A very simple and rough and slow REPL for Java

java-repl.rb

# A very simple and rough and slow REPL for Java, but it's better than making a
# new Java program every time you want to test something.
class JavaREPL
  def initialize(filename = "JavaREPL")
    @filename = filename
    reset
  end

  def repl
    loop { java_print java_eval java_read }
  end

  def java_read
    if @continued_line
      print "  ..> "
    else
      print "java> "
    end

    gets.chomp
  end

  def java_eval(input)
    if java_cmd(input)
      ""
    elsif input[input.length - 1] == ?\\
      @program += "\n" + input[0, input.length - 1]
      @continued_line = true
      ""
    else
      @program += "\n" + input
      @continued_line = false

      @program = @program.strip
      @program.gsub! /^\/\*JavaREPL\*\/System\.out\.println/, 'eat'
      @program.sub! /^(.*?[^;])\z/, '/*JavaREPL*/System.out.println(\1);'

      imports_str = @imports.map { |name| "import #{name};" }.join("\n")

      java = "
        #{imports_str}

        public class #@filename {
          public static void main(String[] args) {
            #@program
          }

          public static void eat(Object obj) {
          }
        }
      "

      File.open("#@filename.java", "w") do |f|
        f << java
      end

      javac_result = `javac #@filename.java 2>&1`
      result = `java #@filename 2>&1`

      `rm -f #@filename.java #@filename.class`

      java_error(javac_result) || result
    end
  end

  def java_print(result)
    puts result unless result.empty?
  end

  def java_cmd(input)
    case input.strip
    when "exit"
      raise StopIteration
      true
    when "reset"
      @program = ""
      true
    when "list"
      puts @program
      true
    when /^import (.+?)$/
      java_import($1)
      true
    when /^unimport (.+?)$/
      java_unimport($1)
      true
    when "error"
      puts @error
      true
    else
      false
    end
  end

  def java_error(result)
    if result =~ /\A#{Regexp.escape @filename}\.java:\d+:(.+?)$/
      error_msg = $1

      @error = result
      @program.sub! /^.+\z/, ''

      if error_msg == " 'void' type not allowed here"
        "Java error: You were probably supposed to put a semicolon on that statement."
      else
        "Java error: #{error_msg}"
      end
    end
  end

  def java_import(name)
    @imports << name unless @imports.include? name
  end

  def java_unimport(name)
    @imports.delete name
  end

  def reset
    @program = ""
    @imports = []
    @continued_line = false
  end
end

if __FILE__ == $0
  JavaREPL.new.repl
end