在Android中，从API级别30开始，DefaultHttpClient、HttpPost等类已被弃用。推荐使用HttpURLConnection或OkHttp库来替代这些类。

以下是使用HttpURLConnection的示例代码：

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class HttpClientExample {
    public static String doPostRequest(String urlString, String requestBody) throws IOException {
        URL url = new URL(urlString);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("POST");
        connection.setRequestProperty("Content-Type", "application/json");
        connection.setDoOutput(true);

        OutputStream outputStream = connection.getOutputStream();
        outputStream.write(requestBody.getBytes());
        outputStream.flush();
        outputStream.close();

        int responseCode = connection.getResponseCode();
        if (responseCode == HttpURLConnection.HTTP_OK) {
            InputStream inputStream = connection.getInputStream();
            BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
            StringBuilder response = new StringBuilder();
            String line;
            while ((line = reader.readLine()) != null) {
                response.append(line);
            }
            reader.close();
            return response.toString();
        } else {
            throw new IOException("HTTP request failed. Response code: " + responseCode);
        }
    }
}

以上代码示例中，doPostRequest方法用于发送POST请求，接收到响应后将其转换为字符串并返回。

如果你想使用OkHttp库，你需要在项目的build.gradle文件中添加以下依赖项：

implementation 'com.squareup.okhttp3:okhttp:4.9.1'

然后，你可以使用以下示例代码：

import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;

public class HttpClientExample {
    public static final MediaType JSON = MediaType.parse("application/json; charset=utf-8");
    public static OkHttpClient client = new OkHttpClient();

    public static String doPostRequest(String urlString, String requestBody) throws IOException {
        RequestBody body = RequestBody.create(requestBody, JSON);
        Request request = new Request.Builder()
                .url(urlString)
                .post(body)
                .build();

        Response response = client.newCall(request).execute();
        return response.body().string();
    }
}

以上代码示例中，doPostRequest方法使用OkHttp发送POST请求，并返回响应体的字符串。

请根据你的具体需求选择使用HttpURLConnection或OkHttp库进行网络请求。