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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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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:

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= vor-/rückwärts,

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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,

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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 4 important time classes for the exam?

java.time.LocalDate
java.time.LocalTime
java.time.LocalDateTime
java.time.ZonedDateTime

Can we call new LocalDate()?

Does this work?:
LocalDate date = LocalDate.of(2020, Month.OCTOBER, 20);
LocalTime time = LocalTime.of(11,12,34);
System.out.println(date.format(DateTimeFormatter.ISO_LOCAL_TIME));
System.out.println(time.format(DateTimeFormatter.ISO_LOCAL_DATE));

No, will throw an exception because of incompatible types

How to add custom text values to dateTimeFormatters?

excape the text by surrounding it with a pair of single quotes: '
DateTimeFormatter.ofPattern("MMM dd', Party''s at' hh:mm);

How to get the users current locale?

Locale.getDefault()

How to get the german language from locale?

Locale.GERMAN

How to get the german region from Locale?

Locale.GERMANY

How to use the locale-builder?

new Locale.Builder().setRegion("US").setLanguage("en").build();

How to set the default locale?

Locale.setDefault(locale)

Does Locale.setDefault change the locale for the system?

No only for that one javaprogramm

What is the NumberFormat-method for getting a general-purpose formatter?

NumberFormat.getInstance()
NumberFormat.getInstance(locale)

What is the NumberFormat-method for getting a general-purpose formatter (with number instance)

NumberFormat.getNumberInstance()
NumberFormat.getNumberInstance(locale)

What is the NumberFormat-method for getting a formatter for formatting monetary amounts?

NumberFormat.getCurrencyInstance()
NumberFormat.getCurrencyInstance(locale)

What are the decimal format symbols?

# - omits the position if no digit exists for it . $2.2
0 - put a 0 in the position if no digit exists for it - $002.20

How to custom format numbers?

new DecimalFormat("###,###,###.0");

How to get a DateTimeFormatter-instance for formatting dates=

DateTimeFormatter.ofLocalizedDate(dateStyle)

How to get a DateTimeFormatter-instance for formatting times?

DateTimeFormatter.ofLocalizedTime(timestyle)

How to get a DateTimeFormatter-instance for formatting dates and times?

DateTimeFormatter.ofLocalizedDateTime(dateStyle, timeStyle)
DateTimeFormatter.ofLocalizedDateTime(dateTimeStyle)

What are possible FormatStyle-parameters?

SHORT, MEDIUM, LONG, FULL

How to get a formatter for a specific locale?

Datetimeformatter.withLocale(locale)

How to set the default locale only for displaying data about the locale?

with Locale.Category value DISPLAY

How to set the default locale only for formatting dates,numbers or currencies?

With Locale.Category value FORMAT

Where is the resourcebundle stored?

in a properties file

What is a properties file?

A textfile that contains a list of key/value pairs

How to get the bundle for a locale?

ResourceBundle.getBundle("name");
ResourceBundle.getBundle("name", locale);

How to get a set of all keys in a ResourceBundle?

resourceBundle.keySet()

What are the steps of getting the proper ResourceBundle?

Look for the resource bundle for the requested locale, followed by the one for the default locale
for each locale, check language/country, followed by just the language
use the default resource bundle if no matching locale can be found

Can java get keys from multiple resource bundles?

Yes, f.e. from the parent resource bundle

How to use params in the resourcebundle?

f.e. with {0} and {1} and then use :
String format = rb.getString("helloByName");
System.out.println(MessageFormat.format(format, "Tammy", "Henry"));

How to set properties in the properties class?

var props = new Properties();
props.setProperty("name", "Ourzoo");

Whats the difference in the properties class between get() and getProperty()?

only getProperty allowes a default value

When to use a supplier? Which function is provided?

when you want to generate or supply values without taking any input.
T get();

When to use a consumer? Whats the function?

when you want to do something with a parameter but not return anything
void accept(T t);

What does a function (the functional iterface) do? Which function impletemns it?

responsible for turning one parameter into a value of a potentially different type and returning it
R apply(T t);

What does a UnaryOperator/BinaryOperator do?

Unary: transforms its value into one of the same type
BinaryOperatior: merges two values into one of the same type

Unary: T apply(T t);
Binary: T apply(T t1, T t2);

What are convenience metthods for Consumer?

Consumer andThen(Consumer);

What are convenience metthods for Function?

Function andThen(Function);
Function compose(Function);

What are convenience metthods for Predicate?

Predicate and(Predicate);
Predicate negate();
Predicate or(Predicate);

Wbhat does the compose function do for the interface function?

chains functional interfaces

What is the optional-method for returning a value and what happens if the optional is empty?

get() , throws exception

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