Builder

Motivation: Constructing objects with many parameters or optional components leads to unwieldy constructors, multiple overloads, hard-to-read call sites, and difficult validation during construction.

Intent: Separate the construction of a complex object from its representation, allowing step-by-step construction.

class HttpRequest private constructor(
    val url: String,
    val method: String,
    val headers: Map<String, String>,
    val body: String?,
) {
    class Builder(private val url: String) {
        private var method = "GET"
        private var headers = mutableMapOf<String, String>()
        private var body: String? = null
 
        fun method(m: String) = apply { this.method = m }
        fun header(k: String, v: String) = apply { headers[k] = v }
        fun body(b: String) = apply { this.body = b }
        fun build() = HttpRequest(url, method, headers, body)
    }
}
 
// Usage
val request = HttpRequest.Builder("https://api.example.com/users")
    .method("POST")
    .header("Content-Type", "application/json")
    .body("""{"name": "Alice"}""")
    .build()