Lectures

Question 1

Where to declare the result of the method?

Answer 1

When the method is created the type of the result is declared.

The result value is passed with the return keyword.

Question 2

What does it mean that Java is statically typed?

Answer 2

The type of a variable is explicitly defined at compile time.

Question 3

What does it mean that Java is strongly typed?

Answer 3

The compiler gives an error when types do not match.

Question 4

How to iterate over each element in a list?

Answer 4

List numbers = new ArrayList<>;

for (Interger number : numbers) {…}

Question 5

What are the values of byte type?

Answer 5

-128 until 127

8 bits

Question 6

What are the values of short type?

Answer 6

-32768 until 32767

16 bits

Question 7

What are the values of int type?

Answer 7

-2^31 until 2^31-1

32 bits

Question 8

What are the values of long type?

Answer 8

-2^63 until 2^63-1

64 bits

Question 9

What are the values of float type?

Answer 9

Real number approximation.

32 bits

Question 10

What are the values of double type?

Answer 10

More precise real number approximation

64 bits

Question 11

What are the values of char type?

Answer 11

A single Unicode character

16 bits

Question 12

What are the values of boolean type?

Answer 12

True or False

Question 13

When can you convert a primitive variable to a different type?

Answer 13

Automatic conversion is only possible if the conversion goes from a more specific to a more general type.

Question 14

What is a static method?

Answer 14

We can write static methods within a class, but we are not allowed to use the current or local instance variables. Only instance variables of objects of whom the reference was passed as an argument can be accessed.

You are never allowed to use this keyword, for example.

public static A_combine(A object1, A object2)

Question 15

What is “this” keyword?

Answer 15

This allows to explicitly refer the current or local instance variables.

Question 16

How to indicate that no object was found while executing a method?

Answer 16

Use the null reference.

Question 17

What is a null reference?

Answer 17

The null reference is used to indicate that there is no object.

Question 18

What is the default value of a instance variable of a non-primitive type?

Answer 18

Instance variables with a non-primitive type always get null as the default value.

Question 19

What happens if we try to call a method on a variable that is null?

Answer 19

will get a NullPointerExceptionand the program will stop.•

Question 20

How to generate a random numbers?

Answer 20

import java.util.Random;

Random ran = new Random(1234);
int myDie = 1+ran.nextInt(6);

Question 21

What is a random seed?

Answer 21

import java.util.Random;

Random ran = new Random(1234);

The argument 1234is called the random seed. You can use any number here. It fixes the random sequence to be generated (so 1234gives one random sequence, 5231another).It is useful to fix the random seed so the results of your run becomes reproducible. If you want to try how your program works with a different random sequence, just change the seed.

Question 22

What are different types of errors in Java?

Answer 22

1. Compiler
Syntax Errors
Type errrs
Structural errors (example with return)
2. Runtime errors
3. Logical errors

Question 23

What are different types of errors in Java?

Answer 23

1. Compiler
Syntax Errors
Type errors
Structural errors (example with return)
2. Runtime errors
Gives an exception
3. Logical errors
The program executes, but you get a wrong answer

Question 24

How to raise an exceptions?

Answer 24

“throw” keyword

if (condition) {
throw new TATATAException("Something went wrong")
}

Question 25

What is the difference between throw and trows?

Answer 25

Throw is used to actually to raise an exception? Throws Indicates something can go wrong in this method by defining throws in the method signature.

Question 26

How to catch an exception?t

Answer 26

``` try - catch if try { ... If there is the problem in the try block, than the catch part is immediately executed. } catch (TypeOfYourException1 e){ System.out.println ("Try again") } catch (TypeOfYourException2 e){ System.out.println ("Try again") } ```

Question 27

What is an interface in Java?

Answer 27

A way to get a polymorphism. Instead of having multiple specific methods that only accept objects of a single class, one can write a single method that works for all kinds of the INTERFACE objects. Example with the games.

Question 28

What is an interface in Java?

Answer 28

A way to get a polymorphism. Instead of having multiple specific methods that only accept objects of a single class, one can write a single method that works for all kinds of the INTERFACE objects. Example with the games. public interface GameValue { }

Question 29

What is an interface in Java?

Answer 29

A way to get a polymorphism. Instead of having multiple specific methods that only accept objects of a single class, one can write a single method that works for all kinds of the INTERFACE objects. Example with the games. A list with methods. public interface GameValue { }

Question 30

What is interface inheritance?

Answer 30

If you need additional features in the interface X you can create a new interface new X and mention that it extends X. public interface new_X extends X {...} Implicit casting from the more general interface to the extended interface is not possible.

Question 31

WHat is a subtype?

Answer 31

public interface BankScore extends GameValue {...} BankScore is a subtype A subtype can always do at least as much as its supertype.

Question 32

WHat is a supertype?

Answer 32

public interface BankScore extends GameValue {...} | GameValue is a supertype.

Question 33

What is class inheritance?

Answer 33

The subclass inherits the methods and instance vars of the superclass. Methods of the superclass as if they are defined in the current class (unless they are private).

Question 34

How many superclasses can a class have?

Answer 34

A class can have at most one direct superclass

Question 35

How many interfaces can a class implement?

Answer 35

A class can implement any non-negative number of interfaces.

Question 36

How many subclasses can a class have?

Answer 36

A class can have any number of subclasses, unless the class is final.

Question 37

How to call another cinstructor of the same class

Answer 37

this ()

Question 38

How to call a constructor of a super class?

Answer 38

super()

Question 39

If the superclass has one or more specific constructors then how to make sure everything is being initialized?

Answer 39

The subclass must call for one of them in its method.

Question 40

How to redefine the body of one or more methods of the superclass?

Answer 40

Use | @ Override

Question 41

When a super keyword should be used?

Answer 41

1. To call the constructor of a superclass. 2. When overriding a method, but still want to use the overridden method of the superclass. super.method()

Question 42

Liskov Substitution Principle

Answer 42

When you write methods that use your class, you don't know whether it will be executed by your original class, or by a subclass implemented by someone else. ``` The LSP: Subclasses of a class should not change the behavior specified in its superclass. Properties that are true for the superclass should sill be true for a subclass. ```

Question 43

Is the type of variable referring to the object or type of the object defines which method is executed in the subclasses of a certain class?

Answer 43

Type of the object

Question 44

How to forbid a method to be overridden by subclasses?

Answer 44

With a final | public final void method () {...}

Question 45

What is the difference between methods / variable of private and public classes?

Answer 45

The public can be called from anywhere (any class) | Private can be called only from within the same class.

Question 46

WHat are protected methods and variables?

Answer 46

