在Java中，可以通过使用多线程或异步编程来实现并行写入文件。下面是使用多线程和异步编程的两个示例解决方法。

示例1：使用多线程实现并行写入文件

import java.io.FileWriter;
import java.io.IOException;

public class ParallelFileWriter implements Runnable {
    private String fileName;
    private String content;

    public ParallelFileWriter(String fileName, String content) {
        this.fileName = fileName;
        this.content = content;
    }

    @Override
    public void run() {
        try {
            FileWriter writer = new FileWriter(fileName);
            writer.write(content);
            writer.close();
            System.out.println("Successfully wrote to file: " + fileName);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        String fileName1 = "file1.txt";
        String content1 = "Content for file 1";
        String fileName2 = "file2.txt";
        String content2 = "Content for file 2";

        Thread thread1 = new Thread(new ParallelFileWriter(fileName1, content1));
        Thread thread2 = new Thread(new ParallelFileWriter(fileName2, content2));

        thread1.start();
        thread2.start();
    }
}

示例2：使用异步编程实现并行写入文件

import java.io.FileWriter;
import java.io.IOException;
import java.util.concurrent.CompletableFuture;

public class ParallelFileWriter {
    public static void main(String[] args) {
        String fileName1 = "file1.txt";
        String content1 = "Content for file 1";
        String fileName2 = "file2.txt";
        String content2 = "Content for file 2";

        CompletableFuture future1 = CompletableFuture.runAsync(() -> {
            try {
                FileWriter writer = new FileWriter(fileName1);
                writer.write(content1);
                writer.close();
                System.out.println("Successfully wrote to file: " + fileName1);
            } catch (IOException e) {
                e.printStackTrace();
            }
        });

        CompletableFuture future2 = CompletableFuture.runAsync(() -> {
            try {
                FileWriter writer = new FileWriter(fileName2);
                writer.write(content2);
                writer.close();
                System.out.println("Successfully wrote to file: " + fileName2);
            } catch (IOException e) {
                e.printStackTrace();
            }
        });

        CompletableFuture allFutures = CompletableFuture.allOf(future1, future2);
        allFutures.join();
    }
}

这两个示例代码都可以实现并行写入文件，其中示例1使用了多线程，示例2使用了CompletableFuture来实现异步编程。在示例1中，创建了两个线程分别写入两个文件；在示例2中，使用了CompletableFuture的runAsync方法来创建两个异步任务，然后通过CompletableFuture.allOf方法等待两个任务完成。两种方法都可以实现并行写入文件的效果。