The toString method of Action directly uses the number field of its enclosing BankAccount object. A nested class can access all members of its enclosing classincluding private fields and methodswithout qualification because it is part of the enclosing class's implementation. An inner class can simply name the members of its enclosing object to use them. The names in the enclosing class are all said to be in scope. The enclosing class can also access the private members of the inner class, but only by an explicit reference to an inner class object such as lastAct. While an object of the inner class is always associated with an object of the enclosing class, the converse is not true. An object of the enclosing class need not have any inner class objects associated with it, or it could have many.
When deposit creates an Action object, a reference to the enclosing BankAccount object is automatically stored in the new Action object. Using this saved reference, the Action object can always refer to the enclosing BankAccount object's number field by the simple name number, as shown in toString. The name of the reference to the enclosing object is this preceded by the enclosing class namea form known as qualified-this. For example, toString could reference the number field of the enclosing BankAccount object explicitly:
return BankAccount.this.number + ": " + act + " " + amount;
The qualified-this reference reinforces the idea that the enclosing object and the inner object are tightly bound as part of the same implementation of the enclosing class. This is further reinforced by the qualified-super reference, which allows access to members of the enclosing instance's superclass that have been hidden, or overridden, by the enclosing class. For example, given a class T that extends S, within T we can invoke the superclass implementation of a method m, by using super.m() in an expression. Similarly, in an inner class of T, we can invoke the same implementation of m using T.super.m() in an expressiona qualified-super referenceand similarly for fields of S hidden by fields in T.
A nested class can have its own nested classes and interfaces. References to enclosing objects can be obtained for any level of nesting in the same way: the name of the class and this. If class X encloses class Y which encloses class Z, code in Z can explicitly access fields of X by using X.this.
The language does not prevent you from deeply nesting classes, but good taste should. A doubly nested class such as Z has three name scopes: itself, its immediate enclosing class Y, and outermost class X. Someone reading the code for Z must understand each class thoroughly to know in which context an identifier is bound and which enclosing object was bound to which nested object. We recommend nesting only one level under most circumstances. Nesting more than two levels invites a readability disaster and should probably never be attempted.
When deposit creates an Action object, a reference to the enclosing BankAccount object is automatically stored in the new Action object. Using this saved reference, the Action object can always refer to the enclosing BankAccount object's number field by the simple name number, as shown in toString. The name of the reference to the enclosing object is this preceded by the enclosing class namea form known as qualified-this. For example, toString could reference the number field of the enclosing BankAccount object explicitly:
return BankAccount.this.number + ": " + act + " " + amount;
The qualified-this reference reinforces the idea that the enclosing object and the inner object are tightly bound as part of the same implementation of the enclosing class. This is further reinforced by the qualified-super reference, which allows access to members of the enclosing instance's superclass that have been hidden, or overridden, by the enclosing class. For example, given a class T that extends S, within T we can invoke the superclass implementation of a method m, by using super.m() in an expression. Similarly, in an inner class of T, we can invoke the same implementation of m using T.super.m() in an expressiona qualified-super referenceand similarly for fields of S hidden by fields in T.
A nested class can have its own nested classes and interfaces. References to enclosing objects can be obtained for any level of nesting in the same way: the name of the class and this. If class X encloses class Y which encloses class Z, code in Z can explicitly access fields of X by using X.this.
The language does not prevent you from deeply nesting classes, but good taste should. A doubly nested class such as Z has three name scopes: itself, its immediate enclosing class Y, and outermost class X. Someone reading the code for Z must understand each class thoroughly to know in which context an identifier is bound and which enclosing object was bound to which nested object. We recommend nesting only one level under most circumstances. Nesting more than two levels invites a readability disaster and should probably never be attempted.
