class MyString(characters : Array[Char])
{
    require(characters != null)
    private val buffer = new Array[Char](characters.length)
    System.arraycopy(characters, 0,  buffer, 0, characters.length)
    def this(input : String) = this(input.toCharArray)
    def length = buffer.length
    override def toString = new String(buffer)
    def substr(pos : Int, len : Int) : MyString =
    {
        require(pos >= 0 && pos < this.length && len > 0 && (pos + len) <= this.length)
        new MyString(buffer.slice(pos, pos + len))
    }

    def substr(pos : Int) : MyString = substr(pos, this.length - pos)
}

var a = new MyString("johnny")
println(a.substr(0))