[{'a':'1','b':'2'},{'c':'3','d':'4'}]"

解決

import net.sf.json.JSONArray;
import net.sf.json.JSONObject;

public static void main(String[] args) {
	
	String invoiceStr = "[{'a':'1','b':'2'},{'c':'3','d':'4'}]";

	JSONArray invoiceArray = JSONArray.fromObject(invoiceStr);
	System.out.println("測試數組: "+invoiceArray.get(0));
	JSONObject invoiceObj = JSONObject.fromObject(invoiceArray.get(1));
	System.out.println("測試json: "+invoiceObj.get("c"));
	
}

結果：

測試數組: {"a":"1","b":"2"}
測試json: 3