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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"?

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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");

}

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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;

}

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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?

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F.e.:

public Animal(List<String> favoriteFoods) {

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

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

}

What is a defensive copy?

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F.e. the copy operation

Does the clone() method copy shallow or deep?

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shallow by default

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

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Throw exception

What can be considered as untrusted data?

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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?

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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 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

When does readResolve run?

After the readObject - method

What is writeReplace for?

F.e. if we want to conrtrol which object is serialized instead of just serializing this

When is writeReplace running?

before writeObject

How to construct sensitive objects?

Making class private
making method private
constructor private

What is a resource leak and how to prevent this?

When the resource is never closed -> close it

Why is reading of files w.o. cheging its size a problem?

The files could be very large -> check the size first to prevent memory issues

What is a inclusion attack?

F.e. billion laughs attack or zip bomb. A file which could expand more and more to become really big

What is the problem with ovwerflowing numbers? How to prevent?

Unpredictable outcomes -> input validation

What is "wasting data structures"?

F.e. the possibility to create a class with a hashCode which always return 42 and put it to a hashmap or the possibility to create very large datastructures

Does ArrayList have a copy method?

What does ArrayList.clone() return?

Object -> be aware to cast it

Where is the clone() Method declared?

In the objectclass

Whats the difference between a black and a whitelist?

Whitelist compares against allowed values (could also throw an exception if not valid)

What does readObject() return?

Object

What does readResolve() return?

Object

Where are the 5 key interfaces for JDBC declared?

In the JDK

Where do we get the concrete classes for JDBC interfaces from?

From the JDBC driver

Which interfaces are implemented within the driver JAR?

Driver, Connection, PreparedStatement, CallableStatement, ResultSet

Do we use the concrete JDBC classes in code?

No, only the interfaces

Whats neccessary in a jdbc url? Which colons?

prefix jdbc + subprotocol for the db + name/address of the database, separated by :

jdbc:postgresql://localhost/zoo

Is it possible to create a jdbc-connection via constructor

No, use DataManager.getConnection for the exam, in real life: DataSource

Which exception is thrown by DriverManager.getConnection?

SqlException (check in examquestions if it is thrown)

What are the benefits of a PreparedStatement?

Performance
Security
Readability
Future use

Is it possible to create a preparedStatement without providing a query as parameter?

No, this will not compile:

var ps = conn.prepareStatement()

How to get the returned Value of ps.execute() ?

Check first if it returned something (returned value will be boolean indicating if returnvalue or updatecount), then get the resultset via ps.getResultset if it is true or ps.getUpdateCount if it is false

What happens with incompatible returntypes? (f.e. SELECT-query but executeUpdate() called)

Will throw a SQLException like: .executeUpdate cannot be called with a statement

What is the returntype and the returnvalues for select/[delete|insert|update] for ps.execute()

boolean, true for select, false for updates

What is the returntype and the returnvalues for select/[delete|insert|update] for ps.executeQuery()?

ResultSet, rows and columns returned for select

What is the returntype and the returnvalues for select/[delete|insert|update] for ps.executeUpdate?

int, number of rows added/changed/removed returned for updates

Where does the bindvariables within a preparedStatement start to count?

start with 1

What happens if not all bindVariables are set in a preparedStatement?

Code compiles but SQLException

What happens if to many bindVariables are provided for a prepared statement?

SQLException like "the number of values assigned is not the same as the number of specified or implied columns"

Can we use the same perparedStatement for multiple executeUpdates? Whats with the bindVariables?

Yes, the perparedStatement will remember the parameters that were already set if not changed

What does resultSet.next() do?

Sets the cursor to the next element, returns boolean indicating if it worked

Where does the counting stat within a resultSet?

at 1

What params does f.e. resultSet.getInt accept?

either 1/2/.. or the columnname

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