可以写一个Java代码示例吗

当然可以。下面是一个简单的Java代码示例，展示了如何使用AWS V4签名来构造一个Cloud Object Storage (COS) 请求头。这个例子假设你已经拥有Access Key ID和Secret Access Key，以及COS服务名称("cos")和区域("ap-shanghai")：

import com.amazonaws.auth.SigningUtils;
import com.amazonaws.services.s3.AmazonS3;
import com.amazonaws.services.s3.AmazonS3ClientBuilder;
import com.amazonaws.services.s3.model.PutObjectRequest;
import java.nio.charset.StandardCharsets;
import java.util.Date;

public class CosV4Signer {

    public static void main(String[] args) {
        String accessKeyId = "your_access_key_id";
        String secretAccessKey = "your_secret_access_key";
        String region = "ap-shanghai";
        String bucketName = "your_bucket_name";
        String objectKey = "your_object_key";
        String content = "Hello, Cloud Object Storage!";

        // 创建Amazon S3客户端
        AmazonS3 s3Client = AmazonS3ClientBuilder.standard()
                .withRegion(region)
                .withCredentials(new BasicAWSCredentials(accessKeyId, secretAccessKey))
                .build();

        // 构造PutObjectRequest
        PutObjectRequest putObjectRequest = new PutObjectRequest(bucketName, objectKey, content);

        // 获取当前日期和时间戳
        Date date = new Date();
        String timestamp = date.toString("yyyyMMdd'T'HHmmssZ");

        // 计算Signature
        String algorithm = "AWS4-HMAC-SHA256";
        String credentialScope = signingScope(date, region, "s3", "aws4_request");
        String signedHeaders = SigningUtils.canonicalizeHeaderNames(
                "host;x-amz-content-sha256;x-amz-date").toLowerCase();
        String stringToSign = createStringToSign(algorithm, timestamp, credentialScope, signedHeaders);
        byte[] signatureBytes = sign(stringToSign, secretAccessKey);
        String signature = Base64.getEncoder().encodeToString(signatureBytes);

        // 设置Authorization header
        putObjectRequest.setMetadata("x-amz-date", timestamp);
        putObjectRequest.setMetadata("x-amz-content-sha256", getSha256Hash(content));
        putObjectRequest.getRequestMetadata().put("Authorization",
                String.format("%s Credential=%s/%s,SignedHeaders=%s,Signature=%s",
                        algorithm, credentialScope, secretAccessKey, signedHeaders, signature));

        // 发送PUT请求
        s3Client.putObject(putObjectRequest);
    }

    private static String signingScope(Date date, String region, String serviceName, String signingType) {
        return date.toString("yyyyMMdd") + "/" + region + "/" + serviceName + "/" + signingType;
    }

    private static String createStringToSign(String algorithm, String timestamp, String scope, String signedHeaders) {
        return String.format("%s\n%s\n%s\n%s", algorithm, timestamp, scope, signedHeaders);
    }

    private static byte[] sign(String message, String secretKey) {
        // 实现实际的HMAC-SHA256签名算法...
    }

    private static String getSha256Hash(String data) {
        // 实现数据的SHA-256哈希...
    }
}

请注意，你需要自己实现`sign()`和`getSha256Hash()`方法，它们分别用于计算HMAC-SHA256签名和内容的SHA-256散列。这是一个基础示例，实际项目中可能还需要考虑错误处理和异常情况。