Constructors in Kotlin

Well init is not body of constructor. It is called after primary constructor with the context of primary constructor.

As given in Official documentation:

The primary constructor cannot contain any code. Initialization code can be placed in initializer blocks, which are prefixed with the init keyword:

class Customer(name: String) {
    init {
        logger.info("Customer initialized with value ${name}")
    }
}

Note that parameters of the primary constructor can be used in the initializer blocks. They can also be used in property initializers declared in the class body:

class Customer(name: String) {
    val customerKey = name.toUpperCase()
}

In fact, for declaring properties and initializing them from the primary constructor, Kotlin has a concise syntax:

class Person(val firstName: String, val lastName: String, var age: Int) {
    // ...
}

As per your question you can add a constructor to accept one parameter like following:

class Person(name: String, surname: String) {

    constructor(name: String) : this(name, "") {
        // constructor body
    }

    init {
        Log.d("App", "Hello");
    }
}

But it doesn’t look right as we are unnecessary passing second argument empty string. So we can order constructor like following:

class Person(name: String) {

    constructor(name: String, surname: String) : this(name) {
        // constructor body
    }

    init {
        Log.d("App", "Hello");
    }
}

Hope it helps.