在Java中下載大文件時，為了避免內存溢出，可以使用以下方法：

1. 使用BufferedInputStream和BufferedOutputStream：這兩個類可以提高文件讀取和寫入的性能。通過使用緩沖區，可以減少對文件的直接訪問次數，從而降低內存占用。

InputStream inputStream = new BufferedInputStream(new FileInputStream("largeFile.txt"));
OutputStream outputStream = new BufferedOutputStream(new FileOutputStream("output.txt"));

2. 使用FileChannel：FileChannel可以實現高效的文件讀寫操作。它可以一次性讀取或寫入多個字節，而不是逐個字符地讀取或寫入。這樣可以減少內存占用。

RandomAccessFile file = new RandomAccessFile("largeFile.txt", "r");
FileChannel channel = file.getChannel();
ByteBuffer buffer = ByteBuffer.allocate(1024 * 1024); // 分配1MB的緩沖區
while (channel.read(buffer) != -1) {
    buffer.flip(); // 切換到讀模式
    // 處理數據
    buffer.compact(); // 切換到寫模式
}
channel.close();
file.close();

3. 使用分塊下載：將大文件分成多個小塊，然后逐個下載這些小塊。這樣可以避免一次性加載整個文件到內存中。

int bufferSize = 1024 * 1024; // 1MB
URL url = new URL("http://example.com/largeFile.txt");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");
connection.setConnectTimeout(5000);
connection.setReadTimeout(5000);

int fileSize = connection.getContentLength();
InputStream inputStream = connection.getInputStream();
OutputStream outputStream = new FileOutputStream("output.txt");

byte[] buffer = new byte[bufferSize];
int bytesRead;
long totalBytesRead = 0;
while ((bytesRead = inputStream.read(buffer)) != -1) {
    totalBytesRead += bytesRead;
    outputStream.write(buffer, 0, bytesRead);
    // 可以在這里更新下載進度
}

outputStream.close();
inputStream.close();
connection.disconnect();

4. 使用多線程：可以將文件分成多個部分，然后使用多個線程同時下載這些部分。這樣可以提高下載速度，同時避免內存溢出。

int numThreads = 4;
int bufferSize = 1024 * 1024; // 1MB
URL url = new URL("http://example.com/largeFile.txt");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");
connection.setConnectTimeout(5000);
connection.setReadTimeout(5000);

int fileSize = connection.getContentLength();
InputStream inputStream = connection.getInputStream();
OutputStream outputStream = new FileOutputStream("output.txt");

byte[] buffer = new byte[bufferSize];
int bytesRead;
long totalBytesRead = 0;
List<DownloadThread> threads = new ArrayList<>();
for (int i = 0; i < numThreads; i++) {
    long start = i * fileSize / numThreads;
    long end = (i + 1) * fileSize / numThreads;
    DownloadThread thread = new DownloadThread(url, start, end, buffer, outputStream);
    threads.add(thread);
    thread.start();
}

for (DownloadThread thread : threads) {
    try {
        thread.join();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

outputStream.close();
inputStream.close();
connection.disconnect();

這些方法可以結合使用，以實現高效且內存友好的大文件下載。