presto-spi/src/main/java/io/prestosql/spi/block/RowBlockBuilder.java/RowBlockBuilder/build
class RowBlockBuilder:    @Override
    public Block build()
    {
        if (currentEntryOpened) {
            throw new IllegalStateException("Current entry must be closed before the block can be built");
        }
        Block[] fieldBlocks = new Block[numFields];
        for (int i = 0; i < numFields; i++) {
            fieldBlocks[i] = fieldBlockBuilders[i].build();
        }
        return createRowBlockInternal(0, positionCount, rowIsNull, fieldBlockOffsets, fieldBlocks);
    }