How do I force when using a constructor with a varargs parameter that at least one parameter has to be passed?

Question

I have admittedly the following poorly implemented class:

public class Fari{
    String name;
    String[] group;

    public Fari(String name, String... group) {
        this.name = name;
        this.group = group;
    }
    // getters, setters
}

This was actually planned in such a way that the name and at least one group label is passed. But i can create a new Fari object with only one parameter without a compile erorr:

Fari f = new Fari("A");

ending in an object with null for the group attribute. Is there a way to force that at least one string should be passed as the varargs parameter?

I could change the parameter String... group to a List<String> but then I have to touch all other classes as well. Apart from that an empty list could be passed, which doesn't really help me. How do I go about this if the created object should have a name and at least one group lable?

If you require parameter to be passed, it is not vararg. You can change constructor as `public Fari(String name, String group, String... otherGroups)` to force at least one group to be specified — uaraven, Aug 04 '19 at 20:38
@Progman During runtime at the at the most no object should exist without a group name. If it can already be checked during compilation it is of course even better — nopens, Aug 04 '19 at 20:42

score 3 · Accepted Answer · answered Aug 04 '19 at 20:38

3

The typical way to solve this would be have

public Fari(String name, String requiredGroup, String... group)

answered Aug 04 '19 at 20:38

Matt Berteaux

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4
12

Thanks for the quick response. is there a way to do this without changing the constructor definition? – nopens Aug 04 '19 at 20:44
@nopens not at compile time. If you just have the varargs you can check it's length at runtime – Matt Berteaux Aug 04 '19 at 20:48

score 0 · Answer 2 · answered Aug 04 '19 at 20:48

You can also check group size and throw error if the size is < 1. Something like this:

public class Fari {

    String name;
    String[] group;

    public Fari(String name, String... group) throws Exception {
        this.name = name;
        if (group.length < 1) throw new Exception();
        this.group = group;
    }
}

Of course you can use your own Exception implementation to identify it in your code.

How do I force when using a constructor with a varargs parameter that at least one parameter has to be passed?

2 Answers2