To ensure that all data buffered in the BufferedWriter is written out to 'alldata.txt', the 'bw.flush()' method should be called. This method forces any buffered output bytes to be written out, which is critical for ensuring that the content read from 'data.txt' is fully written to the 'alldata.txt'. Therefore, writing 'bw.flush();' at line n1 will enable the code to overwrite 'alldata.txt' with 'data.txt'.