It is sometimes useful to visualize methods as being surrounded by a box of "one-way glass." A method can see local variables and parameters that are inside the box. A method can look out through the glass that surrounds it. But no outsider can see into the box.

The picture shows the one-way glass for the example program. The red lines show the box of one-way glass surrounding each method. The method can see out of its box (for example each method can see the instance variable balance) but other methods can't see from the outside into the box of one-way glass.

In processDeposit() the statement can see the variable balance declared as a instance variable. It can also see the parameter amount that is inside its box.

In processCheck() statements can see the instance variable balance, can see the parameter amount, and can see the local variable charge.

The method showCharge() is defective because it contains a statement that attempts to look inside the box that surrounds processCheck().

The names of formal parameters (such as amount) and local variables (such as charge) are only visible from inside the glass box. However, outsiders can call the method and supply values for the formal parameters.