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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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Informatik

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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 are the signatures of PrintStream/PrintWriter.format-methods?

public PrintStream format(String format, Object args...)
public PrintStream format(Locale loc, String format, Object args...)
public PrintWriter format(String format, Object args...)
public PrintWriter format(Locale loc, String format, Object args...)

What happens here?

System.out.format("Food: %d tons", 2.0);

IllegalFormatConversionException -> int expected, floating point provided -> use %f instead

Why is it unnecessary/ unrecommended to close SystemStreams (like System.out)?

The JVM creates and opens them for us -> closing makes them permantly unavailiable for all threads in the remainder of the programm

What happens here?:

try(var out = System.out) {}
System.out.println("Hello");

Prints nothing

What happens here?

try (var err = System.err) {}
System.err.println("Hello");

Prints nothing

What happens here?

var reader = new BufferedReader(new InputStreamReader(System.in));
try(reader) {}
String data = reader.readLine();

IOException at runtime -> most InputStream classes will throw an exception if you try to operate on a closed stream

Which exception is thrown by Files.deleteIfExists?

IOException

Whats the NIO2 replacement for java.io.File.class?

java.nio.file.Path

Do we make a difference in how we treat a instance if it is a file or directory in NIO2?

Does Path support symbolic links?

Yes

Does java.io.File support symbolic Links?

Can we create an instance of Path directly?

No, is a interface

Whats the syntax of the Path factory method?

public static Path of(String first, String... more)

Whats the characteristics of a relative / absolute path for the exam?

A absolute starts with / -> the root directory or with c: .. / d://

a relative would be f.e.: bird/parrot.png

What does the varags in Path.of do?

accepts additional path elements whcih will be combined and automatically be separated by the operation system-depending file separator

What are two ways to get a instance of Path?

Path.of and Paths.get

What are ways to get a Path instance from a Uri?

f.e.: URI a = new URI("file://iceman.txt");
Path b = Path.of(a);
Path c = Paths.get(a);
URI d = b.toUri();

How to get a Path via Filesystems? Whats the syntax?=

FileSystems.getDefault().getPath("/home/zooDirectory");

How to convert a Fileto a Path? How the other way around?

File file = new File("some.png");
Path path = file.toPath();
File backToFile = path.toFile();

Whats the enum value,type and inherited interface for not following symoblic links?

NOFOLLOW_LINKS , LinkOption, CopyOption/OpenOption

Whats the enum value,type and inherited interface for moving a file as atomic file system operation?

ATOMIC_MOVE, StandardCopyOption , CopyOption

Whats the enum value,type and inherited interface for coping existing attributes to new file?

COPY_ATTRIBUTES , StandardCopyOption , CopyOption

Whats the enum value,type and inherited interface for overwriting files if it already exist?

REPLACE_EXISTING , StandardCopyOption , CopyOption

Whats the enum value,type and inherited interface for appending to the end of a file if it is already open for write

APPEND, StandardOpenOption , OpenOption

Whats the enum value,type and inherited interface for creating a new file if it does not exist?

CREATE, StandardOpenOption, OpenOption

Whats the enum value,type and inherited interface for creating a new file if it does not exist or overwrite

CREATE_NEW, StandardOpenOption, OpenOption

Whats the enum value,type and inherited interface for opening for read access?

READ, StandardOpenOption, OpenOption

Whats the enum value,type and inherited interface for erase file and append to beginning if it is already open for write

TRUNCATE_EXISTING, StandardOpenOption, OpenOption

Whats the enum value,type and inherited interface for opening for write access?

WRITE, StandardOpenOption, OpenOption

Whats the enum value,type and inherited interface for following symbolic links?

FOLLOW_LINKS, FileVisitOption

Whats the default behaviour of Files.exists() for symbolicLinks?

checks the target of the smybolic link

Are Path instance mutable?

No, just like string-values

What happens here?
Path p = Path.of("whale");
p.resolve("krill");
System.out.print(p);

prints whale

Whats a rule of thumb when a method declares a IOException within NIO2?

When it requires the paths it operates on to exists

What happens if we call Path.getName with an invalid Index?

Throws an IllegalArgumentException at runtime

Are the references in Path.subpath(int beginIndex, int endIndex) inclusve or exclusive?

inclusive beginindex, exclusive endIndex

What happens with subpath if a invalid index is provided (also with getName(), subpath())

Throws a IllegalArgumentException

When does getParent() return null?

if operated on the root path or at the top of a relative path

What does getRoot return if the Path is a relative path?

Does not traverse outside the current working directory, returns null if it as the top of the relative path

Does getParent resolve pathSymbols like ".."?

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