BookFinder.java contains several methods which can be implemented using java streams. Try to implement them so that the tests pass.
Here are some resources that might help:
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A good summary of non-terminal stream operations: https://www.leveluplunch.com/java/examples/stream-intermediate-operations-example/
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And terminal operations: https://www.leveluplunch.com/java/examples/stream-terminal-operations-example/
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Official documenation: https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html