在S3存储桶中，为当前存储桶的访问策略添加一个允许另一个AWS账户匿名访问该存储桶的条目。使用Java SDK编写代码如下：

import com.amazonaws.auth.BasicAWSCredentials;
import com.amazonaws.services.s3.AmazonS3Client;
import com.amazonaws.services.s3.model.GetObjectRequest;
import com.amazonaws.services.s3.model.S3Object;

public class S3Downloader {

  public static void main(String[] args) {
    String awsAccessKeyId = "YOUR_ACCESS_KEY";
    String awsSecretKey = "YOUR_SECRET_KEY";
    String bucketName = "BUCKET_NAME";
    String key = "OBJECT_KEY";
    String destinationFilePath = "DESTINATION_FILE_PATH";

    // Create BasicAWSCredentials object
    BasicAWSCredentials credentials = new BasicAWSCredentials(awsAccessKeyId, awsSecretKey);

    // Create AmazonS3Client object
    AmazonS3Client s3Client = new AmazonS3Client(credentials);

    // Create GetObjectRequest object with bucket name and object key
    GetObjectRequest getObjectRequest = new GetObjectRequest(bucketName, key);

    // Get S3Object by passing GetObjectRequest object to getObject method of AmazonS3Client
    S3Object s3Object = s3Client.getObject(getObjectRequest);

    // Save the S3Object to the destination file path specified
    s3Client.getObject(new GetObjectRequest(bucketName, key), new File(destinationFilePath));
  }
}