From Java to Kotlin: How Would You Rewrite This Class?

Source: Dev.to

Understanding What This Class Does

Before jumping to Kotlin, let’s break down what this Java class actually contains:

• Two private fields (codigo and nome) that are set via the constructor
• Two private fields with default values (numero = 0 and texto = "EscolaX")

A constructor receives codigo and nome as parameters and assigns them using this.

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Step 1: The Class Declaration

Java

public class AlunoJava { ... }

Kotlin

Classes are public by default, so you don’t need the public keyword:

class AlunoKotlin { ... }

✅ No public needed — less boilerplate from the start.

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Step 2: The Constructor

Java constructor

public AlunoJava(String codigo, String nome) {
    this.codigo = codigo;
    this.nome = nome;
}

Kotlin primary constructor (declared in the class header). Adding val or var creates the fields automatically:

class AlunoKotlin(private val nome: String, private val codigo: String) {
    // nome and codigo are already declared AND assigned here!
}

✅ The constructor declaration, field declaration, and assignment all happen in one line.

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Step 3: val vs var

• val → immutable (like final in Java) — value can’t be reassigned after initialization.
• var → mutable — value can be changed later.

Since the original Java class doesn’t reassign codigo or nome after construction, val is the more faithful and idiomatic translation.

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Step 4: Fields With Default Values

Java fields

private int numero = 0;
private String texto = "EscolaX";

Kotlin equivalents (type inference applies):

private var numero = 0          // inferred as Int
private var texto = "EscolaX"   // inferred as String

✅ We use var here because these fields may be changed later (they weren’t final in Java).

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The Final Result

Putting it all together:

class AlunoKotlin(private val nome: String, private val codigo: String) {
    private var numero = 0
    private var texto = "EscolaX"
}

Side‑by‑side comparison

Java (17 lines)

public class AlunoJava {
    private String codigo;
    private String nome;
    private int numero = 0;
    private String texto = "EscolaX";

    public AlunoJava(String codigo, String nome) {
        this.codigo = codigo;
        this.nome = nome;
    }
}

Kotlin (3 lines)

class AlunoKotlin(private val nome: String, private val codigo: String) {
    private var numero = 0
    private var texto = "EscolaX"
}

Same behavior. A fraction of the code. 🎯

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Key Takeaways

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Conclusion

Kotlin was designed to eliminate Java boilerplate without sacrificing clarity. Once you get used to the primary constructor syntax and the val/var distinction, most simple Java classes translate to Kotlin naturally — and end up much shorter.