public static DataSource parseData(InputStream contents, String identifier)

public static DataSource parseData(InputStream contents, String identifier) {
    if (DataSource1.respondsTo(identifier) {
        return new DataSource1(contents);
    }
    //more ifs. There likely will be about 20 of those.
}

interface DataSource {
    boolean respondsTo(String identifier)
}

class DataSource1 implements DataSource {
    DataSource1(InputStream is) { /* magic goes here */ }
    @Override boolean respondsTo(String identifier) { identifier in ["DS1 idX", "DS1 idY", "DS1 idZ"] }
}

class DataSource2 implements DataSource {
    DataSource2(InputStream is) { /* magic goes here */ }
    @Override boolean respondsTo(String identifier) { identifier in ["DS2 idX", "DS2 idY", "DS2 idZ"] }
}

// ...

class DataSource20 implements DataSource {
    DataSource20(InputStream is) { /* magic goes here */ }
    @Override boolean respondsTo(String identifier) { identifier in ["DS20 idX", "DS20 idY", "DS20 idZ"] }
}

enum DataSourceEnum {
    ds1  (["DS1 idZ",  "DS1 idY",  "DS1 idZ"],  { is -> new DataSource1(is) }),
    ds2  (["DS1 idZ",  "DS1 idY",  "DS1 idZ"],  { is -> new DataSource2(is) }),

    // ...

    ds20 (["DS20 idX", "DS20 idY", "DS20 idZ"], { is -> new DataSource20(is) })

    private final static Map<String, DataSourceEnum> dsMapping = [:]

    final Closure<DataSource> buildDataSource

    private DataSourceEnum(List<String> identifiers, Closure<DataSource> ctor) {
        DataSourceEnum.dsMapping += identifiers.collectEntries { id -> [(id):this] }
        this.buildDataSource = ctor
    }

    static DataSourceEnum identify(String id) { dsMapping[id] }
}

DataSource parseData(InputStream contents, String identifier) {
    DataSourceEnum.identify(identifier)?.buildDataSource(contents)
}