问题描述
java.util.Queue<TreeNode> queue = new java.util.LinkedList<TreeNode>();
LinkedList实现队列。不应该排队在右边的上面的语句和LinkedList左边？
LinkedList implements Queue. Shouldn't Queue be on the right side of the above statement and LinkedList the left?
推荐答案
在Java中，你可以分配一个值为相同类型或更一般类型的变量。在你的例子中， new LinkedList< TreeNode>（）是一个值。因为 LinkedList 实现 队列，它比 Queue 。即A LinkedList 是 队列。
In Java, you can assign a value to a variable of the same type or a more general type. In your example, new LinkedList<TreeNode>() is a value. Since LinkedList implements Queue, it's more specific than Queue. i.e. A LinkedList is a Queue.
例如，这三个都是有效的
For instance, all three of these are valid
Object o = new LinkedList<TreeNode>();
Queue<TreeNode> queue = new LinkedList<TreeNode>();
LinkedList<TreeNode> list = new LinkedList<TreeNode>();
但是你不能写任何这些，因为它们的分配不正确。
But you can't write any of these because they're incorrect assignment.
LinkedList<TreeNode> o = new Object();
LinkedList<TreeNode> queue = new Queue<TreeNode>();
该示例中的第二个也是无效的，因为Queue是一个接口，您不能实例化（新的）接口，因为它不是具体类型
P.S. the second one in that example is also invalid because Queue is an interface, and you can't instantiate (new) an interface because it's not a concrete type
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