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Diese Karteikarten bieten einen umfassenden Überblick über die Java-Programmierung, insbesondere zu Themen wie Dateiverarbeitung, Funktionen und Sammlungen. Sie decken wichtige Konzepte wie das Arbeiten mit Pfaden, Dateien und Strings ab, sowie das Verhalten von Sammlungen und deren Sortiermethoden. Die Karteikarten sind ideal für Entwickler, die ihr Wissen in Java vertiefen möchten, insbesondere in Bezug auf die Handhabung von Sammlungen, das Sortieren und Vergleichen von Objekten sowie die Verwendung von Stream-APIs und Collectors.

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496

Utilisateurs

Langue

Allemand

Catégorie

Informatique

Niveau

Autres

Créé / Mis à jour

06.12.2020 / 24.01.2021

Fichiers de cartes similaires

When can classes be used for datastructures that require comparision?

if it implements comparable

When to use a comparator?

to specify that you want to use a different order than the object itslefs provides

Whats the signature of Comparable-interface?

public interface Comparable<T> {
int compareTo( T o);
}

When is a Cast needed within the compareTo-method?

when dealing with legacy code or code that does not use generics

When implementing compareTo, where do we have to pay attention to?

keeping compareTo and equals consistent

Can comparable or comparator be used without a import statement?

Comparator yes, comparable not

Whats the package name, whats the method name, whats the number of parameters, has the interface to be implemented by class compaing in class Comparable?

java.lang / compareTo() / 1 / Yes

Whats the package name, whats the method name, whats the number of parameters, has the interface to be implemented by class compaing in class Comparator?

java.util / compare() / 2 / no

Whats the static comparator-method for comparing by results of a function that returns any object?

comparing(function)

Whats the static comparator-method for comparing by the results of a function that returns a double?

comparingDouble(function)

Whats the static comparator-method for comparing by the results of a function that returns an int?

comparingInt(function)

Whats the static comparator-method for comparing by the results of a function that returns a long?

comparingLong(function)

Whats the static comparator-method for sorting using the order specified by the comparable implementation on the object itself?

naturalOrder()

Whats the static comparator-method for sorting using the reverse of the order specified by the Comparable implementation on the object itself?

reverseOrder()

Whats the default-method in comparator for reversing the order of the chained comparator?

reversed()

Whats the default-method in comparator if the previous comparator returns 0 se this comparator that returns a double/int/long , otherwise return the value form the previous comparator

thenComparingDouble(function) / thenComparingInt(function) / thenComparingLong(function)

When can we use Collections.sort()?

When the objects to be sorted are Comparable

What are the preconditions for Collections.binarySearch?

Must be sorted

What cant be done with genericTypes?

calling a constructor
creating an array of that generic type
calling instanceof
using a primitive type as a generic type parameter
creating a static variable as a generic type parameter

What happns if you add list.removeIf on a list which was created by list.of()?

unsupportedOperation

What is the "principle of least privilege"?

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

Limiting access as much as possible

What is the problem here? How to fix?

public class GrasshopperCage {

public static void openLock(ComboLocks comboLocks, String combo) {

if(comboLocks.isComboValid("grasshopper", combo)) System.out.println("open");

}

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

a attacker could override the method isComboValid and pass it to openLock. (fix it by with final class) f.e:

public boolean isComboValid(String animal, String combo) {

var valid = super.isComboValid(animal, combo);

if(valid) { // email the password to s.b.}

return valid;

}

Is this a immutable object?:

public final class Animal {

private final ArrayList<String> favoriteFoods;

public Animal() {

this.favoriteFoods = new ArrayList<String>();

this.favoriteFoods.add("Apples");

}

public List<String> getFavoriteFoods() {

return favoriteFoods;

}

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

No, the returned list can be modified. Better implement methods for getting properties, f.e.:

public String getFavoriteFoodsElement(int index) {

return favoriteFoods.get(index);

}

What is a copy constructor?

