Whats wrong here?: new BufferedInputStream(new FileReader("z.txt")); new BufferedWriter(new FileOutpuitStream("z.txt")); new ObjectInputStream(new FileOutputStream("z.txt")); new BufferedInputStream(new InputStream());

Mix reader/writer " Mix outputStream with InputStream InputStream is a abstract class

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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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Lernkarten

How to rename a file/directory with a File-Object?

boolean renameTo(File dest)

What is the benefit of a BufferedOutput-Stream?

Can write a large chunk of bytes at a time, reducing round-tips and improve perfomance

Which two sets of stream-classes are difined in the java.io.API? How does the names end?

Byte-streams that end in InputStream/OutputStream
Character streams that end in Reader/Writer

Are Reader/Writers still streams?

Yes, I/O Streams

What are advantages of using a characterstream even if byte streams can do everything?

They are specificially focused on managing character/stringdata -> no need to worry about the underlying characterencoding of the file

Does the class PrintReader exist compliant to PrintWriter?

Is there a InputStream class for PrintStream?

Npo

What are the four abstract parents of all Streams?

InputStream, OutputStream, Reader, Writer

Whats wrong here?:

new BufferedInputStream(new FileReader("z.txt"));
new BufferedWriter(new FileOutpuitStream("z.txt"));
new ObjectInputStream(new FileOutputStream("z.txt"));
new BufferedInputStream(new InputStream());

Mix reader/writer
"
Mix outputStream with InputStream
InputStream is a abstract class

Whats the stream-class for reading file data as bytes? Low/Highlvl?

FileInputStream - low

Whats the stream-class for writing file data as bytes? Low/Highlvl?

FileOutputStream - low

Whats the stream-class for reading file data as characters? Low/Highlvl?

FileReader -low

Whats the stream-class for writing file data as characters? Low/Highlvl?

FileWriter - low

Whats the stream-class for reading byte data from exsting InputStream in buffered manner? Low/Highlvl?

BufferedInputStream - high

Whats the stream-class for writing byte data to an existing outputstream in a buffered manner? Low/Highlvl?

BufferedOutputStream - High

Whats the stream-class for reading character data from exsting Reader in a buffered manner? Low/Highlvl?

BufferedReader - high

Whats the stream-class for writing character data to an existing writer in a buffered manner? Low/Highlvl?

BufferedWriter

Whats the stream-class for deserializing primitive java datay types and graphs of java objects from an existing InputStream ? Low/Highlvl?

ObjectInputStream - High

Whats the stream-class for serializing primitive java data types and graphs of java objects to an existing outputstream? Low/Highlvl?

ObjectOutputStream - High

Whats the stream-class for writing formatted representations of java objects to a binary stream? Low/Highlvl?

PrintStream - High

Whats the stream-class for writing formatted representations of objects to a character stream? Low/Highlvl?

PrintWriter - High

Does OutputStream/Writer.write and IntputStream/Reader.read() use byte-values?

No, ints: void write(int b); int read()

Whats the signature of InputStream/reader.read?

public int read() throws IOException

Whats the Sginature of OutputStream/writer.write()?

public void write(int b) throws IOException

How to check if an InputStream is at the end?

will return -1

What does offset 5, length 3 mean for a OutputStream?

should read up to 3 bytes and put them into the array stating with position 5

Are there read/write methods for Output/Input-stream/Reader/writer with offset and length?

Yes, overloaded, f.e.:
read(byte[] b, int offset, int length)
write(char[] c, int offset, int length)

How to close a IOStream?

public void close() throws IOException

Where should a stream which is passed around be closed?

In the method that created it

How to check if a stream supports mark()?

public boolean markSupported()

What dopes the readlimit mean in mark?

Instructs the stream that we expect to call reset() after at most x-bytes. if later: it may throws an exception, depending on the stream class

What does the returnparam of skip tells us?

How many values actuell got skipped

Whats the signature for closing a stream and releasing resoruces ? For what stream class(es)?

void close() - All streams

Whats the signature for reading a single byte/return -1 if no bytes are availiable? For what stream class(es)?

int read() -- all input-streams

Whats the signature for reading values into a buffer, returning the number of bytes read? For what stream class(es)?

int read(byte[] b) / int read(char[] c) : InputStream / Reader

Whats the signature for reading up to length values into a buffer starging from offset, returning number of bytes read? For what stream class(es)?

int read(byte[] b, int offset, int length) - InputStream
int read(char[] c, int offset, int length) - Reader

Whats the signature for writing a single byte? For what stream class(es)?

void write(int) - all output streams

Whats the signature for writing an array of values into the stream? For what stream class(es)?

void write(byte[] b) - OutputStream
void write(char[] c) - Writer

Whats the signature for writing length values from an array into the stream, starting with an offset index? For what stream class(es)?

void write(byte[] b, int offset, int length) - OutputStream
void write(char[] c, int offset, int length) - Writer

Whats the signature for returning true if the stream supports mark? For what stream class(es)?

boolean markSupported() - All input streams

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