Java Streams functional transform / enrich data

I want to write a DataX to List<DataY> transform/enrich function

class DataX {
    private String a;
    private List<String> c;
}

class DataY {
    private String a;
    private String b;
    private String cx;
}

JSON representation

Input

{
    "a": "abc-123",
    "c": ["c1", "c2", "c3", "c4", "c5"]
}

As processing each element of c the value is checked against a lookup table (can be a database, local cache, or REST lookup) and some get mapped from 1 to many.

c1 -> c1 
c2 -> c2 
c3 -> c31, c32, c33 
c4 -> c4 
c5 -> c51, c52

Expected output,

[
    {
        "a": "abc-123",
        "b": "xyz-1",
        "cx": "c1"
    },
    {
        "a": "abc-123",
        "b": "xyz-2",
        "cx": "c2"
    },
    {
        "a": "abc-123",
        "b": "xyz-3",
        "cx": "c31"
    },
    {
        "a": "abc-123",
        "b": "xyz-4",
        "cx": "c32"
    },
    {
        "a": "abc-123",
        "b": "xyz-5",
        "cx": "c33"
    },
    {
        "a": "abc-123",
        "b": "xyz-6",
        "cx": "c4"
    },
    {
        "a": "abc-123",
        "b": "xyz-7",
        "cx": "c51"
    },
    {
        "a": "abc-123",
        "b": "xyz-8",
        "cx": "c52"
    }
]

> Note: Value b represent UUIDs generated via UUID.randomUUID() method

My attempt

public class Test {
    public static void main(String[] args) {
        DataX dataX = new DataX("abc-123", Arrays.asList("c1", "c2", "c3", "c4", "c5"));

        List<DataY> collect = dataX.getC().stream()
                .map(c -> new DataY(dataX.getA(), UUID.randomUUID().toString(), c))
                .map(dataY -> {       // use flatMap here?
                    if (isOneToMany(dataY.getCx()))
                        return operationOneToMany(dataY.getCx());
                    else
                        return dataY.getCx();
                })
                .collect(Collectors.toList());
    }

    private static boolean isOneToMany(String input) {
        return (Arrays.asList("c3", "c5").contains(input));
    }

    // Shall I apply this to each element, and return list of 1 or list of many elements 
    private static Optional<List<String>> operationOneToMany(String input) {
        if ("c3".equals(input))
            return Optional.of(Arrays.asList("c31", "c32", "c33"));
        else if ("c5".equals(input))
            return Optional.of(Arrays.asList("c51", "c52"));
        else
            return Optional.empty();
    }

}

> BTW, it's not working

Working code but not sure if optimised

public class Test2 {
    public static void main(String[] args) {
        DataX dataX = new DataX("abc-123", Arrays.asList("c1", "c2", "c3", "c4", "c5"));
        System.out.println("dataX = " + dataX);

        List<DataY> collect = dataX.getC().stream()
                .map(c -> operationOneToMany(c))
                .flatMap(e -> e.stream())
                .map(c -> new DataY(dataX.getA(), UUID.randomUUID().toString(), c))
                .collect(Collectors.toList());

        System.out.println("collect = " + collect);
    }

    private static List<String> operationOneToMany(String input) {
        if ("c3".equals(input))
            return Arrays.asList("c31", "c32", "c33");
        else if ("c5".equals(input))
            return Arrays.asList("c51", "c52");
        else
            return Arrays.asList(input);
    }

}

As suggested by @Rono

public class Test2 {
    public static void main(String[] args) {
        DataX dataX = new DataX("abc-123", Arrays.asList("c1", "c2", "c3", "c4", "c5"));
        System.out.println("dataX = " + dataX);

        List<DataY> collect = dataX.getC().stream()
                .flatMap(e -> operationOneToMany(e).stream())
                .map(c -> new DataY(dataX.getA(), UUID.randomUUID().toString(), c))
                .collect(Collectors.toList());

        System.out.println("collect = " + collect);
    }

    private static List<String> operationOneToMany(String input) {
        if ("c3".equals(input))
            return Arrays.asList("c31", "c32", "c33");
        else if ("c5".equals(input))
            return Arrays.asList("c51", "c52");
        else
            return Arrays.asList(input);
    }

}