副作用持ちだけどAccumulableIterationCallback作れる。

AccumulableIterationCallback.java

import javax.annotation.Nonnull;

import org.seasar.doma.jdbc.IterationContext;
import org.seasar.doma.jdbc.PostIterationCallback;

import com.google.common.base.Optional;

// ThreadUnsafe
// 同一インスタンスの再利用不可
public abstract class AccumulableIterationCallback<ACCUM, RECORD> implements PostIterationCallback<ACCUM, RECORD> {

    private Optional<ACCUM> accumulator = Optional.absent();
    
    public abstract ACCUM initialValue();
    
    public abstract ACCUM iterate(@Nonnull final ACCUM accumulator, @Nonnull final RECORD record, @Nonnull final IterationContext context);

    @Override
    public final ACCUM iterate(@Nonnull final RECORD record, @Nonnull final IterationContext context) {
        final ACCUM accum = accumulator.or(initialValue());
        final ACCUM result = iterate(accum, record, context);
        accumulator = Optional.of(result);
        return result;
    }

    @Override
    public ACCUM postIterate(@Nonnull final ACCUM accum, @Nonnull final IterationContext context) {
        return accumulator.or(initialValue());
    }
    
}