使用Java调用RESTful API接口

要使用Java调用RESTful API接口，可以使用Java内置的HttpURLConnection类或者第三方库如Apache HttpClient等。

以下是使用HttpURLConnection类的示例代码：

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

public class RestClient {
    public static void main(String[] args) throws IOException {
        String url = "http://api.example.com/users";
        URL obj = new URL(url);
        HttpURLConnection con = (HttpURLConnection) obj.openConnection();

        //设置请求方法
        con.setRequestMethod("GET");

        //添加请求头
        con.setRequestProperty("User-Agent", "Mozilla/5.0");

        int responseCode = con.getResponseCode();
        System.out.println("Response Code : " + responseCode);

        BufferedReader in = new BufferedReader(new InputStreamReader(con.getInputStream()));
        String inputLine;
        StringBuffer response = new StringBuffer();

        while ((inputLine = in.readLine()) != null) {
            response.append(inputLine);
        }
        in.close();

        System.out.println(response.toString());
    }
}

以上代码通过HttpURLConnection发送GET请求，并输出返回的字符串。可以根据需要修改请求方法和添加请求头。