可以有以下几种方法，但是都只能合并文本文件，对有一定格式的文件(比如：word、excel、bmp等)虽然数据加上去了，表现为文件大小的改变，但是打开文件后看不出结果来。

代码一：import java.io.*;

public class Test1 {

public static void main(String[] args){

try {

java.io.File file = new java.io.File("123.txt");

java.io.FileOutputStream fos = new java.io.FileOutputStream(file);

FileInputStream fis = new FileInputStream(new File("haha.txt"));

FileInputStream fis1 = new FileInputStream(new File("abc.txt"));

byte[] b = new byte[1];

while((fis.read(b)) != -1){

fos.write(b);

}

while((fis1.read(b)) != -1){

fos.write(b);

}

fos.flush();

System.out.println("success!");

}

catch(Exception e){System.out.println("error: " + e);}

}

}

代码二(与代码一基本相似，只是先合并输入流)：

import java.io.*;

public class Test2 {

public static void main(String[] args){

try {

java.io.File file = new java.io.File("123.txt");

java.io.FileOutputStream fos = new java.io.FileOutputStream(file);FileInputStream fis = new FileInputStream(new File("haha.txt"));

FileInputStream fis1 = new FileInputStream(new File("abc.txt"));

SequenceInputStream sis = new SequenceInputStream(fis,fis1);              byte[] b = new byte[1];

while((sis.read(b)) != -1){

fos.write(b);

}fos.flush();

System.out.println("success!");

}

catch(Exception e){System.out.println("error: " + e);}

}

}

代码三(使用RandomFile)：

import java.io.*;

public class Test3{

public static void main(String[] args){

try {

RandomAccessFile outt= new RandomAccessFile(new java.io.File("123.doc"),"rw");

RandomAccessFile inn= new RandomAccessFile(new File("haha.doc"),"r");

int c;

while((c=inn.read())!=-1)

outt.write(c);

RandomAccessFile inn1= new RandomAccessFile(new File("abc.doc"),"r");

while((c=inn1.read())!=-1)

outt.write(c);

System.out.println("success!");

}

catch(Exception e){System.out.println("error: " + e);}

}

}