Closures in Groovy - Implementing in equivalent Java

Question

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?

Answer 1

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 ;-)