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→Exceptions
===With apt===
===With apt===
<nowiki>sudo apt-add-repository ppa:webupd8team/java
<nowiki>sudo apt-add-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java8-installer</nowiki>
Also ensure your JAVA_HOME variable has been set to:
Also ensure your JAVA_HOME variable has been set to:
System.out.println("My integer is: " + myInt);
System.out.println("My integer is: " + myInt);
}
}
}</source>
}</source><syntaxhighlight lang="java">
//Java 11
" ".isBlank(); // --> true
" lr ".strip(); //Java 11
" lr ".stripLeading().replace(" ", "@");
" lr ".stripTrailing().replace(" ", "@");
"line1\nline2\nline3".lines().forEach(System.out::println);
//Java 12
"UPPER".transform(s -> s.substring(2));
//Java 13
"My name is %s. My age is %d".formatted("Rafa", 40);
//Java 14
</syntaxhighlight>
==Enum type==
==Enum type==
See code example:<br />
See code example:<br />
words.contains("Dog");
words.contains("Dog");
words.indexOf("Cat");
words.indexOf("Cat");
</syntaxhighlight>
</syntaxhighlight>List.of() and List.copyOf() creates unmodifiable lists copyOf is introduced on Java 10
===ArrayList===
===ArrayList===
===Reading text files===
===Reading text files===
See: [https://github.com/rafahsolis/javaTutorial/blob/master/ReadTextFile.java ReadTextFile.java]
See: [https://github.com/rafahsolis/javaTutorial/blob/master/ReadTextFile.java ReadTextFile.java]
===List files in dir===
<syntaxhighlight lang="java">
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.function.Predicate;
Path currentDirectory = Paths.get(".");
Files.list(currentDirectory).forEach(System.out::println);
int levels = 5;
Files.walk(currentDirectory, levels).forEach(System.out::println);
// list only .java files
Predicate<? super Path> predicate = path -> String.valueOf(path).contains(".java");
Files.walk(currentDirectory, levels).filter(predicate).forEach(System.out::println);
// Search files
BiPredicate<Path, BasicFileAttributes> javaMatcher = (path, attributes) -> String.valueOf(path).contains(".java");
Files.find(currentDirectory, levels, javaMatcher).forEach(System.out::println);
BiPredicate<Path, BasicFileAttributes> dirMatcher = (path, attributes) -> attributes.isDirectory();
Files.find(currentDirectory, levels, dirMatcher).forEach(System.out::println);
</syntaxhighlight>
===Reading files (Try-With-Resources)===
===Reading files (Try-With-Resources)===
This requires at least Java 7<br />
This requires at least Java 7<br />
}</source>
}</source>
===with nio===
<syntaxhighlight lang="java">
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;
public class FileReadRunner {
public static void main(String[] args) throws IOException {
Path pathFileToRead = Paths.get("./resources/data.txt");
List<String> lines= Files.readAllLines(pathFileToRead);
System.out.println(lines);
}
}
public class FileReadLineByLineRunner {
public static void main(String[] args) throws IOException {
Path pathFileToRead = Paths.get("./resources/data.txt");
Files.lines(pathFileToRead).forEach(System.out::println);
Files.lines(pathFileToRead).map(String::toLowerCase).forEach(System.out::println);
Files.lines(pathFileToRead).map(String::toLowerCase).filter(str -> str.contains("a")).forEach(System.out::println);
}
}
</syntaxhighlight><br />
===Writing files===
===Writing files===
<source lang="java">
<source lang="java">
}
}
}</source>
}</source><syntaxhighlight lang="java">
import java.io.IOException;
public class FileWriteRunner {
public static void main(String[] args){
Path filePath = Paths.get("./resources/filetowrite.txt");
List<String> list = List.of("Apfel", "Junge", "Hund");
Files.write(filePath, list);
}
}
</syntaxhighlight>
===readString writeString (Java 11)===
<syntaxhighlight lang="java">
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
public class FileReadWriteRunner {
public static void main(String[] args) throws IOException {
// Read
Path path = Paths.get("./file.txt");
String fileContent = Files.readString(path);
System.out.println(fileContent);
// Write
String newFileContent = fileContent.replace("Line", "Lines");
Path path = Paths.get("./newfile.txt");
Files.writeString(newFilePath, newFileContent);
}
}
</syntaxhighlight>
==Anonymous class==
==Anonymous class==
==Exceptions==
==Exceptions==
[http://docs.oracle.com/javase/8/docs/api/java/lang/Exception.html Class Exception]
[http://docs.oracle.com/javase/8/docs/api/java/lang/Exception.html Class Exception]
[https://github.com/in28minutes/in28minutes-initiatives/blob/master/The-in28Minutes-TroubleshootingGuide-And-FAQ/quick-start.md Exception Troubleshooting]
===trows===
===trows===
<source lang="java">
<source lang="java">
public class HandlingExceptions {
public class HandlingExceptions {
public static void main(String[] args) trows FileNotFoundException {
File file = new File("test.txt");
FileReader fr = new FileReader(file);
throw new RuntimeException("Example exception throwing");
}
}
// To throw Exception instead of RuntimeException you need to explicit specify your class throws an exception:
public class HandlingExceptions throws Exception {
public static void main(String[] args) trows FileNotFoundException {
public static void main(String[] args) trows FileNotFoundException {
File file = new File("test.txt");
File file = new File("test.txt");
FileReader fr = new FileReader(file);
FileReader fr = new FileReader(file);
throw new Exception("Example exception throwing");
throw new Exception("Example exception throwing");
}
}
// Custom exception
class CustomException extends Exception {
}
class CustomException1 extends Exception {
public CustomException1()
}
public class CustomExceptionRunner {
public void static main(String[], args) throws CustomException {
throw new CustomException("Some text");
}
}
}</source>
}</source>
}
}
}</source>
}</source>
===Try with resources (similar to python with)===
<syntaxhighlight lang="java">
import java.util.Scanner;
public class TryWithResourcesRunner {
public static void main(String[] args){
try (Scanner scanner = new Scanner(System.in)){
int[] numbers = {12, 3, 4, 5};
int number = numbers[21];
}
// this will throw an exception but scanner will be closed
}
}
</syntaxhighlight>
===catch multiple exceptions===
<syntaxhighlight lang="java">
try {
// code
} catch (IOException | SQLException ex) {
ex.printStackTrace();
}
</syntaxhighlight>
===Runtime exceptions===
===Runtime exceptions===
The compiler does not force you to handle them.
