Question

java.util.Queue<TreeNode> queue = new java.util.LinkedList<TreeNode>();

LinkedList implements Queue. Shouldn’t Queue be on the right side of the above statement and LinkedList the left?

Answer 1

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.

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>();

But you can’t write any of these because they’re incorrect assignment.

LinkedList<TreeNode> o = new Object();
LinkedList<TreeNode> queue = new Queue<TreeNode>();

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