Type constraints with upper bounds in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a type constraint with an upper bound in Kotlin?

It is a way to limit a generic type to be a subtype of a specific class or interface using the <T : UpperBound> syntax.

beginner

How do you declare a generic function in Kotlin that only accepts types extending Number?

Use fun <T : Number> functionName(param: T) { ... } to restrict T to Number or its subclasses.

intermediate

Why use upper bounds in Kotlin generics?

To ensure the generic type has certain properties or functions, so you can safely call them inside the generic code.

beginner

What happens if you try to use a type that does not meet the upper bound constraint?

The Kotlin compiler will show an error because the type does not satisfy the required subtype relationship.

intermediate

Example: Explain this declaration: fun <T : Comparable<T>> sortList(list: List<T>)

This means T must implement Comparable of T, so the list elements can be compared and sorted safely.

What does mean in Kotlin?

A T must be a supertype of Number

B T can be any type

C T must be a subtype of Number

D T must be exactly Number

The syntax restricts T to be Number or any subclass of Number.

Which of these types can be used for T in fun printLength(text: T)?

A String

B Int

C List

D Boolean

String implements CharSequence, so it satisfies the upper bound constraint.

What error occurs if you pass a type not meeting the upper bound?

A Warning only

B Runtime exception

C No error

D Compile-time error

Kotlin enforces type constraints at compile time, so it will not compile.

How do you specify multiple upper bounds in Kotlin?

A fun <T : Interface1 & Interface2>

B fun <T> where T : Interface1, T : Interface2

C fun <T : Interface1, Interface2>

D fun <T : Interface1; Interface2>

Use multiple constraints separated by commas with where clause: fun where T : Interface1, T : Interface2

What is the benefit of using upper bounds in generics?

A Allows calling specific methods safely on generic types

B Makes code slower

C Removes type safety

D Allows any type to be used

Upper bounds ensure the generic type has certain methods or properties, enabling safe calls.

Explain how to use upper bounds in Kotlin generics and why they are useful.

Think about limiting types to those that have certain features.

Describe a situation where you would use multiple upper bounds in Kotlin generics.

Consider when a type must have more than one trait.