Calling Java method from JRuby throws "no constructor for arguments error"

Question

I'm trying to initialize this class from the JaCop constraint programming library from jRuby. I am using the correct type of arguments, but for some reason I keep getting this error message:

  NameError: no constructor for arguments
    (
      org.jruby.RubyArray,
      org.jruby.RubyArray,
      org.jruby.RubyArray,
      JaCoP.core.IntVar,
      JaCoP.core.IntVar)
    on Java::JaCoPConstraintsKnapsack::Knapsack
    (root) at rb/knapsack.rb:24

The code that it points to is this:

k = Jacop::Knapsack.new(@values, @weights, quantity, knapsackCapacity, knapsackProfit)

The signature of the Java class constructor is this:

public Knapsack(int[] profits,
                int[] weights,
                IntVar[] quantity,
                IntVar knapsackCapacity,
                IntVar knapsackProfit)

I don't understand why jRuby complains that constructor is not found because it should be able to find this.

kares · Accepted Answer · 2013-07-15 07:11:30Z

3

JRuby can not always guess "complicated" method args for you, you might want to try :

Jacop::Knapsack.new(@values, @weights, quantity.to_java(JaCoP.core.IntVar), knapsackCapacity, knapsackProfit)

or even helping with the int[] cast (should not be necessary) :

Jacop::Knapsack.new(@values.to_java(:int), @weights.to_java(:int), quantity.to_java(JaCoP.core.IntVar), knapsackCapacity, knapsackProfit)

answered Jul 15, 2013 at 7:11

kares

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1 Comment

picardo Over a year ago

You were absolutely right. I had to cast the ints as well to make it work.

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