合并多个文件的内容

合并的结果应该是这样的：

现在是这样的

public static void main(String[] args) {
        List<Object> list = new ArrayList<>();
        File file1 = new File("d:\\ti.txt");
        File file2 = new File("d:\\titi.txt");
        list.add(file1);
        list.add(file2);
        FileWriter writer = null;
        BufferedReader reader = null;
        try {
            for (int i = 0; i < list.size(); i++) {
                File f = (File) list.get(i);
                writer = new FileWriter("d:\\concatenation.txt");
                reader = new BufferedReader(new FileReader(f));

                String l;
                while ((l = reader.readLine()) != null) {
                    writer.write(l);
                }
            }
        writer.flush();
        writer.close();
        reader.close();

        } catch (IOException e) {
            e.printStackTrace();
        }

    }
    

运行，得到结果如下

怎么只有第二个文件的内容？

原来这一句writer = new FileWriter("d:\\concatenation.txt");写在循环里了，因为writer是引用，所以writer是先指向了一个new FileWriter("d:\\concatenation.txt")地址，循环一下，又指向了一个新的new FileWriter("d:\\concatenation.txt")地址，这时文件只有文件二了,所以写入的只有文件二的内容了

总的目标是一个，所以要写在循环外

public static void main(String[] args) {
        List<Object> list = new ArrayList<>();
        File file1 = new File("d:\\ti.txt");
        File file2 = new File("d:\\titi.txt");
        list.add(file1);
        list.add(file2);
        FileWriter writer = null;
        BufferedReader reader = null;
        try {
            writer = new FileWriter("d:\\concatenation.txt");
            for (int i = 0; i < list.size(); i++) {
                File f = (File) list.get(i);
                reader = new BufferedReader(new FileReader(f));
                String l;
                while ((l = reader.readLine()) != null) {
                    writer.write(l);
                }
            }
        writer.flush();
        writer.close();
        reader.close();

        } catch (IOException e) {
            e.printStackTrace();
        }

    }