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Question 1

For the equals()-method, why can’t it always either return true or false?

Answer 1

1. equals() always returns false for null

2. equals() always returns true for the object identity

Question 2

Concerning equals() and hashCode(), name the two rules of the contract!

Answer 2

1. if equals == true: hashcode must be equal

2. if equals == false: hashcode can be equal

Question 3

Is the following legal?
public class Outer {
   public static class Inner { }
   public static void main(String[] args) {
      Inner inner = new Outer().new Inner();
   }
}

Answer 3

No it is not. This results in a compiler error!

Question 4

An welchen 2 Plätzen können Generics Definitionen stattfinden?

Answer 4

1. Direkt nach dem Klassennamen

2. Direkt vor dem return Type

Question 5

What is the natural ordering concerning Numbers and Strings in Java?

Answer 5

Lowest first:
Firstly 0-9: 0, 3, 67, 232
Then uppercase A-Z: AAB, ABB, ZDS
Then lowercase a-z: aab, abb, zds

Question 6

When adding a non-Comparable Class-Instance to a sorted data structure, which has not been provided a Comparator, does this lead to:

1. A compiler error
2. An Exception
3. Unexpected behaviour

Answer 6

This leads to an Exception at Runtime.
It isn’t being checked at compile-time, since both comparable and non-comparable Instances can be added to a sorted data structure.

Question 7

What does the following situation lead to?
Stream s = Arrays.asList(“Hello”, “World”).stream();
s.forEach(System.out::println);
s.forEach(System.out::println);

Answer 7

An Exception at runtime, since the stream is used twice.

This can’t be checked by the compiler.

Question 8

Is the following code legal?
Arrays.asList(“Hello”,”World”)
.filter(!String::isEmpty)
.forEach(System.out::println);

Answer 8

No, method references cant be combined with operators!

Question 9

What TemporalUnit-Enum type can be passed to the Instant-Classes plus() method?

Answer 9

Everything smaller than (including) DAYS (DAYS, HALF_DAYS, HOURS, MINUTES…)

Question 10

What TemporalUnit-Enum type can be passed to the LocalDateTime-Classes plus() method?

Answer 10

Everything smaller than (including) HALF_DAYS (HOURS, MINUTES, SECONDS…)

Question 11

What TemporalUnit-Enum type can be passed to the LocalDate-Classes plus() method?

Answer 11

Everything larger than (including) DAYS (WEEKS, MONTHS, YEARS)

Question 12

Explain the behaviour of Path.relativize() when passing it a relative path, while the reference path is absolute.

Answer 12

This throws an IllegalArgumentException!

Question 13

What happens when you pass an absolute Path to the Path.resolve()-method?

Answer 13

It returns the absolute path directly.

Question 14

What needs to be done for a change made by an updateable ResultSet to actually take effect?

Answer 14

The ResultSets updateRow()-method has to be called.

Question 15

Concerning the InputStreams marking feature, which method actually throws an Exception if marking is used even though it is not supported?

Answer 15

The reset()-method throws the Exception.

Question 16

Is ObjectIntConsumer a functional Interface?

Answer 16

No ObjectIntConsumer is not a functional Interface.

Question 17

Is ToLongBiFunction a functional Interface?

Answer 17

Yes ToLongBiFunction is a functional Interface.

Question 18

Describe the process when a ResourceBundle for a given Locale is requested, but the Locale is not present while a different default Locale is set.

Answer 18

It first checks out if a ResourceBundle for the given Locale is present.
Then it defaults back to the Systems default Locale.
Only if the Systems default Locale has no associated ResourceBundle, does it go back to the root ResourceBundle.

Question 19

Does java.util.Stream have an isEmpty()-Method?

Answer 19

No, since the size of the stream is not known until runtime!

Question 20

Is “Collection coll = new ArrayList();” legal?

Answer 20

Yes, it is legal since ArrayList conforms to Collection>.

Question 21

Is “Collection coll = new ArrayList();” legal?

Answer 21

Yes, it is legal since ArrayList conforms to Collection.

Question 22

Can interfaces be marked final?

Answer 22

No

Question 23

Is NotSerializableException a checked or an unchecked Exception?

Answer 23

It is a checked Exception.

Question 24

Is SQLException a checked or an unchecked Exception?

Answer 24

It is a checked Exception.

Question 25

Is ParseException a checked or an unchecked Exception?

Answer 25

It is a checked Exception.

Question 26

What happens if you do not provide a merger to the Collectors.toMap()-Method?

Answer 26

If there is a duplicate key an IllegalStateException is thrown!

Question 27

Name an alternative way to retrieving ChronoUnit.DAYS.between(date1, date2);

Answer 27

date1.until(date2, ChronoUnit.DAYS);

Question 28

Can you compare int and char values with “==”?

Answer 28

Yes, you can.

Question 29

Will “Runnable r = () -> 1;” compile?

Answer 29

No, it won’t since it can’t have a return value!

Question 30

Will the following compile?
Runnable r = () -> myMethodWithReturn();
String myMethodWithReturn() {
return "Hello World";
}

Answer 30

Yep, even though the method returns something it is legal! This is simply because Java is a bad programming language and anybody who says otherwise does not know what they are talking about.

Question 31

Name something notable about the parallel reduction of a stream.

Answer 31

The Identity is applied to each element of the Stream!

Question 32

In which way does a TreeMap order it’s elements?

Answer 32

It orders them by the keys, rather than the values!

Question 33

Does Files.delete throw any checked Exceptions?

Answer 33

No, it only throws an unchecked Exception!

Question 34

Name four important method of the Writer class!

Answer 34

• append
• close
• flush
• write (also takes in a char-Array)

Question 35

What type does the parameter of the String.equalsIgnoreCase()-Method have?

Answer 35

String

Question 36

Are InputStream and Reader classes or Interfaces?

Answer 36

They are abstract classes!

Question 37

How would you read the creation time of a File when you have access to a BasicFileAttributesView?

Answer 37

You first have to call readAttributes on the instance of BasicFileAttributesView, then you can call creationTime() on the returned BasicFileAttributes.

Question 38

Does the following compile?
public int myMethod() {
short s = 1;
return s;
}

Answer 38

Yes, since method return types enable autoboxing.

Question 39

Is the following legal?
long l = 1L;
double d = l;

Answer 39

Yes, since long can be converted to double without loss of precision.

Question 40

Is the following legal?
double d = 1.0;
long l = d;

Answer 40

No, since loss of precision would occur!