Question

interface TypeConverter<T, E> {
     T convert(E e);
}
class CollectionUtil() {
     public static <E> List<T> convertToList(List<E> fromList, TypeConverter<T, E> conv) {
     {
            if(fromList== null) return null;
            List<T> newList = new ArrayList<T>(fromList.size())  
            for(E e : fromList) 
            {
                newList.add(conv.convert(e));
            }
            return newList;
     } 
}

Above code explains converting from List of String to List of Integer by implementing TypeConverter interface for String, Integer. Are there already any collections conversion utility methods exists in any API like list to set and so on?

Answer 1

In Guava:

List<String> strings = Arrays.asList("1", "2", "3");
List<Integer> integers =
        Lists.transform(strings, new Function<String, Integer>() {
            @Override
            public Integer apply(String input) {
                return Integer.parseInt(input);
            }
        });

Maven

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>13.0</version>
</dependency>

In Collections 15 package:

final List<Integer> integers =
        ListUtils.transformedList(strings, new Transformer<String, Integer>() {
            @Override
            public Integer transform(String s) {
                return Integer.parseInt(s);
            }
        });

Maven

<dependency>
    <groupId>net.sourceforge.collections</groupId>
    <artifactId>collections-generic</artifactId>
    <version>4.01</version>
</dependency>

In Functional Java

List<Integer> integers = List.list("1", "2", "3").map(new F<String, Integer>() {
    @Override
    public Integer f(String s) {
        return Integer.parseInt(s);
    }
});

Maven

<dependency>
    <groupId>org.functionaljava</groupId>
    <artifactId>functionaljava</artifactId>
    <version>3.1</version>
</dependency>

Note that this library uses different set of collections.

…bonus in Scala:

Seq("1", "2", "3") map {_.toInt}