Certain kinds of expression are counted as statements. You write any expression, put a semicolon after it, and voila, it's an expression statement. In particular, an assignment, method invocation, the creation of a object by calling a constructor (looks like a method call, and the keyword “new” is used so we can tell them apart), and pre-increment, post-increment, and decrement are all expressions that can be statements.
a = b; // assignment
w.setSize(200,100); // method invocation
new WarningWindow(f); // instance creation
++i; // pre-increment
An expression statement is executed by evaluating the expression.
If the expression is an instance creation, you usually save a reference to it so that you can access fields and invoke methods on that object. For example, you have:
foo = new WarningWindow(f); // instance creation
not:
new WarningWindow(f); // instance, but no ref saved
However, there are certain classes we'll see later that you can instantiate for which you don't necessarily need to save a reference. The two main examples are Threads and inner (nested) classes. They are fine doing their work without further input from you. So, while you usually keep a reference to a newly instantiated class, you might occasionally see no reference kept.