Gemfile

 source "https://rubygems.org"
 gem "benchmark-ips"

merge_benchmark.rb

require "benchmark/ips"

class Java::JavaUtil::LinkedHashMap
  # Java 8 Map implements a merge method with a different signature from
  # the Ruby Hash#merge. see https://github.com/jruby/jruby/issues/1249
  if ENV_JAVA['java.specification.version'] >= '1.8'
    def merge_ruby(other)
      # commented is a Java implementation, need to bench to see which is most efficient
      # and if this implementation respects the Ruby merge contract.
      #

      dup.merge!(other)
    end

    def merge_java(other)
      duped = Java::JavaUtil::LinkedHashMap.new(self)
      duped.putAll(Java::JavaUtil::LinkedHashMap.new(other))
      duped
    end

    def merge_java2(other)
      duped = self.clone
      duped.putAll(Java::JavaUtil::LinkedHashMap.new(other))
      duped
    end
  end
end



m1 = { "hello" => "maximun", "World" => "bye", "nested2" => [1, 2, 3, 4], "array" => [1] }
m2 = { "World" => "Uncool", "nested" => { "complex" => "structure" }, "array" => [2,3] }

Benchmark.ips do |x|
  x.warmup = 5 

  x.report("merge ruby") do
    m = Java::JavaUtil::LinkedHashMap.new(m1)
    m.merge_ruby(m2)
  end

  x.report("merge java") do
    m = Java::JavaUtil::LinkedHashMap.new(m1)
    m.merge_java(m2)
  end
  
  x.report("merge java 2") do
    m = Java::JavaUtil::LinkedHashMap.new(m1)
    m.merge_java2(m2)
  end

  x.compare!
end