Why can Method4 be called being not static?

Just like non-static fields can be used to set the values of other non-static fields (see my explanation on your other post) non-static methods can call other non static methods. Some explanations: Fields are executed when they are created (static fields are created when a class is loaded into memory, non-static fields are created when an object is created and are unique to only that object). Methods are executed when they are called. Static methods can be called without any objects, and non-static methods require an object to be created and used to call them. Static methods exist before any object is created. This means that they can access any other static method or field, however the non-static methods and fields do not exist and therefore can not simply be accessed. Because they are executed when called, you can create an object inside a static method and then use that object to access that object's fields and methods. In this code you can see that the static method2() creates a new Solution object which is then used to call the non static method3(). This is legal because an object was created and that object contained the method3() that was accessed.