Scala Cookbook

5.3. Setting Default Values for Method Parameters

Problem

You want to set default values for method parameters so the method can optionally be called without those parameters having to be assigned.

Solution

Specify the default value for parameters in the method signature. In the following code, the timeout field is assigned a default value of 5000, and the protocol field is given a default value of "http":

class Connection {
  def makeConnection(timeout: Int = 5000, protocol:  = "http") {
    println("timeout = %d, protocol = %s".format(timeout, protocol))
    // more code here
  }
}

This method can now be called in the following ways:

c.makeConnection()
c.makeConnection(2000)
c.makeConnection(3000, "https")

The results are demonstrated in the REPL:

scala> val c = new Connection
c: Connection = Connection@385db088

scala> c.makeConnection()
timeout = 5000, protocol = http

scala> c.makeConnection(2000)
timeout = 2000, protocol = http

scala> c.makeConnection(3000, "https")
timeout = 3000, protocol = https

Note that empty parentheses are used in the first example. Attempting to call this method without parentheses results in an error:

scala> c.makeConnection
<console>:10: error: missing arguments for method makeConnection in Connection;
follow this method with `_' to treat it as a partially applied function
              c.makeConnection
                ^

The reason for this error is discussed in Recipe 9.6.

If you like to call methods with the names of the method parameters, the method makeConnection can also be called in these ways: