Description

An import statement is considered unused if:

It is not referenced in the file. The algorithm does not support wild-card imports like import java.io.*;. Most IDE's provide very sophisticated checks for imports that handle wild-card imports.
It is a duplicate of another import. This is when a class is imported more than once.
The class imported is from the java.lang package. For example importing java.lang.String.
The class imported is from the same package.
Optionally: it is referenced in Javadoc comments. This check is on by default, but it is considered bad practice to introduce a compile time dependency for documentation purposes only. As an example, the import java.util.Date would be considered referenced with the Javadoc comment {@link Date}. The alternative to avoid introducing a compile time dependency would be to write the Javadoc comment as {@link java.util.Date}.

The main limitation of this check is handling the case where an imported type has the same name as a declaration, such as a member variable.

For example, in the following case the import java.awt.Component will not be flagged as unused:

import java.awt.Component;
class FooBar {
    private Object Component; // a bad practice in my opinion
    ...
}

Examples

To configure the check:

<module name="UnusedImports"/>