How could this be rewritten? private static void threeDigit(Optional optional) { if(optional.isPresent() { var num = optional.get(); var string = "" + num; if(string.length() == 3) System.out.println(string); } }

private static void threeDigiht(Optional optional) { optional.map(n -> ""+n) .filter(s -> s.length() == 3) .ifPresent(System.out::println); }

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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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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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For which functions is the method valid and what is the method for returning an object containing numerous stream statistics such as the average, min, max, etc?

IntSummaryStatistics/LongSummaryStatistics/DoubleSummaryStatistics summaryStatistics()
IntStream/LongStream/DoubleStream

Whats the diffierence between OptionalDouble and Optional<Double>?

OptionalDouble is for primitives and Optional<Doubloe> for the double wrapper class

What are the optionals for primitives?

OptionalDouble, OptionalInt, OptionalLong

How could this be rewritten?
private static void threeDigit(Optional<Integer> optional) {
if(optional.isPresent() {
var num = optional.get();
var string = "" + num;
if(string.length() == 3)
System.out.println(string);
}
}

private static void threeDigiht(Optional<Integer> optional) {
optional.map(n -> ""+n)
.filter(s -> s.length() == 3)
.ifPresent(System.out::println);
}

How to flat two optionals?

optional.flatMap

What is the Collector and whats the return value for calculationg the average for our three core primitive types?

Double averagingDouble(ToDoubleFunction f)
Double averagingInt(ToIntFunction f)
Double averagingLong(ToLongFunction f)

What is the Collector and whats the return value for counting the number of elements?

Long counting()

What is the Collector and whats the return value for creating a map grouping by the specified function with the optional map type spplier and optional downstream collector?

Map<K, List<T>> groupingBy(Function f)
Map<K, List<T>> groupingBy(Function f, Collector dc)
Map<K, List<T>> groupingBy(Function f, Supplier s, Collector dc)

What is the Collector and whats the return value for creating a single string using cs as a delemiter between elements if one is specified?

String joinig(CharSequence cs)

What is the Collector and whats the return value for finding the largest/smallest elements?

Optional<T> maxBy(Coparator c)
Optional<T> minBy(Comparator c)

What is the Collector and whats the return value for adding another level of collectors?

Collector mapping(Function f, Collector dc)

What is the Collector and whats the return value for creating a map grouping by the specified predicate with the optional further downstream collector?

Map<Boolean, List<T>> partitioningBy(Predicate p)
Map<Boolean, List<T>> partitioningBy(Perdicate p, Collector dc)

What is the Collector and whats the return value for calculating average, min, max, ...

DoubleSummaryStatistics summarizingDouble(ToDoubleFunction f)
IntSummaryStatistics summarizingInt(ToIntFunction f)
LongSummaryStatistics summarizingLong(ToLongFunction f)

What is the Collector and whats the return value for calculating the sum for our three core primitive types?

Double summingDouble(ToDoubleFunction f)
Integer summingInt(ToIntFunction f)
Long summingLong(ToLongFunction f)

What is the Collector and whats the return value for creating an arbitrary tpye of liust or set?

List toList()
Set toSet()

What is the Collector and whats the return value for creating a collection of the specified type?

Collection toCollection(Supplier s)

What is the Collector and whats the return value for creating a map using functions to map the keys, values, an optional merge function and an optional map type supplier?

Map toMap(Function k, Function v)
Map toMap(Function k, Function v, BinaryOperator m)
Map toMap(Function k, Function v, BinaryOperator m, Supplier s)

What happens here? How To fix?
var ohMy = Stream.of("lions", "tigers", "bears";
Map<Integer, String> map = ohMy.collect(Collectors.toMap(String::length, k-> k));

java.lang.IlllegalStateException: Duplicate key 5 (two have the same length)
var ohMy = Stream.of("lions", "tigers", "bears"M
Map<Integer, Stream> map = ohMy.collect(Collectors.toMap(String::length, k->k, (s1,s2) -> s1 + "," + s2));

Could the function you call in groupingBy return null?

no, it does not alllow null keys

How to change the type of the map returned by Collectors.groupingBy?

F.e.:
Collectors.groupingBy( String:: length,
TreeMap::new, Collectors.toSet());

How to change the type of map returned by partitioningBy?

We cannot change the type returned by parititionBy?

How to add a second collector?

With Collectors.mapping, F.e.
Collectors.groupingBy( String::length,
Collectors.mapping( s-> s.charAt(0), Collectors.minBy((a,b)->a-b))));

Is it possible to use multiple parameters with intance method references?

Yes

What happens here?
var heights = new ArrayList<Integer>();
heights.add(null);
int h = heights.get(0);

NullPointerException -> we try to unbox null into a int primitive

Are duplicates allowed in lists?

yes

What happens with remove() on a list and a index passed which does not exists?

IndexOutOfBoundsException

When is ArrayList a good choice?

when you are reading more often than(or the same amount as) writing to the ArrayLList

When is LinkedList a good choice?

when yo will be using it as a queue

What are the factorymethods to create a list?

Arrays.asList(varargs)
List.of(varargs)
List.copyOf(collection)

Is it possible to add / replace / delete elements on a list which is created by Arrays.asList(varargs)?

No, Yes, No

Is it possible to add / replace / delete elements on a list which is created by List.of(varargs)?

No, No, No

Is it possible to add / replace / delete elements on a list which is created by List.copyOf(collection)

No, No, No

Is Map.of with two or more key-value-pairs possible?

Yes but key and value per set has to exist.
F.e. Valud: Map.of("key1","value1","key2","value2");
invalid: Map.of("key1","value1", "key2");

Can the type List contain duplicate elements / elements always ordered / has keys and values / must add/remove in specific order?

Yes / Yes / No / No

Can the type Map contain duplicate elements / elements always ordered / has keys and values / must add/remove in specific order?

Yes / No / Yes / No

Can the type queue contain duplicate elements / elements always ordered / has keys and values / must add/remove in specific order?

Yes / Yes / No / Yes

Can the type Set contain duplicate elements / elements always ordered / has keys and values / must add/remove in specific order?

No / No / No / No

Is a type ArrayList sorted? / calls hashcode? / calls compareTo?

No / No / No

Is a type HashMap sorted? / calls hashcode? / calls compareTo?

No / Yes / No

Is a type HashSet sorted? / calls hashcode? / calls compareTo?

No / Yes / No

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