Equality Workbook

Practice problems for In Kotlin, == Is the One You Actually Want. Each takes a minute or two. Write your own answer first, then click Show answer — nothing here is a trick question, just direct practice of the syntax from the lesson.

== versus ===

1. Value or identity

Write code that builds two Strings with equal contents but as different objects, then both comparisons — annotate what each returns.

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val a = "hello"
val b = StringBuilder("hel").append("lo").toString()

a == b    // true  — equal contents (calls .equals())
a === b   // false — different objects

2. Null-safe comparison

Given a: String? and b: String?, write the comparison that’s true when they hold equal text or are both null — with no explicit null check.

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a == b

== calls .equals() and handles null on both sides, so null == null is true and neither side throws.

3. Null-safe by default

Given a: String? and b: String?, compare them for value equality. What happens if both are null?

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a == b

== handles null on both sides without throwing — null == null is true. No Objects.equals wrapper needed.

4. Detect a fresh copy

Write code that confirms toList() produced a new object whose contents still equal the original.

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val a = listOf(1, 2, 3)
val b = a.toList()

a == b    // true  — same contents
a === b   // false — toList() allocated a new list

defining equality

5. Free value equality

Declare a type Point so that Point(1, 2) == Point(1, 2) is true without writing equals yourself.

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data class Point(val x: Int, val y: Int)

6. Works correctly in a HashSet

Declare a Point type so two equal points are deduplicated by a HashSet (its equals and hashCode must agree). Show the resulting size.

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data class Point(val x: Int, val y: Int)

hashSetOf(Point(1, 1), Point(1, 1)).size   // 1

data class generates equals and a matching hashCode — the contract hash-based collections rely on.

ordering

7. Make a type comparable

Make Version(val major: Int, val minor: Int) implement Comparable so < and > work, comparing major first then minor.

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class Version(val major: Int, val minor: Int) : Comparable<Version> {
    override fun compareTo(other: Version) =
        compareValuesBy(this, other, { it.major }, { it.minor })
}

8. Use the operator

Given the Version above, write the expression comparing whether Version(2, 1) is greater than Version(2, 0).

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Version(2, 1) > Version(2, 0)   // true

> delegates to compareTo.

9. Sort by a key

Given a list people of objects with an age, return them sorted youngest first.

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people.sortedBy { it.age }

10. Sort by two keys

Sort people by lastName, then firstName as a tiebreaker.

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people.sortedWith(compareBy({ it.lastName }, { it.firstName }))

Back to the lesson, In Kotlin, == Is the One You Actually Want, or on to the next one: destructuring.