StringBuilder sb_a = new StringBuilder(); // for a
StringBuilder sb_b = new StringBuilder(); // for b
然後讀文件：
Path aFile = Paths.get("a.txt");
try (BufferedReader reader = Files.newBufferedReader( aFile, Charset.defaultCharset())) {
String lineFromFile = "";
while((lineFromFile = reader.readLine()) != null){
sb_a.append(lineFromFile);
}
} catch(IOException exception) {
System.out.println("Error while reading file");
}
b.txt也是同壹個方法
跟著以妳自定義的方法去比較兩個怎樣合拼，暫存為String merged = ...;
最後寫回文件：
Path abFile = Paths.get("ab.txt");
try (BufferedWriter writer = Files.newBufferedWriter( abFile, Charset.defaultCharset())) {
writer.append(merged);
writer.newLine(); // optional
writer.flush();
} catch(IOException exception) {
System.out.println("Error writing to file");
}