``` Can be called from the same class Can be called from within subclasses Can be called from within the same package. ```

Question 47

What is the instanceof operator?

Answer 47

It is used to check whether it is safe to cast a supertype to subtype.

Question 48

What is the cosmic superclass?

Answer 48

Object is the class of all classes that do not have an explicit superclass defined on it. There are number of methods defined in the Object class. Close, finalize getClass etc. Important methods : equals(), hashCode(), toString().

Question 49

What is the difference between == and .equals?

Answer 49

== is the objects are exactly the same. (for a non-primitive variables are considered different, since they have different mempry locations). equals checks if the content of the object is the same.

Question 50

How to write a class?

Answer 50

Own .java file Basic syntax Def vars, constructors and methods

Question 51

Difference between instance vars, class (static) vars and local vars?

Answer 51

instance vars - individual for each object of a class. class (static) vars - single global var, correspond to a class. single value local vars - live locally in a method.

Question 52

Difference between static and regular methods?

Answer 52

regular methods - operate in an object directly. | static methods - do not have an instance they are working on, but work with objects in genral.

Question 53

What do Accessor methods do

Answer 53

Only read information from the object.

Question 54

What do Mutator methods do

Answer 54

Changes the state / info stored in the object

Question 55

JAVA is a strongly typed language. what does it mean?

Answer 55

All variables and expression have a type, which must be declared

Question 56

What is a distinction between primitive types and non-primitive types?

Answer 56

8 primitive types Primitive variable - hold actual values Non-prim vars - hold references There is a non-prim for each primitive variables.

Question 57

What is a distinction between primitive types and non-primitive types?

Answer 57

8 primitive types Primitive variable - hold actual values. Cannot be null Non-prim vars - hold references. Can be null if unassigned There is a non-prim for each primitive variables.

Question 58

What is implicit casting?

Answer 58

Allows to convert vars safely: int to long.

Question 59

What is explicit casting?

Answer 59

Allows to convert vars unsafely: long to int. | How?

Question 60

What are generic types?

Answer 60

... Allows to create the ArrayList

Question 61

What is the inharitence between subclasses and superclasses?

Answer 61

subclasses are of the type of their superclasses, the other way around needs explicit casting.

Question 62

How to visualize the relations between types?

Answer 62

Class inharitance digram

Question 63

What are immutable objects?

Answer 63

String, Non-prim prim vars (Integer, Boolean)

Question 64

WHat is the name of the calsses, whose objects can be modified?

Answer 64

Mutable

Question 65

What is a Memory State Digram?

Answer 65

Shows references and assignments in the class.

Question 66

Which keyword calls an exception?

Answer 66

trow

Question 67

What is a type of an exeption object?

Answer 67

Throwables

Question 68

How to indicate that a method can throw an exception?

Answer 68

Using throws keyword in the method header

Question 69

What is the difference between checked and unchecked exceptions?

Answer 69

unchecked exceptions are error and tuntimeexception. We are not too concerned with it. checked exceptions - all other exceptions. need to be caught or thrown to the calles When you are working with files we have to take care of IOExeption FileNotFoundExeption using try - catch w

Question 70

How to achieve polymorphysm?

Answer 70

Interfaces A class implements interface with implements keyword. Inheritance Indicated with the keyword extends. Can be for classes and interfaces. There is a relation beween superclass and the subclass relation.

Question 71

Abstruct classes

Answer 71

Used in case we want to implement part of a class, we leave some methods open for subclasses to implemnt. ``` Called with the keyword abstract in the header. New can be only on the subclass of an abstruct class. ```

Question 72

What is the class object?

Answer 72

If a class is not explicitly extending another clas, it implicitly extends Object.

Question 73

hashCode()

Answer 73

gives memore adress in a form of a number

Question 74

equals()

Answer 74

uses memory adress of an object to determine wheather two objects are equal.

Question 75

Comperator interface

Answer 75

Compares two objects

Question 76

Comparable interface

Answer 76

Compare this object to another

Question 77

IS there an index / order in a set?

Answer 77

No. There is no order in the set. | Enhanced-loop should be used.