将特定列表复制到另一个列表

Question

我想将特定数据从一个列表复制到另一个列表。它显示

ERROR:
java.lang.NullPointerException
    at controllers.WebForms.getMenu(WebForms.java:134)
    at Routes$$anonfun$routes$1$$anonfun$applyOrElse$2$$anonfun$apply$2.apply(routes_routing.scala:222)

       List<menu> allMenus = menu.find.all();

       List<menu> selectedMenu=null;
       System.out.println("selected menu:"+selectedMenu);//prints: null
       int count = 0;
       for (models.menu m : allMenus) {
           System.out.println("m:"+m);
           if (m.r_id == r_id) {
               count=1;
               System.out.println("r_id:"+m.r_id+"   "+m.item);
//prints: r_id:1   Noodles
               selectedMenu.add(m);
//throws NULLPointer Exception
               System.out.println("selected menu inside loop:"+selectedMenu);
           }
       }

Answer 1

List<menu> selectedMenu = new ArrayList<>();。在对列表执行操作之前，必须先初始化列表。

Answer 2

列出selectedMenu = new ArrayList＆lt;＆gt;（）;