Question

我正在尝试创建数组列表，动态大小，添加元素并在完成后迭代它们。目前我的代码是：

    String activities = "SELECT DISTINCT phoneactivity from ContextTable030313";
    ArrayList<String> myActivities = new ArrayList();

    Cursor cursor2 = db.rawQuery(activities, null);
    // looping through all rows and adding to list
    if (cursor2.moveToFirst()) {
        do {
            myActivities.add(cursor.getString(cursor2.getColumnIndex("ts")));

        } while (cursor2.moveToNext());
    }

然而它无法在do循环中运行。我相信我正在宣布错误的内容，并收到以下警告：

- ArrayList is a raw type. References to generic type ArrayList<E> should be parameterized
    - Avoid object allocations during draw/layout operations (preallocate and reuse instead)
    - Type safety: The expression of type ArrayList needs unchecked conversion to conform to 
     ArrayList<String>

但我不明白为什么这不起作用。

Answer 1

问题似乎是：

类型安全性：ArrayList类型的表达式需要取消选中转换为符合 ArrayList的

指出这行代码：

ArrayList<String> myActivities = new ArrayList();

您应该将该行更改为：

ArrayList<String> myActivities = new ArrayList<String>();

另外，您可以替换此代码段：

  if (cursor2.moveToFirst()) {
        do {
            myActivities.add(cursor.getString(cursor2.getColumnIndex("ts")));

        } while (cursor2.moveToNext());
    }

有了这个：

  while (cursor2.moveToFirst()) {       
        myActivities.add(cursor.getString(cursor2.getColumnIndex("ts")));       
    }

Answer 2

更改您的arrayList声明，使其在构造函数调用期间接受参数化类型。

ArrayList<String> myActivities = new ArrayList<String>();

如果您使用java 7或更高版本：

ArrayList<String> myActivities = new ArrayList<>();

由于* 类型推断 *，您不需要在构造函数调用期间声明参数化类型，但仍需要声明空<＆lt;＆gt;。

在Java中创建并迭代字符串列表

2 个答案: