Accessors, Mutators, and the static Keyword

The only doors into a private object

An accessor (getter) returns a copy of some state and changes nothing: public int getCopies() { return copies; }. A mutator (setter) changes state and normally returns void. The reason a mutator beats a public field is that a mutator can refuse: it validates before it writes, so the object can never enter an impossible state.

A local variable declared inside a method exists only until the method returns, and it is not given a default value — using it before assignment is a compile-time error. An instance variable, by contrast, is initialised to the zero value of its type and lives as long as the object does.

static: one copy for the whole class

A static variable belongs to the class, not to any object: every object shares the same single copy, so a counter incremented in a constructor records how many objects have ever been made. A static method is called on the class — Widget.getMade() — and therefore has no this. That single fact explains the whole rule: a static method cannot read an instance variable or call an instance method directly, because there is no particular object to read from.

The reverse direction is always legal: an instance method may freely use static variables and static methods, since the one shared copy exists no matter which object is asking.