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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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Erstellt / Aktualisiert

06.12.2020 / 24.01.2021

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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"?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

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");

}

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

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;

}

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

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?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

F.e.:

public Animal(List<String> favoriteFoods) {

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

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

}

What is a defensive copy?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

F.e. the copy operation

Does the clone() method copy shallow or deep?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

shallow by default

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

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

Throw exception

What can be considered as untrusted data?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

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?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

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?

Tastatur-Befehle:

= drehen,

= vor-/rückwärts,

= scrollen

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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What happens here? Symbolic link from /zebra to /horse and current working direectory is /horse/schedule
System.out.println(Paths.get("/zebra/food.txt").toRealPath());
System.out.println(Paths.get(".././food.txt").toRealPath());

Both times the output is: /horse/food.txt

Does files have methods like mkdir/renameTo?

No but move() and createDirectory()

Whats the signature of Files.exists?

public static boolean exists(Path path, LinkOption... options)

Does Files.isSameFile follow symbolicLinks?

Yes

Can Files.isSameFile also check directories?

yes

What happens in isSameFile when a Path not exists? Are there exceptions to this?

Will trigger an exception, specialcase: If two path objects are equal in terms of equals() then it wont check if the file exists

Does Files.isSameFile compare the contents?

How to create a Directory with Files-class? (Methods)

public static Path createDirectory(Path dir, FileAttribute<?>... attrs) throws IOException
public static Path createDirectories(Path dir, FileAttribute<?>... attrs) throws IOException

What happens with createDirectories if the directories already exists?

Will simply doe nothing -> createDirectory would throw an excetion

What happens with createDirectory if it already exists? What if the path leading up tp the directory does not exists?

throw an exception

What is the signature of Files.copy?

public static Path copy(Path source, Path target, CopyOption... options) throws IOException

Whats the default behavoiur of copy if the target already exists?

Throws an exception

What happens here if food.txt and /enclosure exists?
var file = Paths.get("food.txt");
var directory = Paths.get("/enclosure");
Files.copy(file, directory);

Throws an exception -> enclosure already exists -> Path would have to be "/enclosure/food.txt"

What happens with Files.delete if a directory is not empty?

Throws an exception

What happens if delete/deleteIf is called on a symbolic link?

The link will be deleted, not the target

What does newBufferedReader() do?

Reads the file specified at the Path location

What does Files.newBufferedWriter(Path path) do?

Writes to a file specified at the Path location

What is the signature of Files.readallLines?

public static List<String> readAllLines(Path path) throws IOException

Whats the problem with Files.readAllLines()?

May trigger an outofMemoryError

What is a regular file?

One that contains content -> no symbolic links

Does Files.isRegularFile check the target of symbolicLinks?

Yes

Which of these functions in Files will throw an exceiption if the path does not exist?
- isHedden, isReadable, isWritable, isExecutable

isHidden

What happens with Files.size on directories?

is undefined, depends on the filesystem

Whats the difference between BasicFileAttributes and BasicFileAttributeView?

BasicFileAttributes are readOnly, BasicFileAttributeView is also updatable

Whats the signature of readAttributes?

public static <A extends BasicFileAttributes> A readAttributes(Path path, Class<A> type, LinkOption... options) throws IOException

Signature of Files.getFileAttributeView?

public static <V extends FileAttributeView> V getFileAttributeView(Path path, Class<V> type, LinkOption... options)

Can all file attributes be changed?

No, f.e. you can't change a file to a directory

Whats the signature of the File.list method?

public static Stream<Path> list(Path dir) throws IOException

Are symbolic links follow when using Files.list?

How deep traverses Files.list()?

only a single directory

Is Files.walk breadth-first or depth-first?

depth-first

Syntax of Files.walk()?

public static Stream<Path> walk(Path start, FileVisitOption... options) throws IOException
public static Stream<Path> walk(Path start, int maxDepth, FileVisitOption... options) throws IOException

Is Files.walk lazy?

Yes

Why is the getSize method outsourced here?
private long getSize(Path p) {
try{return Files.size(p);} catch(IOException e) {}
return 0L;
}
public long getPathSize(Path source) throws IOException {
try(var s= Files.walk(source)) {
return s.parallel().filter(p-> !Files.isDirectory(p))
.mapToLong(this::getSize)
.sum();
}
}

Files.size() declares IOException and we'd rather not put a try/catch block inside a lambda expression

Whats the minimum depth for Files.walk to get a non-zero result?

at least 1

Does Files.walk traverse symbolic links by default?

How to follow symbolic links with Files.walk?

try(var s = Files.walk(source, FileVisitOption.FOLLOW_LINKS))..

What happens if Files.walk follows symbolic links and there is a link to root?

Every file in the system will be searched

What happens with Files.walk if it follows symbolic links but a Path is visited twice?

It will throw a FileSystemLoopException

Whats the syntax of Files.find?

public static Stream<Path> find(Path start, int maxDepth, BiPredicate<Path, BasicFileAttributes> matcher, FileVisitOption... options) throws IOException

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