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java强制转换ArrayList <StringBuilder> 到ArrayList <String>

[英]java cast ArrayList<StringBuilder> to ArrayList<String>

原文 2014-09-26 01:52:47 0 1 java

I'm writing a method in java which requires ArrayList<String> as its output. 我正在用java写一个方法，它需要ArrayList<String>作为其输出。 In this method, String concatenation is frequently used. 在此方法中，经常使用字符串连接。 I use StringBuilder instead of String since String is more expensive in concatenation. 我使用StringBuilder而不是String，因为String在连接中更昂贵。

Before returning result, how could I cast all elements in ArrayList<StringBuilder> to ArrayList<String> ? 返回结果之前，如何将ArrayList<StringBuilder>所有元素转换为ArrayList<String> ？

1 个解决方案

how could I cast all elements in ArrayList<StringBuilder> to ArrayList<String> ? 我如何将ArrayList<StringBuilder>所有元素转换为ArrayList<String> ？

You can't and you shouldn't. 你不能，你不应该。 Make it an ArrayList of one or the other. 使它成为另一个的ArrayList。

Or if you must translate, then you'll need a for loop to iterate through the collection, building the new collection. 或者，如果您必须翻译，那么您将需要一个for循环来遍历集合，从而构建新的集合。

or I suppose you could declare the variable as an ArrayList<? extends CharSequence> 或者我想您可以将变量声明为ArrayList<? extends CharSequence> ArrayList<? extends CharSequence> -- nope that wouldn't work as we're nixed since we can't add(...) anything into a covariant structure, so we wouldn't be able to add anything to this list, just read from it. ArrayList<? extends CharSequence> -不，因为我们无法add(...)任何东西add(...)到协变结构中，所以它不起作用，所以我们不能将任何东西添加到此列表中，只需从它。