String comparison (equals, compareTo) in Kotlin - Mini Project: Build & Apply

String comparison (equals, compareTo)

📖 Scenario: You are creating a simple program to compare two names to see if they are the same or which one comes first in alphabetical order.

🎯 Goal: Build a Kotlin program that compares two strings using equals and compareTo methods and shows the results.

📋 What You'll Learn

Create two string variables with exact names and values

Create a boolean variable to store the result of equals comparison

Use compareTo to find the alphabetical order difference

Print the results exactly as instructed

💡 Why This Matters

🌍 Real World

String comparison is useful in sorting names, searching text, and validating user input in many apps.

💼 Career

Understanding string comparison helps in writing code for data processing, user authentication, and UI sorting features.

1

Create two string variables

Create two string variables called name1 and name2 with the exact values "Alice" and "Bob" respectively.

Kotlin

# Your code here

Hint

Use val to create variables and assign the exact string values.

2

Create a boolean variable for equals comparison

Create a boolean variable called areEqual that stores the result of name1.equals(name2).

Kotlin

val name1 = "Alice"
val name2 = "Bob"
# Your code here

Hint

Use the equals method to compare name1 and name2.

3

Compare strings alphabetically using compareTo

Create an integer variable called compareResult that stores the result of name1.compareTo(name2).

Kotlin

val name1 = "Alice"
val name2 = "Bob"
val areEqual = name1.equals(name2)
# Your code here

Hint

Use the compareTo method to compare name1 and name2 alphabetically.

4

Print the comparison results

Print the values of areEqual and compareResult using two separate println statements.

Kotlin

val name1 = "Alice"
val name2 = "Bob"
val areEqual = name1.equals(name2)
val compareResult = name1.compareTo(name2)
# Your code here

Hint

Use println to show the boolean and integer results on separate lines.

Practice

1. Which Kotlin method should you use to check if two strings have exactly the same characters and case?
val a = "Hello"
val b = "hello"

easy

A. a.equals(b)

B. a.compareTo(b)

C. a === b

D. a.equals(b, ignoreCase = true)

Solution

1. Step 1: Understand equals() method

   The equals() method checks if two strings are exactly the same including case.

2. Step 2: Compare equals() with other options

   compareTo() returns an order number, === checks reference equality (not content), equals(ignoreCase=true) ignores case, so only equals() matches exact case-sensitive check.

3. Final Answer:

   a.equals(b) -> Option A

4. Quick Check:

   Use equals() for exact string match [OK]

Hint: Use equals() for exact case-sensitive string match [OK]

Common Mistakes:

• Using compareTo() to check equality
• Assuming === checks content equality
• Using equals(ignoreCase=true) when case matters

2. Which of the following is the correct syntax to compare two strings str1 and str2 for equality ignoring case in Kotlin?

easy

A. str1.equals(str2, ignoreCase = true)

B. str1.compareTo(str2, ignoreCase = true)

C. str1.equals(str2)

D. str1 == str2

Solution

1. Step 1: Recall equals() with ignoreCase parameter

   In Kotlin, equals() can take ignoreCase=true to compare strings ignoring case differences.

2. Step 2: Check other options

   A: equals() without ignoreCase is case-sensitive, B: compareTo(ignoreCase=true) returns Int not Boolean, D: == is case-sensitive.

3. Final Answer:

   str1.equals(str2, ignoreCase = true) -> Option A

4. Quick Check:

   Use equals(ignoreCase=true) to ignore case [OK]

Hint: Use equals(ignoreCase=true) to compare ignoring case [OK]

Common Mistakes:

• Using compareTo(ignoreCase=true) directly (returns Int, not Boolean)
• Using == which is case-sensitive
• Forgetting to set ignoreCase=true

3. What will be the output of the following Kotlin code?

val s1 = "apple"
val s2 = "banana"
println(s1.compareTo(s2))

medium

A. A positive number

B. A negative number

C. 0

D. Compilation error

Solution

1. Step 1: Understand compareTo() behavior

   compareTo() returns a negative number if the first string is lexicographically less than the second.

2. Step 2: Compare "apple" and "banana" lex order

   "apple" comes before "banana" alphabetically, so compareTo returns a negative number.

3. Final Answer:

   A negative number -> Option B

4. Quick Check:

   "apple" < "banana" means compareTo() < 0 [OK]

Hint: compareTo() < 0 means first string is before second [OK]

Common Mistakes:

• Assuming compareTo() returns boolean
• Confusing positive and negative return values
• Thinking compareTo() returns 0 for different strings

4. The following Kotlin code is intended to check if two strings are equal ignoring case, but it does not compile. What is the error?

val a = "Test"
val b = "test"
if (a.compareTo(b, ignoreCase = true)) {
    println("Equal")
} else {
    println("Not Equal")
}

medium

A. Missing parentheses in if condition

B. ignoreCase is not a valid parameter for compareTo()

C. Strings cannot be compared with compareTo()

D. compareTo() returns Int, cannot be used as Boolean

Solution

1. Step 1: Check compareTo() return type

   compareTo() returns an Int, not a Boolean, so it cannot be used directly in if condition.

2. Step 2: Understand how to check equality with compareTo()

   To check equality ignoring case, compareTo() == 0 should be used, not just compareTo() alone.

3. Final Answer:

   compareTo() returns Int, cannot be used as Boolean -> Option D

4. Quick Check:

   compareTo() returns Int, use == 0 to check equality [OK]

Hint: compareTo() returns Int; compare with 0 for equality [OK]

Common Mistakes:

• Using compareTo() directly as Boolean
• Forgetting to compare result with 0
• Assuming ignoreCase is invalid parameter (it is valid)

5. You want to sort a list of strings ignoring case in Kotlin. Which code snippet correctly sorts the list val fruits = listOf("Banana", "apple", "Cherry") ignoring case?

hard

A. fruits.sortedBy { it }

B. fruits.sortedBy(String.CASE_INSENSITIVE_ORDER)

C. fruits.sortedWith(String.CASE_INSENSITIVE_ORDER)

D. fruits.sorted()

Solution

1. Step 1: Understand sorting ignoring case

   To sort strings ignoring case, use a comparator that compares strings case-insensitively.

2. Step 2: Check options for case-insensitive sorting

   A and D are case-sensitive. C: sortedBy expects key selector, not Comparator (compile error). B uses built-in case-insensitive Comparator correctly.

3. Final Answer:

   fruits.sortedWith(String.CASE_INSENSITIVE_ORDER) -> Option C

4. Quick Check:

   Use sortedWith(String.CASE_INSENSITIVE_ORDER) for case-insensitive sort [OK]

Hint: Use sortedWith(String.CASE_INSENSITIVE_ORDER) to ignore case in sort [OK]

Common Mistakes:

• Using sorted() which is case-sensitive
• Using sortedBy { it } which is case-sensitive
• Confusing sortedBy (selector) with sortedWith (Comparator)