Java: Using Generics for String Parsing to Improve DRYness

Question

Suppose I need to parse space delimited lists of numbers where some lists contain integers and some lists contain doubles. What would be a good way to generalize the following functions to reduce repetition? I was thinking this might be a good use case for Generics?

public static ArrayList<Integer> stringToIntList(String str)
{
    String[] strs = str.split("\\s");
    ArrayList<Integer> output = new ArrayList<Integer>();


    for (String val : strs)
    {
        output.add(Integer.parseInt(val));
    }

    return output;
}

public static ArrayList<Double> stringToDoubleList(String str)
{
    String[] strs = str.split("\\s");
    ArrayList<Double> output = new ArrayList<Double>();


    for (String val : strs)
    {
        output.add(Double.parseDouble(val));
    }

    return output;
}

Andy Turner · Accepted Answer · 2016-03-29 15:28:14Z

4

You can do this with generics if you have some sort of function (e.g. java.util.function.Function or com.google.common.base.Function) which takes a String and returns the desired type.

static <T> ArrayList<T> stringToList(
    String str, Function<? super String, ? extends T> fn) {
  String[] strs = str.split("\\s");
  ArrayList<T> output = new ArrayList<>();
  for (String val : strs)
  {
      output.add(fn.apply(val));
  }
  return output;
 }

answered Mar 29, 2016 at 15:28

Andy Turner

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Comments

user140547 · Accepted Answer · 2016-03-29 21:53:49Z

3

With Java 8, it can be quite concise:

static <T> List<T> convertToList(
        String string, Function<String, T> function) {
    return Pattern.compile("\\s").splitAsStream(string)
            .map(function).collect(Collectors.toList());
}

public static void main(String[] args) {
    String s = "3 4";
    System.out.println(convertToList(s,Double::parseDouble)); // [3.0, 4.0]
    System.out.println(convertToList(s,Integer::parseInt)); // [3, 4]
}

You can make the Pattern also a constant when it does not change.

edited Mar 29, 2016 at 21:53

answered Mar 29, 2016 at 15:37

user140547

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6 Comments

evan.oman Over a year ago

When I try this, I get an error saying that the function is returning a List<java.lang.Object> rather than a List<T>. Do I need a cast there or something?

user140547 Over a year ago

@evan058 how do you call it? You need to pass a function which returns T, but if T is Object you get List<Object>

evan.oman Over a year ago

I am using IntelliJ and the IDE is throwing that error. I have just defined the function and it says: Incompatible Types: Required: List<T>; Found List<java.lang.Object>

evan.oman Over a year ago

Ah, you need to do Collectors.<T>toList() rather than Collectors.toList()

evan.oman Over a year ago

Even still when I try to run ArrayList<Double> dubTimes = convertToList(times, Double::parseDouble) I get an error "Bad return type in method reference, cannot convert double to T" Andy Turner's answer works with this kind of method call

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