Cannot import java.util.concurrent.locks ... WHY?

Why is Xcode unable to find the java.util.concurrent.locks package. The class browser knows it exists. For example the entry for the ReentrantLock class looks like this in the browser class window:

ReentrantLock (java.util.concurrent.locks)

Xcode knows about other java.util packages such as java.util.ResourceBundle which I have been accessed successfully in other parts of my projecgt.

Here is a source file and the resulting compiler error:

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The source file:
// Foo.java
import java.util.ResourceBundle;
import java.util.concurrent.locks;
public class Foo { }

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The compiler error:

compile:
Compiling 2 source files to /Users/Terry/Desktop/JAVA/PROJECTS/Logic/bin
/Users/Terry/Desktop/JAVA/PROJECTS/Logic/src/Foo.java:3: cannot find symbol
symbol : class locks
location: package java.util.concurrent
import java.util.concurrent.locks;
^
1 error

BUILD FAILED

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Help or hints would be greatly appreciated!