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

F.e.:

public Animal(List<String> favoriteFoods) {

if(favoriteFoods == null) throw new Runtimeexception("..");

this.favoriteFoods = new ArrayList<String>(favoriteFoods);

}

What is a defensive copy?

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

F.e. the copy operation

Does the clone() method copy shallow or deep?

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

shallow by default

What happens if clone is called on an object which doesnt implement cloneable?

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

Throw exception

What can be considered as untrusted data?

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

F.e. user inout, reading from files, retrieving data from databases. Any data that did not originate form your program

what is command injection? How to prevent?

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

F.e. typing ".." here (for going back one directory). Prevent this with whitelists

Console console = System.console();

String dirName = console.readLine();

Path path = Paths.get("c:/data/diets/" + dirName);

try(Stream<Path> stream = Files.walk(path)) {

stream.filter(p -> p.toString().endsWith(".txt")).forEach(System.out::println);

}

How to prevent comand injection with lists? which one to prefer?

Commandes clavier:

= tourner,

= avant/arrière,

= faire défiler

Whitelists/blacklists: better whitelists

What are sensitive contexts which should be avoided to ensure confidental informations doenst escape?

* logfiles

* printing exceptions or stacktraces

* System.out and System.err messages

* writing to data files

Why should confidenial data be stored in char[] instead of string?

java wont place it in the stringpool and therefore it wount live long in the memory

How to prevent command injections?

Whitelists/blacklists/limiting file acces with permissions

What does defense in depth mean?

To apply multiple techniques to protect an application

How to specify which fields get serialized? (two ways)

transient or with an array:

private static final ObjectStreamField[] serialPersistentFields = { new ObjectStreamField("name", String.class) };

What is the opposite of transient?

The array:

private static final ObjectStream[] serialPersistentFields = {new ObjectStreamField("name", String.class) };

What modifiers are required for serialPersistentFields?

private static final

How to ensure that fields are enrcrypted during serialization?

implementing writeObject(ObjectOutputStream s) and readObject(ObjectInputStream s) and doing the encrpytion here

What exceotion is thrown when a field is marked with transient/is not in serialPersistentFields?

IllegalArguemntException

What is readResolve for?

When we want to control f.e. if a new object is created during deserialization

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Cartes-fiches

Whats wrong here?
Lock lock = new ReentrantLock();
if(lock.tryLock()) {
try {
lock.lock();
} finally {
lock.unlock();
}
}

Two times locked but only one released

What are the differences between ReentrantLock and a synchronized block?

the support the same futures but but ReentrantLock adds following:

Ability to request a lock without blocking
Ability to reqeust a lock while blocking for a specfied amount of time
create lock with fairnessproperty

What are the constructors of a CyclicBarrier?

CyclicBarrier(int threads)
CyclicBarrier(int threads, Runnable barrierAction)

What are the most important methods of a CyclicBarrier?

public int await() throws InterruptedException,BrokenBarrierException
public int await(long timeout, TimeUnit unit) throws InterruptedException, BrokenBarrierException,TimeoutException
public void reset()
public int getNumberWaiting()

What happens if we have a fixedThreadPool with two threads but wating for 3 threads within a cyclicBarrier?

The code will hang indefinitely

When does a memorry consistency error occur?

when two threads ahve incosistent views of what should be the same data

What will happen here?
var foodData = new HashMap<String,Integer>();
foodData.put("penguin",1);
foodData.put("flamingo",2);
for(String key: foodData.keySet())
foodData.remove(key);

A ConcurrentModificationException will be thrown

What er the concurrenct counterparts to TreeSet and ThreeMap?

