导入json.org并尝试以下操作：

JSONObject json = new JSONObject(); //creates main json
json.put("Name", "sam");
json.put("Id", 1234);

JSONObject valuesJson = new JSONObject(); //another object
valuesJson.put("Car", "Maruti");

json.put("Values", valuesJson); //puts a json inside another

String jsonString = json.toString();

//next, saves the file:
PrintWriter out1 = null;
try {
    out1 = new PrintWriter(new FileWriter("C:\\Users\\YOURNAME\\Documents\\json.txt"));
    out1.write(jsonString);
} catch (Exception ex) {
    System.out.println("error: " + ex.toString());
}

不要忘记正确设置文件路径！