How to use generics properly for a Holder

Question

I am trying to create a Holder class for different objects to be used in my application, I ended up with this code that works fine until some extent, the builder pattern works fine for the optional fields, but I guess this holder could be refactored to accept any arbitrarily number of parameter

package pojos;

public class Holder<T, R, S, U> {
private final T t;
private final R r;

private final S s;
private final U u;


private Holder(final Builder<T, R, S, U> builder) {
    this.t = builder.t;
    this.r = builder.r;
    this.s = builder.s;
    this.u = builder.u;
}


public T getField1() {
    return this.t;
}

public R getField2() {
    return this.r;
}

public S getField3() {
    return this.s;
}

public U getField4() {
    return this.u;
}


public static class Builder<T, R, S, U> {
    private T t;
    private R r;
    private S s;
    private U u;


    public Builder field1(final T t) {
        this.t = t;
        return this;
    }

    public Builder field2(final R r) {
        this.r = r;
        return this;
    }

    public Builder field3(final S s) {
        this.s = s;
        return this;
    }

    public Builder field4(final U u) {
        this.u = u;
        return this;
    }

    public Holder<T, R, S, U> build() {
        return new Holder<>(this);
    }

    public Builder<T, R, S, U> copy(final Holder<T, R, S, U> rowMappingsHolder) {
        this.t = rowMappingsHolder.getField1();
        this.r = rowMappingsHolder.getField2();
        this.s = rowMappingsHolder.getField3();
        this.u = rowMappingsHolder.getField4();
        return this;
    }


}

}

Example of usage:

protected Holder<Row, Map<Integer, String>, Void, Void> getRowMapHolder(Row row, Map<Integer,String> map) {
    return (Holder<Row, Map<Integer, String>, Void, Void>) new Holder.Builder<Row, Map<Integer, String>,Void, Void>().field1(row).field2(map).build();
}

Any ideas?

Regards

~Marco

Answer 1

Thanks to Andy's comment and Google Autovalue, a good solution arose:

So we can create different classes that have meaning, no more "field1", "field2"...

package pojos;

import com.google.auto.value.AutoValue;
import org.apache.poi.ss.usermodel.Row;

import java.util.Map;

@AutoValue
public abstract class RowMapHolder {

    public abstract Row row();

    public abstract Map<Integer,String> mapping();

    public static RowMapHolder create(Row row, Map<Integer, String> mapping) {
        return new AutoValue_RowMapHolder(row, mapping);
    }

}

or

package pojos;

import com.google.auto.value.AutoValue;

import java.util.List;
import java.util.Map;

@AutoValue
public abstract class KeyValuesMapHolder {

    public abstract List<KeyValue<String,String>> keyValues();
    public abstract Map<Integer,String> mapping();

    public static KeyValuesMapHolder create(List<KeyValue<String, String>> keyValues, Map<Integer, String> mapping) {
        return new AutoValue_KeyValuesMapHolder(keyValues, mapping);
    }

}