ConcurrentSkipListSet and ConcurrentSkipListMap

Whats special about CopyOnWriteArrayList and CopyOnWriteArraySet?

tey copy all of their elements to a new underlying structure anytime an element is added, modified or removed from the collection

What does "modifing" mean in the context of CopyOnWriteArrayList and copyOnWriteArraySet?

when the reference in the collection is changed

What happens here?
List<Integer> favNumbers = new CopyOnWriteArrayList<>(List.of(4,3,42));
for(var n: favNumbers) {
System.out.print(n + " ");
favNumbers.add(9);
}
System.out.println();
System.out.println("Size: " + favNumbers.size());

4 3 42
Size 6

What is special about BlockingQueue?

Its like a regular queue but it includes Methods that will wait a specfic amount of time to complete an operation

Whats the syntax for adding an item to the queue, wating the specified time and returning false if the time elapses before space is availiable in BlockingQueue?

boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException

Whats the syntax for retrieving and removing an item from the queue, waiting for the specified time and returing null if the time elapses before the item is availiable in BlockingQueue?

E poll(long timeout, TimeUnit unt) throws InterruptedException

Whats a difference between concurrent and snychronized collections regarding the exceptions?

unlike the concurrent collections the snychronized collections also throw an exception if they are modified witihn an iterator by a single thread

How to create a parallelStream form an existing stream?

Calling parallel()

How to create a parallel stream from a collection?

calling parallelStream()

When Stream.concat is called on two parallel streams, is the result still parallel?

yes

When Stream.flatmap is called on two parallel streams, is the result still parallel?

What does parallel decomposition mean?

process of taking a task, breaking it up into smaller pieces that can be performed concurrently and then reassembling the results

Does forEachOrdered makes the whole stream serialized?

What does the streamoperation reduce() do?

combines a stream into a single object

What is the syntax of stream.reduce with parallel streams?

<U> U reduce(U identity, BiFunction<U, ? super T, U> accumulator, BinaryOperator<U> combiner)

Why is this problematic?
System.out.println(List.of(1,2,3,4,5,6)
.parallelStream()
.reduce(0, (a,b) -> (a-b)));

It may output -21,3 or some other value

Whats the syntax of Stream.collect with parallel Streams?

<R> R collect(Supplier<R> supplier, BiConsumer<R, ? super T> accumulator, BiConsumer<R,R> combiner)

What are the requirements to execute Stream.collect in parallell?

the stream is parallel
parameter of collect ooperation has the Characteristics.CONCURRENT
either the stream is unordered or the collector has the characteristic Characteristics.UNORDERED

What does "transitiv" do witihn a module-info.java?

Indicates the module and that all modules that use this module are dependent on another module

Which types of modules exists?

named, unnamed, automatic

What is the characteristics of a named module?

has the name inside the module-info and is on the module path

What is the characteristics of an automatic module?

is on the module path, has no module-info, is simply a regular jar-file on the module-path and treated as a module

How does code which is referencing an automatic module treat it?

As if there is a module-ionfo file and exports all packages. it determines the modulename

What are the two ways of determining the name of a automatic module?

* set a property called Automatic-Module-Name in the MANIFEST.MF file

* Java determines it by passing it off the filename of the jar file

Describe the algorithm of creating the name for a automatic file

If MANIFEST.MF contains an Automatic-Module-Name use that. Otherwise:
Remove file extension from jar name
remove any version info from the end of the name
replace any remaining chars other than letters and numbers with dots
replace any sequences of dots with a single dot
remove the dot if it is the first or last char of the result

What is the charactersitics of a unamed module?

its on the classpath, a regular jar and could contain a module-info.java file but would be ignored as long as the module is on the classpath

Can code from the classpath access the module path? Whats about the other way around?

Yes but code on the module path is unable to read from the classpath

Is it possible to use only a part of the JDK?

Yes

Which module is availiable by default to all modular applications?

java.base

What are important comman modules?

java.base
java.desktop
java.logging
java.sql
java.xml

How to provide details about unsupported apis with jdeps?

with --jdk-internals

Which is the easiest MigrationStrategy for Modules?

bottum-up-migration

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