Answer 1

A Java expression consists of variables, operators, literals, and method calls. To know more about method calls, learn Java methods.

You can a null statement is when a statement is syntactically required but not semantically required. The most common case is to omit one of the parts of a for loop, like this example, although most people find that usage odd and hard to read. Like your book says, it's rarely used.

more info:

https://www.programiz.com/java-programming/expressions-statements-blocks