Java Data Types
byte/short/int/long |
2^ (8/ 16/ 32/ 64) |
float [32bit] / double [64bit] |
1.0f / 1.0d, 1.0 |
char [16bit] |
"U", "±" |
String |
"Hello World" |
Literals
2 ⇒ int |
2.0F ⇒ float |
2L ⇒ long |
2.0 ; 2.0D ⇒ double |
"u" ⇒ String |
'u' ⇒ char |
true ⇒ boolean |
false ⇒ boolean |
number literal modifiers are case-insesitive
Inc- / Decrement
a++ |
++a |
a-- |
--a |
➜ return a++ / a-- |
➜ return a |
Assignment shortcuts: x op= y
Exp: x += 2;
Variables
int i = 5, j;
int a = 3, b = a + 1;
final int c; //=> Constant
c = 22; // may be init after
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Comments
// Single Line
/* Multi Line */
/** Docu String */
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Array
int[] a; //Declaration
a = new int[5] //Dimension
int[] a = new int[5];
int[] b = {10, 20, 30};
int[][] matrix = new int[2][3];
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Init Values: 0, "\0", false, null
Array Methods
int[] a;
a.length; //length of array
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ArrayList
ArrayList<Double> nums = new ArrayList<>();
nums.add(2.3);
nums.size() == 1
double a = nums.get(0);
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Type Casting
int a, b;
a = (double) b * a //b => double
a = b * (double) a //a => double
a = (double) (a * b)
/* (a * b) will be calculated
with int logic then
type-casted to double */
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Strings
1. Strings are a class in Java.
2. Concating is possible without type-cast
String a = "Num: " + 5;
3. Parsing:
int i = Integer.parseInt("22");
float f = Float.parseFloat("1.3");
boolean b = Boolean.parseBoolean("True"); |
String Functions
s.equals(String s2) -> bool |
s.toLowerCase() |
s.toUpperCase() |
s.replace(char old, char new) |
s.replace(String old, String new) |
s.indexOf(String s) //-1 if not availabe |
s.lastIndexOf(String s) |
s.split(String delimiter) -> String[] |
all functions return the modified String. They don't modify the original String.
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Bitwise Operations
int |
boolean |
| |
! |
NOT |
& |
&& |
AND |
| |
|| |
OR |
^ |
^ |
XOR |
Biwise Shifts
~ |
Complement |
<< |
Shift left |
>> |
Shift right |
>>> |
Shift right Zero fill |
Arithmetic Operator Results (+ - * / % )
Both Arguments |
byte, short, int |
⇒ int |
One, or Both Arguments |
long |
⇒ long |
float |
⇒ float |
double |
⇒ double |
Examples: byte * byte ⇒ int; double + float ⇒ double
Java Operations Order
1. Members |
() [] . |
2. Multiplikation |
* / % |
3. Addition |
+ - |
4. XOR |
^ |
5. Logical AND |
&& |
6. Logical OR |
|| |
a++, a-- counts as 3. Addition
int a = 5, b = 8;
int c = a * b++ //c is 40;
Java Naming Convention
Constants:
MAX, PI, MIN_TIME
Variables, Methods, Packages:
xVal, int1Arr, date, showDate
Classes:
Date, DatabaseHelper |
File IO
File file = new File("text.txt");
file.exsits();
file.createNewFile();
file.delete();
BufferdReader = new BufferedReader(file);
while ((line = input.readLine()) != null) {
//statements with line
}
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There are other classes, but just use BufferdReader
abstract / Interface
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abstract Method public abstract fun();
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abstract Class public abstract class Test{}
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Interface Like abstract class, but with only abstract functions. You don't need abstract for these
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Abstract Classes and Methods are without implementation.
You use implements for Interfaces
UI
// Pop-Up Box:
import javax.swing.*;
String out = JOptionPane.showInputDialog("Inp: ");
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Regular Expressions (Regex)
Pattern p = Pattern.compile("\\w+");
Matcher m = p.matcher("abc");
while (m.find()) {
m.group()
}
//true if whole string matches
m.matches() -> bool
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Bolierplate
//Syntax for a standalone application in Java:
class <classname>
{
public static void main(String args[]) {
statements;
————————;
————————;
}
}
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Bolierplate cont.
Steps to run the above application:
1. Type the program in IntelliJ or notepad. Save the file with a .java extension.
2. The file name should be the same as the class, which has the main method.
3. To compile the program, using javac compiler, type the following on the command line:
Syntax: javac <filename.java>
Example: javac abc.java
4. After compilation, run the program using the Java interpreter.
Syntax: java <filaname> (without the .java extension)
Example: java abc
5. The program output will be displayed on the command line |
Java Statements
If Statement
if ( expression ) {
statements
} else if ( expression ) {
statements
} else {
statements
}
While Loop
while ( expression ) {
statements
}
Do-While Loop
do {
statements
} while ( expression );
For Loop
for ( int i = 0; i < max; ++i) {
statements
}
For Each Loop
for ( vartype var : collection ) {
statements
}
Switch Statement
switch ( expression ) {
case value:
statements
break;
case value2:
statements
break;
default:
statements
}
Exception Handling
try {
statements;
} catch (ExceptionType e1) {
statements;
} catch (Exception e2) {
catch-all statements;
} finally {
statements;
}
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IntelliJ Emmets
psvm |
public static void main(String[] args) {} |
sout |
System.out.println(); |
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