JAVA中如何将一个json形式的字符串转为json对象

import java.io.*;
import org.json.*;  

public class Demo {
    public static void main(String[] args) throws Exception {
        String str = "{\"a\":\"b\", \"c\":\"d\"}";
        JSONObject a = new JSONObject(str);
        System.out.println(a); // {"c":"d","a":"b"}
        System.out.println(a.get("c")); // d
    }
}