = 24; // 🚫 Typ nur bei Deklaration
numberOfStudents = 24;

double uninitialised;

---

Operationen

🐍
3 + 'Python' -> 💥

3 + "Java" -> "3Java"

String + ??? = String

3 + 2 + "!"  -> "5!"

---

Arithmetik

🐍
8 / 3 = 2.6666666666666665

8 / 3 = 2

int / int = int

int n = 2;
System.out.println(++n);    // n += 1, dann Ausgabe
System.out.println(n++);    // Ausgabe, dann n =+ 1
System.out.println(n);

3
3
4

---

if

if (grade == 1) {
    System.out.println("Sehr gut");
}

if (i % 2 == 0) {
    System.out.println("Gerade");
} else {
    System.out.println("Ungerade");
}

if (condition) {
    // true
} else {
    // false
}

if (condition) {
    // true
} else if (otherCondition) {
    // condition false, otherCondition true
} else {
    // condition false, otherCondition false
}

---

and/or/not

🐍
if month == 2 and day == 30 or month > 12:
    raise ValueError('Illegal date')

if (month == 2 && day == 30 || month > 12) {
    throw new IllegalArgumentException("Illegal date");
}

🐍
not(7 / 2 == 3)

!(7 / 2 == 3)

---

switch/case

🐍
if grade == 'Sehr gut':
    result = 1
elif grade == 'Gut':
    result = 2
...
else:
    raise ValueError('Invalid grade: ' + grade)

int result = switch (grade) {
    case "Sehr gut", "sehr gut" -> 1;
    case "Gut" -> 2;
    ...
    default -> 
        throw new IllegalArgumentException("Invalid grade: " + grade);
};

---

for

for (int i = 0; i < 3; i++) {
    System.out.println(i);
}

0
1
2

for (int i = 0; i < 4; i+=2) {
    System.out.println(i);
}

0
2

---

for (init; condition; update) {
    // code
}

init;
if (condition) {
    // code
}
update;
if (condition) {
    // code
}
update;
if (condition) {
    // code
}
...

while, break, continue -> wie in Python

---

Strings

char oneCharacter = 'c';
String text = "Hello World";
String number = "42";

🐍
as_int = int(number)

int asInt = Integer.parseInt(number);

---

Stringformatierung

🐍
pi = 3.1415926
print(f'pi = {pi:06.2f}')

pi = 003.14

double pi = 3.1415926;
System.out.printf("pi = %06.2f", pi);
String asString = String.format("pi = %06.2f", pi);

---

Einlesen

🐍
input_string = input('Age: ')
age = int(input_string)

Scanner scanner = new Scanner(System.in);
System.out.println("Age: ");
int age = scanner.nextInt();

---

Manipulieren

🐍
len('String') -> 6

"String".length() -> 6

🐍
'String'[1] -> 't'

"String".charAt(1) -> 't'

🐍
'String'[2:5] -> 'rin'

"String".substring(2,5) -> "rin"

🐍
'Seperated by delimiter'.split(' ') 
    -> ['Seperated', 'by', 'delimiter']

"Seperated#by#delimiter".split("#")
    -> ["Seperated", "by", "delimiter"]

---

try / catch / throw

🐍
age = input('Age: ')
try:
    age = int(age)
except ValueError as error:
    print('Illegal age: ' + age)

String input = scanner.nextLine();
int age = 0;
try {
    age = Integer.parseInt(input);
} catch (NumberFormatException e) {
    System.out.println("Illegal age: " + input);
}

🐍
raise ValueError('Error accessing ' + database)

throw new IllegalArgumentException('Error accessing ' + database)

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Arrays

• Syntax ähnlich Python

• Definierte Länge

• Typisiert

• Hilfsmethoden unter Arrays. verfügbar

🐍
list = [42]
list.append(3)
print(list[1]) -> 3

🐍
list = [42, 'nein', list]

int[] array = new int[3];
System.out.println(array) -> [I@6acbcfc0
System.out.println(Arrays.toString(array)) -> [0, 0, 0]
array[0] = 42;

---

Mehr Dimensionen

int[][] matrix = new int[3][2];
for (int row = 0; row < matrix.length; row++) {
    for (int col = 0; col < matrix[row].length; col++) {
        matrix[row][col] = 10 * row + col;
    }
}

	col
row	0	1
0	0	1
1	10	11
2	20	21

---

for-in

🐍
for item in list:
    print(item)

for (int i = 0; i < array.length; i++) {
    int item = array[i];
    System.out.println(item);
}

for (int item : array) {
    System.out.println(item);
}

---

Methoden/Routinen/Funktionen

🐍
def foo(number: int) -> float:
    """Takes an integer, returns a float"""
    return 'Hello World'

foo('42')

/**
* Takes an integer, returns a double
*/
double foo(int number) {
    return "Hello World"; 🚫
}

double foo(int number) {
    return 1.0;
}

foo("42"); 🚫

void methodReturnsNothing(String s) {
    System.out.println(s);
}

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Command Line Arguments

Vorteile gegenüber GUIs:

• automatisierbar

• weniger Aufwand

---

☕

public class CLI {

    public static void main(String[] args) {
        System.out.println(Arrays.toString(args));
    }
}

java CLI These are command line arguments

[These, are, command, line, arguments]

---

🐍

import sys

if __name__ == '__main__':
    print(sys.argv)

python CLI These are command line arguments

['cli.py', 'These', 'are', 'command', 'line', 'arguments']