Safe call operator (?.) in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the safe call operator (?.) do in Kotlin?

It allows you to safely access a property or call a method on an object that might be null, preventing a NullPointerException by returning null instead of crashing.

beginner

How does the safe call operator (?.) behave when the object is null?

When the object is null, the safe call operator returns null immediately without executing the method or property access.

beginner

Example: What is the output of this Kotlin code? val name: String? = null println(name?.length)

The output is 'null' because 'name' is null, so 'name?.length' safely returns null instead of throwing an error.

intermediate

Can the safe call operator (?.) be chained in Kotlin?

Yes, you can chain multiple safe calls like 'obj?.property?.method()' to safely navigate through multiple nullable references.

intermediate

What is the difference between the safe call operator (?.) and the non-null asserted call (!!) in Kotlin?

The safe call operator returns null if the object is null, avoiding exceptions. The non-null asserted call (!!) throws a NullPointerException if the object is null.

What does the safe call operator (?.) return if the object is null?

A null

B Throws NullPointerException

C Empty string

D Zero

The safe call operator returns null when the object is null, preventing exceptions.

Which Kotlin operator throws an exception if the object is null?

A ?.

B ?

C !!

D ?:

The '!!' operator throws a NullPointerException if the object is null.

How would you safely access the length of a nullable String 'text'?

A text.length

B length?.text

C text!!

D text?.length

Using 'text?.length' safely accesses length if 'text' is not null.

Can you chain safe calls like 'obj?.property?.method()' in Kotlin?

A Yes

B No

C Only for properties

D Only for methods

Safe calls can be chained to safely navigate multiple nullable references.

What is the output of this code? val user: User? = null println(user?.name ?: "Unknown")

A null

B Unknown

C Throws exception

D Empty string

The Elvis operator (?:) provides 'Unknown' when 'user?.name' is null.

Explain how the safe call operator (?.) helps prevent null pointer exceptions in Kotlin.

Think about what happens when you try to access something on a null object.

Describe a situation where chaining safe calls (?.) would be useful.

Imagine you have an object inside another object, both could be null.