通用方法<?>类型参数

Question

In a library for charts I found the following class:

public class SeriesBuilder<T> {
    private T[] data;

    private SeriesBuilder() {
    }

    public static SeriesBuilder<?> get() {
        return new SeriesBuilder();
    }
    
    public SeriesBuilder<T> withData(T... data) {
        this.data = data;
        return this;
    }

    public Series<T> build() {
        Series<T> series = new Series<T>();
        series.setData(data);
        return series;
    }
}

Using code:

SeriesBuilder.get()
             .withData(<<<<<???>>>>)
             .build()

I'm not able to find out how to use the class because of the <?> Type. I can't find an argument that fullfills the signature. How to use this?

Answer 1

I'm not able to find out how to use the class because of the <?> Type. I can't find an argument that fullfills the signature. How to use this?

You pretty much can't use it. There is no way to obtain a SeriesBuilder instance except via SeriesBuilder.get() , and the type parameter of an instance that you obtain that way is unknown -- in fact, what get actually instantiates is the raw type. You should be able to produce a series of null s if you wish.

There cannot even be any subclasses of SeriesBuilder (that might be more usable), because its only constructor is private.

Without putting too fine a point on it, this SeriesBuilder is pretty much an abomination. There is an argument to be made for preferring factory methods (such as SeriesBuilder.get() ) over constructors, but that implementation is terrible. In addition to the type parameter issues, it does not initialize the resulting object to a valid state. That has to be done separately via withData() , so what is the point of get() supposed to be?

I'm inclined to think that whoever wrote that was looking for something that would have been better expressed via this variation on withData() :

    public static <T> SeriesBuilder<T> withData(T ... data) {
        SeriesBuilder<T> builder = new SeriesBuilder<>();

        builder.data = data;
        return builder;
    }

You might use that as

SomeType item1 = /* value */;
SomeType item2 = /* value */;
SomeType item3 = /* value */;

Series<SomeType> series =
        SeriesBuilder.withData(item1, item2, item3)
                     .build();