Programmers should have access to Math.sqrt (), Math.random () and other methods, but they should never be able to write "Math myObject = new Math ();" But why make a public method getInstance ()?

The Java compiler must create synthetic methods on nested classes when their attributes specified with the private modifier are accessed by the enclosing class. The next code sample indicates this ...

See the Java 101 Java interfaces tutorial for a more in-depth introduction to using interfaces in your Java programs, including where and where not to use default, static, and private methods.