Lecture 1

Question 1

What are primitive types (mention some)?

Answer 1

Types that are already defined by the program (int, float, double, char, boolean)

Question 2

What are non-primitive types (mention some)?

Answer 2

Types that the programmer defines for the program (arrays, strings, classes)

Question 3

Primitive types:
Describe whole numbers and decimal numbers

Answer 3

Whole numbers: long, byte, int, short
Decimal numbers: float, double (default)

Question 4

What is a value literal?

Answer 4

The letter at the end of a variable initialisation:
float 1.2f (without f, it will be interpreted as double)
double 1.6 or 1.6d (interpreted as double, since it’s default (d optional))

Question 5

What is a local variable?

Answer 5

Variables that are declared inside the body of a method

class Guru99 {
static int a = 1; //static variable
int data = 99; //instance variable
void method() {
int b = 90; //local variable
}
}

Question 6

What is an instance variable?

Answer 6

Defined without the static keyword. They are defined outside a method declaration and are object specific

class Guru99 {
static int a = 1; //static variable
int data = 99; //instance variable
void method() {
int b = 90; //local variable
}
}

Question 7

What is a static variable?

Answer 7

Initialised only once, at the start of the program execution. They should be initialised first (before any instance variables)

class Guru99 {
static int a = 1; //static variable
int data = 99; //instance variable
void method() {
int b = 90; //local variable
}
}

Question 8

What does null reference mean?

Answer 8

null (used for non-primitive types) will reference to nowhere, since it has not been set to target a proper element

Question 9

Write the skeleton of an “if” statement.

Answer 9

boolean condition = true; // has to be true or false
// …

if (condition) {
// … happens if true (then block, mandatory)
} else {
// … happens if false (else block, optional)
}

Question 10

Write the skeleton of a “while” statement.

Answer 10

boolean condition = true;

while (condition) {
// Perform operations repeatedly
// Modify condition to eventually end loop
}

(if the condition is initially false, it will not execute, and will repeat as long as the condition holds)

Question 11

Write the skeleton of a “for” loop statement. What does the (initialiser, condition, increment/decrement) (IN THAT ORDER) do?

Answer 11

e.g.:
for (int i = 0; i < 10; i++) {
// Perform operation ten times
}

Initialiser: one time before the first loop
Condition: checked every time before loop
Increment/Decrement: every time
after loop

Question 12

Write the skeleton of a “for each” loop

Answer 12

for (type variableName : arrayName) {
// code block to be executed
}

Question 13

Can you use “==” (equality operator) on Strings?

Answer 13

NO. Instead use “equals()”.
Example:
if (message.equals(“Hello World”)){
//…
}

Question 14

What is Class-based OOP and what does it contain?

Answer 14

You have a Class, that Class contains Data and Actions on that Data. Then you have an Object that is an Instance of the whole Class

Question 15

What is a Class?

Answer 15

Represents many objects that belong to the Class. Will group data and actions on that data for data which logically belong closely together (high coherence)

Question 16

What is an object (instance of a class)?vend tilbage

Answer 16

Is the property of a specific element. A class is the super, and an object is inside of the class.

Question 17

How do you create an new object/instance of a class. Give an example:

Answer 17

You use the “new” keyword and reference to the object with a new variable name.

class Car {
// …
}

class CarProgram {
public static void main(String[] args){
Car myCar = new Car();
(my car is the new object you’re creating from the Car class)

Question 18

What is an attribute?vend tilbage til

Answer 18

A data element in a class, sometimes known as field (DATA)

Question 19

What is a method ?

Answer 19

a function on data, sometimes known as operation (ACTION ON DATA)

Question 20

What is a member?

Answer 20

Umbrella term for attributes and methods (ATTRIBUTE, MEMBER, CONSTRUCTOR)

Question 21

How to write the “main()”?

Answer 21

public static void main(String[] args)
(public static is mandatory)

Question 22

Describe attributes.

Answer 22

They’re declared within a class.
Consists of modifiers (optional), type, name.

Question 23

Give an example of attributes.

Answer 23

class Car {
private String licensePlate;
private double gasLevel;
}
(modifier, type, name)

Question 24

What is in a method.

Answer 24

Signature:
- Visibility modifiers (optional)
- Return type (possibly void)
- Name
- Parameter(s) (optional) Name and Type

Body:
- Code
- return statement (at least one if not void)

Question 25

Give an example of methods.

Answer 25

class Car {
double getGasLevel() {
return gasLevel;
}
public void refuel(double amount) {
// …
}
}

Question 26

How do you make comments?

Answer 26

single line comment = //
multi line comment = /* and */
java documentation = /** and */

Question 27

What does the visibility modifier public do?

Answer 27

public (+) = Everyone can see this class, attribute, method, constructor, and it’s therefore accessible by any other class.

Question 28

What does the visibility modifier private do?

Answer 28

private (-) = Can be used for subclasses (only, not a super), attributes, methods and constructors, making them only accessible within the declared class.

Question 29

What does the visibility modifier protected do?

Answer 29

protected (#) = Can be used for subclasses (only, not a super), attributes, methods and constructors, making them accessible in the same package and subclasses.

Question 30

What is static?vend tilbage til

Answer 30

It’s a non-access modifier used for methods and attributes. Static methods/attributes can be accessed without creating an object of a class.

Question 31

What is void?

Answer 31

The void keyword specifies that a method should not have a return value.

Question 32

Why is code duplication bad?

Answer 32

It gives you more to maintain, and one modification means you have to change everything else. Inheritance can solve this problem.

Question 33

What is inheritance?

Answer 33

A sub-class inherits all the members of a super class (given the visibility modifier)

Question 34

How do you use inheritance? Give an example.

Answer 34

You use the “extends” keyword, like this:
class Car {
String licensePlate;
void drive() {
// …
}
}

class GasolineCar extends Car {
double gasLevel;
// …
}

class ElectricCar extends Car {
double batteryLevel;
// …
}

GasolineCar extends Car and GasolineCar inherits from Car.
Car is the super class of GasolineCar and GasolineCar is a subclass of Car

Question 35

What are packages?

Answer 35

A logically grouping of coherent classes. Packages allow for different classes to have the same name since they’re in different packages. They can be nested.

Question 36

Give an example of packages, and an example in code.

Answer 36

Vehicles: Car, motorbike, gasoline car, electric car
Biology (nested):
Animals: Mammal, reptile, bird
Plants: Flower, bush, tree

package vehicles;
import vehicles.Car;
class GasolineCar extends Car {
// …
}

Question 37

What is encapsulation?

Answer 37

It regulates access to members from the outside. We put the class in a capsule to protect it. We use visibility modifiers for this.

Question 38

Is encapsulation/visibility modifiers a security mechanism?

Answer 38

NO, they’re a design mechanism, as people with bad intentions can still get around this fact.

Question 39

How do you achieve encapsulation?

Answer 39

Declare class variables/attributes as private and by providing public get and set methods to access and update the value of a private variable. By using get and set methods, we can make it accessible.

Question 40

What are the benefits of OOP?

Answer 40

Abstraction, encapsulation, inheritance, polymorphism

Question 41

What is required from a variable to be made in a class?

Answer 41

They must be declared and they must have a type.