The compiler does not force you to handle them.
</syntaxhighlight>
</syntaxhighlight>
==Concurrency==
===syncrhronized===
If there are multiple synchronized methods on an instance only one of them can be run by threads at one point in time<syntaxhighlight lang="java">
public class Counter {
private int i = 0;
synchronized public void increment() { // synchronized keyword: only one thread can run at same time
i++; // get; increment; set (Not atomic, not thread safe if not synchronized)
}
public int getI(){
return i;
}
}
</syntaxhighlight>
===Locks===
Solve the multiple synchronized methods on same instance (apart from ReentrantLock thre are TryLock and TryForLock)<syntaxhighlight lang="java">
import java.util.concurrent.locks.ReentrantLock;
public class BiCounterWithLock {
private int i = 0;
private int j = 0;
Lock lockForI = new ReentrantLock();
Lock lockForJ = new ReentrantLock();
public void incrementI() {
lockForI.lock();
i++;
lockForI.unlock();
}
public int getI(){
return i;
}
public void incrementJ() {
lockForJ.lock();
j++;
lockForJ.unlock();
}
public int getJ(){
return j;
}
}
</syntaxhighlight>
===Atomic classes===
Solves the same problem as locks but for simple operators (AtomicInteger, AtomicBoolean, AtomicIntegerArray, AtomicLong, AtomicBoolean....)<syntaxhighlight lang="java">
import java.util.concurrent.atomic.AtomicInteger;
public class BiCounterWithLock {
private AtomicInteger i = new AtomicInteger();
private AtomicInteger j = new AtomicInteger();
public void incrementI() {
i.incrementAndGet();
}
public int getI(){
return i.get();
}
public void incrementJ() {
j.incrementAndGet();
}
public int getJ(){
return j.get();
}
}
</syntaxhighlight>
===Concurrent collections===
<syntaxhighlight lang="java">
import java.util.Hashtable;
import java.util.Map;
import java.util.concurrent.atomic.LongAdder;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ConcurrentHashMap;
// Not thread safe!!!
public class MapRunner{
Map<Character, LongAdder> occurrances = new HashTable<>();
String str = "ABCD ABCD ABCD";
for (char character:str.toCharArray()){
LongAdder longAdder = ocurrances.get(character);
if (longAdder==null) {
longAdder = new LongAdder();
}
longAdder.increment();
occurrances.put(character, longAdder);
}
System.out.println(occurrances);
}
// Thread safe
public class ConcurrentMapRunner{
ConcurrentMap<Character, LongAdder> occurrances = new ConcurrentHashMap<>();
String str = "ABCD ABCD ABCD";
for (char character:str.toCharArray()){
occurances.computeIfAbsent(character, ch -> new LongAdder()).increment();
}
System.out.println(occurrances);
}
</syntaxhighlight>
===CopyOnWriteArrayList===
Expensive write, fast concurrent read, synchronized writes, free reads. <syntaxhighlight lang="java">
import java.utils.List;
import java.util.concurrent.CopyOnWriteArrayList;
public class CopyOnWriteArrayListRunner {
List<String> list = new CopyOnWriteArrayList<>();
// Threads -3
list.add("Ant");
list.add("Bat");
list.add("Cat");
// Threads - 10000
for (String element:list){
System.out.println(element);
}
}
</syntaxhighlight>
==Java tips==
===Static imports===
<syntaxhighlight lang="java">
import static java.lang.System.out;
import static java.util.Collections.*;
out.println("System.out.printl System not needed");
sort(new ArrayList<String>());
</syntaxhighlight>
===Blocks===
<syntaxhighlight lang="java">
pubublic class BlocksRunner {
public static void main(String[] args){
{
int i;
System.out.println("i can be accessed here: " + i);
}
System.out.println("i can not be accessed here")
}
}
</syntaxhighlight>
===equals and hashCode methods from an object===
2 objects only obj.equals if they are the same object, but equals can be overriden (generate hashcode and equals)
Hash function should return the same value if 2 objects are equal
===Access modifiers===
public: accessible every where.
protected: accessible in same package or subclasses.
