Javaで始めるパーザコンビネータの作り方（１） ~ Hello, World! ref: http://qiita.com/kmizu/items/ecf3f345a10c6daab495

file0.scala

import com.github.kmizu.scomb.SCombinator

object IntegerParser extends SCombinator[Int] {
  override def root: P[Int] = (digit.+).map{ case digits => digits.mkString.toInt }
  lazy val digit: P[String] = set('0'to'9')

  def main(args: Array[String]): Unit = {
    assert(parse("100") == Success(100))
  }
}

file1.java

public interface Parser<T> {
    ParseResult<T> invoke(String input);
}

file2.java

import java.util.function.*;
public interface ParseResult<T> {
    public static class Success<T> implements ParseResult<T> {
        public final T value;
        public final String next;
        Success(T value, String next) {
            this.value = value;
            this.next = next;
        }

        @Override
        public <U> ParseResult<U> map(Function<T, U> fn) {
            return new Success<U>(fn.apply(value), next);
        }
    }

    public static class Failure<T> implements ParseResult<T> {      
        public final String message;
        public final String next;
        public Failure(String message, String next) {
            this.message = message;
            this.next = next;
        }
        @Override
        public <U> ParseResult<U> map(Function1<T, U> fn) {
            return (ParseResult<U>)this;
        }       
    }

    <U> ParseResult<U> map(Function<T, U> fn);
}

file3.java

public class StringParser implements Parser<String> {
    public final String literal;
    public StringParser(String literal) {
        this.literal = literal;
    }

    @Override
    public ParseResult<String> invoke(String input) {
        if(input.startsWith(literal)) {
            return new ParseResult.Success<>(literal, input.substring(literal.length()));
        }else {
            return new ParseResult.Failure<>("expect: " + literal, input);
        }
    }
}

file4.java

public interface Parser<T> {
    ParseResult<T> invoke(String input);
        static Parser<String> string(String literal) {
        return new StringParser(literal);
    }
}

file5.java

public class Main {
  public static void main(String[] args) {
    ParseResult<String> result = Parser.string("Hello, World!").invoke("Hello, World!");
    System.out.println((ParseResult.Success<String>)result).value);
  }
}