Lecture 2 and 3 objects

Question 1

What do objects do?

Answer 1

Objects allow us to bring together related variables and methods into a single thing.
eg a Scanner object.

Question 2

What are objects defined by?

Answer 2

Objects are defined by a class. Classes do not always need to include the main method. In general classes have:

1) a name (starting with a capital letter)
2) some variables (known as class attributes)
3) methods
4) some special methods, called constructors

If you don’t create a constructor, the system will create one for you with no arguments.

Each class is defined in its own specific file

Question 3

How do we use a class that we create?

Answer 3

We have to create another class to call it

Question 4

How do you create a new object? What is the general format if a class already exists?

Answer 4

= new (input1, input2);

input1 and input2 are the arguments passed to the constructor.

From there, you can call methods within the class by:
.();

You can change the value of the attributes in a class, from another class, by:

. = ;

Question 5

How can you tell if a method is a constructor by looking at it?
There are two main tell-tales.

Answer 5

The name of the constructor is the same as the name of the class and there is no return type.

e.g. public Circle(double r){
radius = r;
}

Question 6

What does “private” mean?

Answer 6

These classes, attributes, and methods can only be used by the class in which they are defined.

Question 7

What does “protected” mean when defining a class?

Answer 7

These classes, attributes and methods can be used only by other classes in the same package or that inherit from this class

Question 8

What are getter and setter methods?

Answer 8

These are public methods that allow other programmers to access the class and change variables, when the class attributes are private.

Question 9

Why is it a convention to make the class attributes private?

Answer 9

The person who created the class has control over other people accessing and modifying the variables.

Question 10

Can you generate getters and setters in Eclipse?

Answer 10

In eclipse, you can do:

Source –> generate getters and setters

Question 11

Why do we use “this”?

e.g this.radius = radius

Answer 11

Often the arguments to a method are named the same as the class attributes, but these are not the same thing. If we say this.radius, we are saying that the radius is the class attribute (not the input argument to our constructor). Using “this” avoids ambiguity.

Similar to “Self” in Python.

Question 12

What does static mean?

Answer 12

Static is an optional keyword that can be used to create variables or methods that will exist independently of any instances created for the class.

Therefore, they can be accessed without creating an instance of the class (i.e. an object).

Question 13

What’s another word for decorator?

Answer 13

modifier

Question 14

Why is the main method static?

Answer 14

It involves creating objects from other classes and isn’t applied to an object itself.

Question 15

What is the reference of an object?

Answer 15

You can think of references like the “address” of a “house”

We can have many references going to the same object.
A single reference can only refer to one object.

Question 16

What does it mean when methods have been overloaded?

Answer 16

They have the same name but different arguments.

Can be anything.
Eg., System out println()
or it could just be a method name used more than once with different arguments

Question 17

what are the two types of classes?

Answer 17

Objects and static

Question 18

When we create a new object, we also create a reference to that object. Is it the same when we create variables of a primitive type?

Answer 18

No, when we create a variable to a primitive type, we’re just creating a single value. An object has got lots of values associated with it (e.g. name and age) so we just store the reference (i.e. the “address”)

Question 19

What happens if we test for equality of two references that are:

a) the same object
b) multiple objects with the same values for their attributes

Answer 19

a) Equality is true if they refer to the same object.

b) Equality is false if multiple objects with the same values for their attributes

Question 20

What is meant by the equals(Object o) method?

Answer 20

All objects have a method to check if an object equals another object.

Question 21

How can we inherit a class from another existing class, so that we automatically inherit everything and can add extra attributes and methods?

Answer 21

We can use extends in the class declaration, e.g.
public class B extends A (){
super();
}

B has all the methods and attributes that A has but a different toString method.

We need to change the attributes from private to protected in the A class.

Question 22

Can immutable objects be changed once created?

Answer 22

No.

All wrapper classes including String, Integer etc are all immutable.

Question 23

Are Strings objects? What happens when we check for equality of string a and string b:

String a = “hello”
String b = “hel” + “lo”

Answer 23

They are not equal as they are different objects.

Question 24

what’s the most common way of drawing classes (in particular the hierarchy of super classes and subclasses)?

Answer 24

UML diagrams

Question 25

What’s an example of an immutable object?

Answer 25

A String is immutable. For example, if we have
String b = “Hel”;
a=b;
b+=”lo”;
You think that we are just changing the string b, but in fact we are creating a new object. a still points to “Hel” the original object.

Question 26

What are abstract classes?

Answer 26

Abstract classes are classes that you can’t make objects from.

Question 27

What are interfaces?

Answer 27

Interfaces define a set of methods. Any class that implements the interface must have these methods in it.

In Java, any class can only extend one other class but your class can implement as many interfaces as you like.

This allows other bits of code to know formally what you’re going to do

Question 28

What is polymorphism?

Answer 28

We can store an object in a reference for its parent class. Useful when we get to arrays.

Question 29

What does instanceof do?

Answer 29

instanceof checks whether an instance is of a specific type or not

Question 30

What is the Object class?

Answer 30

Every single class inherits from the Object class.

Question 31

How do we use equals(Object o)?

Answer 31

Since we inherit the equals(Object o) method from the Object class, we don’t need to pass in a type.

1) check that the Object equals the class that you are in
2) If so, we need to create a new reference and cast the class type to o
3) From there we can use methods etc in the class

Question 32

How does Java know to use a toString() method when trying to print out an object?

Answer 32

System.out.println() is part of Print Stream object

Question 33

In the Book class with two class attributes int code and String name, what would a copy constructor be?

Answer 33

Add this to the Book class:

public Book(Book b){
code = b.code;
name = b.name;
}

In the main class, suppose you create a new book:

Book b1 = new Book(35, "pooh bear");
Book b2= new Book(b1); //copy constructor

Depending on the equals method, they could be equal

Question 34

If we have

String a = “hel”
a = “hel” + “lo”

Does the original “hel” disappear?

Answer 34

Yes, the original object disappears usually with the garbage collector (which comes round periodically)

Question 35

Can a private attribute be inherited by the new class?

Answer 35

Yes, but we can’t access it unless it’s changed to “protected”

Question 36

Are there any arguments for the super class?

Answer 36

No. The default constructor of the super class doesn’t have any

Question 37

Describe immutable objects in Java?

Answer 37

Once immutable objects are created, its object values and state can not be changed.

For example, primitive objects such as int, long, float, double, all legacy classes, Wrapper class, String class, etc.

It only supports get() method to pass the value of the object.

Question 38

Give an example of a method that can be overridden?

Answer 38

A common example is a toString method.