javagroovyclosures

Closures in Groovy - Implementing in equivalent Java


I have the following Groovy script:

def n = ["1","2","3"]
println n.collect { v -> v.toInteger()*2 }

And I would like to translate it to equivalent Java code (abstracting from the meaning of "collect" --- does not matter what it does for now).

I wrote the following:

class X {
   X() {
          object[] n = new object[]{"1","2","3"};

          object anonymousBlock(object v) { return v.toInteger()*2; }

          System.out.println(n.collect(???));
   }
}

What should I pass as argument to collect using the representation above?


Solution

  • With Java 8, you can do:

    List<String> strings = Arrays.asList( "1","2","3" ) ;
    List<Integer> numbers = strings.stream()
                                   .map( Integer::parseInt )
                                   .map( (i) -> i * 2 )
                                   .collect( Collectors.toList() ) ;
    

    With Java 7, one approach is something like this:

    First, define an interface to convert from one type to another:

    static interface Mapper<T,U> {
        U apply( T value ) ;
    }
    

    Then, we can define a class that delegates to an iterator and applies the Mapper.apply method to each element as it is returned:

    static class Collector<T,U> implements Iterator<U> {
        Iterator<T> delegate ;
        Mapper<T,U> mapper ;
    
        public Collector( Iterable<T> elements, Mapper<T,U> c ) {
            delegate = elements.iterator() ;
            this.mapper = c ;
        }
    
        @Override
        public void remove() { delegate.remove() ; }
        @Override
        public boolean hasNext() { return delegate.hasNext() ; }
        @Override
        public U next() { return mapper.apply( delegate.next() ) ; }
    }
    

    Then, we can call it, using something like:

    List<String> strings = Arrays.asList( "1","2","3" ) ;
    
    // Create our mapping Iterator
    Iterator<Integer> iter = new Collector<>( strings, new Mapper<String,Integer>() {
        @Override
        public Integer apply( String v ) {
            return Integer.parseInt( v ) * 2 ;
        }
    } ) ;
    
    // Collect back from iterator into a List
    List<Integer> numbers = new ArrayList<>() ;
    while( iter.hasNext() ) {
        numbers.add( iter.next() ) ;
    }
    

    Java 8 and Groovy FTW ;-)