Common rationales

Abstraction not needed

Abstract modifier in a class is unnecessary if the class has no abstract methods.

abstract class Message {
    fun send() {
        // implementation
    }
}

Business logic first

Built-in methods like toString, hashCode and equals are often placed at the end of the class. They are unrelated to the rest of the class and can be distracting when reading the source code.

class User(val firstName: String, val lastName: String) {
    var isAdmin: Boolean = false

    override fun toString(): String = "$lastName, $firstName"

    override fun hashCode(): Int = Objects.hash(firstName, lastName)

    override fun equals(other: Any?): Boolean {
        if (this === other) return true
        if (other !is Robot) return false
        return firstName == other.firstName && lastName == other.lastName
    }

    fun promote() {
        isAdmin = true
    }
}

Clean comment

Comment and block comment bodies should not have leading or trailing blank lines. No consecutive blank lines are allowed in the comment body.

/**
 *
 * The main function.
 *
 */
fun main() {
    // initialize the logger
    //
    //
    // and print the starting message
    val logger = Logger()
    logger.info("Starting the application")
}

Imports not optimized

Remove imports that are not used in the file.

import java.lang.String

val names = arrayListOf<String>()

Instance not allowed

To prevent instantiating a utility class, put a final modifier on the class and add a private constructor.

final class Strings {
    private Strings() {}

    static String capitalize(String str) {
        return str.substring(0, 1).toUpperCase() + str.substring(1);
    }
}

Purposeful name

When declaring string and primitive variables, it is tempting to use simple names based on their type. In fact, they are usually recommendations given by the IDE. But these names are not descriptive and hard to understand what their purpose is.

val string = "Alice"

val int = 30

Trailing elvis

In a multiline statement, the elvis operator should align with chained method calls instead of the same column.

val bananas =
    fruits
        .filter { it.type == BANANA }
        .takeUnless { it.isEmpty() } ?: return