Generics

Question 1

What is Type erasure?

Answer 1

Process of converting generics to Objects

Question 2

Static and generics?

Answer 2

Not permitted. static T

Question 3

Primitives and generics?

Answer 3

Not permitted. < int >

Question 4

Instantiating a generic?

Answer 4

Not permitted. new T()

Question 5

Checking instanceof a generic?

Answer 5

Not permitted.

Question 6

Using generic in ArrayList?

Answer 6

Not permitted. new ArrayList< T >

Question 7

What generics actually become after compile?

Answer 7

Object

Question 8

What types of wildcard generics do we have?

Answer 8

Unbounded, upper bounded, lower bounded

Question 9

What are unbound generics?

Answer 9

<?>

Question 10

Can we pass multiple generics?

Answer 10

Yes. <T, U>

Question 11

Can wildcard generics be used as return types?

Answer 11

No

Question 12

Where do we use wildcard generics?

Answer 12

Mostly as parameters or as local variable types

Question 13

Why would we use wildcard generics?

Answer 13

To define code that can be more universal

Question 14

At what level are generics resolved?

Answer 14

At instance level

Question 15

Why cannot we use generics with static?

Answer 15

Because static is resolved at class level, not instance level like generics

Question 16

Where can static be used with generics?

Answer 16

On static methods with < T >

Question 17

What does “public static < T >T identity(T t)” mean?

Answer 17

It is a method of name “identity”, where we use type parameter < T >, which defines return type T and method parameter T.

Question 18

What does “public static T noGood(T t)” mean?

Answer 18

No good. Compile error, since we dont tell compiler that we want to use generic type. We define object T as return type, and object T as parameter type.

Question 19

What does “public static < T > void sink(T t)” mean?

Answer 19

Method that has no return type, but we define type parameter < T > for method parameter T t

Question 20

Where can T be used at?

Answer 20

As member types, as parameter argument types, as method parameter types

Question 21

In what types of classes can generics be used?

Answer 21

In classes, records, interfaces

Question 22

Can wildcard generics be used with generic types?

Answer 22

Yes. <? extends T>

Question 23

Can we use upper and lower bounds on generic types?

Answer 23

Yes. < T extends SomeClass >

Question 24

Where can we use upper and lower bound generic types?

Answer 24

At same places (reference types) where we can use generic types

Question 25

When to use “<? extends T>”

Answer 25

When we want to make collection read only

Question 26

When to use <? super T> (lower bound wildcard)

Answer 26

When we want to make collection write only

Question 27

What should we watch out regarding generics and method parameters?

Answer 27

Watch for overloading problems - eg. List< T > becomes List< Object >, so we cannot have two methods with those in same signature

Question 28

What should we watch out regarding generics and return types when overriding?

Answer 28

Return type must be covariant, so watch out when overriding methods with generics

Question 29

Why “lower bound wildcard” as name, or “upper bound wildcard”?

Answer 29

Wildcard can be any class.
Bound means depicts that something linked or depends on something.
Regarding lower or upper, imagine class diagrams.

Lower bound, or super, is drawn from down to up in hierarchy.

Upper bound, or extends, is drawn from up to down.

Question 30

Can wildcards be used when instantianting objects?

Answer 30

No, Java requires to be type specific. References can use wildcards tho.