思路：将第一个文件追加到第二个文件末尾，再将第二个文件反写至文件一并覆盖。
package zh.codegym.task.task18.task1819;
/*
合并文件
*/
import java.io.*;
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
String filename1 = bufferedReader.readLine();
String filename2 = bufferedReader.readLine();
bufferedReader.close();
FileInputStream fileInputStream1 = new FileInputStream(filename1);
FileOutputStream fileOutputStream2 = new FileOutputStream(filename2, true);
while (fileInputStream1.available() > 0) {
int read = fileInputStream1.read();
fileOutputStream2.write(read);
}
fileInputStream1.close();
fileOutputStream2.close();
FileInputStream fileInputStream2 = new FileInputStream(filename2);
FileOutputStream fileOutputStream1 = new FileOutputStream(filename1);
while (fileInputStream2.available() > 0) {
int read = fileInputStream2.read();
fileOutputStream1.write(read);
}
fileInputStream2.close();
fileOutputStream1.close();
}
}