(default): only inside the class or package.
private: only inside the class.
===non access modifier===
====final====
final classes can not be extended.
final methods can not be overridden.
final variable can not change (Constants).
final argument can not be changed.
====static====
static class variables are common to all class instances.
static methods can be called from class, no need for instance, inside static methods instance variables or methods are not accessible.
====Constants====
public static final
====Enums====
like: java.time.Month, java.time.DayOfWeek<syntaxhighlight lang="java">
enum Season {
WINTER, SPRING, SUMMER, FALL;
// to store in db
private int value;
WINTER(1), SPRING(2), SUMMER(3), FALL(4);
private Season(int value){
this.value = value;
}
}
public class EnumRunner{
public static void main(String[] args){
Season season = Season.WINTER;
Season season1 = Season.valueOf("WINTER");
System.out.print(season1);
System.out.print(Season.SPRING.ordinal());
System.out.print(Season.SPRING.getValue());
System.out.print(Arrays.toString(Season.values()));
}
}
</syntaxhighlight>
====module-info.java====
requires: allows usage of other modules
requires transitive: allows usage of other modules and modules that use this module will also have access to it
exports: allows other packages to use your classes
<br />
==Predicate Not==
Java 11<syntaxhighlight lang="java">
import java.util.List;
import java.util.function.Predicate;
public class PredicateNotRunner {
public static boolean isEven(Integer number){
return number&2==0;
}
public static void main(String[] args){
List<Integer> numbers = List.of(3, 4, 5, 57, 65, 88);
// Predicate<Integer> evenNumberPredicate = number -> number%2==0;
// numbers.stream().filter(evenNumberPredicate).forEach(System.out::println);
numbers.stream().filter(Predicate.not(PredicateNotRunner::isEven)).forEach(System.out::println);
}
}
</syntaxhighlight>
==Sort strings==
<syntaxhighlight lang="java">
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Comparator;
class LengthComparator implements Comparator<String> {
@Override
public int compare(String str1, String str2){
return Integer.compare(str1.length(), str2.length());
}
}
public class AnonymousClassRunner{
public static void main(String args){
List<String> animals = new ArrayList<String>(List.of("Ant", "Cat", "Ball", "Elephant"));
// Collections.sort(animals); //Alphabetical order
Collections.sort(animals, new LengthComparator()); //Length order
System.out.println(animals);
}
}
</syntaxhighlight>
==Logger==
<syntaxhighlight lang="java">
import java.util.logging.Logger
public class BubbleSort {
public List<String> sort(List<String> names){
return names;
}
}
public class MySortingUtil {
public List<String> sort(List<String> names){
BubbleSort bubbleSort = new BubbleSort();
return bubbleSort.sort(names);
}
}
public class MySortingUtilConsumer {
private static Logger logger = Logger.getLogger(MySortingUtilConsumer.class.getName());
public static void main(String args){
MySortingUtil util = MySortingUtil();
List<String> sorted = util.sort(List.of("Elephant", "Bat", "Mosquito"));
logger.info(sorted.toString());
}
}
</syntaxhighlight><br />
==Eclipse shortcuts==
==Eclipse shortcuts==
Ctrl + Space → Autocomplete.<br />
Ctrl + Space → Autocomplete.<br />
==Notes==
==Notes==
Hibernate → Java ORM framework for web design
Hibernate → Java ORM framework for web design
==java cli==
<syntaxhighlight lang="bash">
# Compile class
javac someclass.java
# List modules
java --list-modules
# Java shell
jshell
# Run java application
java -jar someclass.jar
# List module dependencies
java -d java.sql
</syntaxhighlight>
==Frameworks==
[[Java: Spring|Spring]]
==Java and Eclipse troubleshooting==
====Default Home Folder for JDK====
*Windows: C:\Program Files\Java\jdk-{version}
**Example for JDK 16 - C:\Program Files\Java\jdk-16
**Example for JDK 17 - C:\Program Files\Java\jdk-17
*Mac: /Library/Java/JavaVirtualMachines/jdk-{version}.jdk/Contents/Home
**Example for JDK 16 - /Library/Java/JavaVirtualMachines/jdk-16.jdk/Contents/Home
**Example for JDK 17 - /Library/Java/JavaVirtualMachines/jdk-17.jdk/Contents/Home
====Default Home Folder for JDK====
*Windows: C:\Program Files\Java\jdk-{version}\bin
**Example for JDK 16 - C:\Program Files\Java\jdk-16\bin
*Mac: /Library/Java/JavaVirtualMachines/jdk-{version}.jdk/Contents/Home/bin
**Example for JDK 16 - /Library/Java/JavaVirtualMachines/jdk-16.jdk/Contents/Home/bin
[[Category:Java]]