Objects

Question 1

What is a field?

Answer 1

A variable that belongs to a class and make up the state of the class

Question 2

What is a class?

Answer 2

Defines the states

Question 3

What is an object?

Answer 3

An individual instance of a class

Question 4

What is the default constructor?

Answer 4

Provided by Java by default, takes no parameters.

Called using new.

Dog bc = new Dog();

Question 5

What is an initializing constructor?

Answer 5

Used to initialize fields other than default.

public Wizard (String n, int a) {
name = n;
age = a;
}

Question 6

Why do you use this?

Answer 6

Using this.variableName always refers to the field with the specified name, even
if another variable is closer in scope.
This is called disambiguation (it eliminates ambiguity).

Question 7

What is a parameterless constructor and why use it?

Answer 7

Once a constructor is defined, Java no longer provides default, but if you want to still have one without parameters, you need to write it!

Question 8

What is a method?

Answer 8

A function that is a member of a class. May use fields of class. No need to pass a reference to the object.

Question 9

What is encapsulation?

Answer 9

An object acts like a container, holding state and behavior in place. An object can protect access to fiends and methods using access modifiers.W

Question 10

What are access modifiers?

Answer 10

A keyword placed before a class, method or field and determines its visibility.

Question 11

What is an accessor?

Answer 11

Defined for each instance variable that the programmer wants
to make visible outside of the object.

Aka a getter.

Question 12

What is a mutator?

Answer 12

Defined for each instance variable that the programmer wants to
make modifiable from outside of the object.

Aka a setter.

Question 13

Why override toString()?

Answer 13

There’s a default, but it’s not useful.

Question 14

What is the equals(Object) method?

Answer 14

he default implementation works exactly the same as the == operator; it only returns true if an object is compared to itself (it compares identity, i.e. the object’s address).
The author of a class may choose to write their own implementation, typically by following this pattern:
○ Since any type of object may be passed as an argument to the equals(Object) method, you
must first check the make sure that the object is an instance of the same class.
○ Java is a statically typed language: before you can access the state and behavior in the object, you must cast it into a variable of the correct type.
○ Compare each of the important fields in both
instances to see if they match.
This kind of comparison is called
“deep equality.”

@Override
public boolean equals(Object c) {
if(c instanceof Car) {
Car car = (Car) c;
return car.vin == this.vin;
} else {
return false;
}
}

Question 15

What is the difference between actual and reference type?

Answer 15

The actual type is specified when new is
used to create the new object.

The reference type determines which state
and behavior can be used, regardless of
what the actual type is.

Object obj = new Wizard(); can’t use methods set by Wizard since its ref type is Object.

Question 16

What is a static method?

Answer 16

A static member belongs to the class itself (not an object).
○ There is one copy of the method or field.
○ It is important to note that each individual object does not get its own copy of a static field; changes made to a static field
affect the one and only copy of that field.
● static methods exist in a static context, and can only access or use other methods or fields that are also static.
○ Trying to use non-static methods or fields will cause a syntax error.