Here is an example of using the DeferredFileOutputStream class from the Apache Commons IO library in Java to write data to a file, while keeping the data in memory until a certain threshold is reached:

import java.io.File;
import java.io.IOException;
import org.apache.commons.io.output.DeferredFileOutputStream;

public class DeferredFileOutputStreamExample {
    public static void main(String[] args) {
        // specify the file to write to
        File file = new File("/path/to/file.txt");

        // specify the threshold size
        int threshold = 8192;

        try (DeferredFileOutputStream out = new DeferredFileOutputStream(threshold, file)) {
            // write data to the stream
            for (int i = 0; i < 1000000; i++) {
                out.write("This is some text to be written to the file.\n".getBytes());
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

In this example, the DeferredFileOutputStream class is used to write data to a file, while keeping the data in memory until a certain threshold is reached. The DeferredFileOutputStream is created by passing the threshold size and the file to write to as arguments. In this example, the threshold size is set to 8192 bytes, and any data written to the stream will be kept in memory until 8192 bytes have been written. Once the threshold is reached, the data is written to the file and any further data is written directly to the file.

The DeferredFileOutputStream class provides a way to write large amounts of data to a file without consuming too much memory. It can be useful in situations where you need to process a large amount of data and write it to a file, but you don't want to keep the entire data set in memory. The threshold value should be set to a value that balances the memory usage and the performance.