As with class methods and variables, a static nested class is associated with its outer class. And like static class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class: it can use them only through an object reference. Note that a static nested class interacts with the instance members of its outer class just like any other top-level class. The static nested class StaticNestedClass can't directly access outerField because it's an instance variable of the enclosing class, OuterClass.
The Java compiler generates an error at the highlighted statement:. Similarly, the top-level class TopLevelClass can't directly access outerField either. If a declaration of a type such as a member variable or a parameter name in a particular scope such as an inner class or a method definition has the same name as another declaration in the enclosing scope, then the declaration shadows the declaration of the enclosing scope.
You cannot refer to a shadowed declaration by its name alone. The following example, ShadowTest , demonstrates this:. This example defines three variables named x : the member variable of the class ShadowTest , the member variable of the inner class FirstLevel , and the parameter in the method methodInFirstLevel.
The variable x defined as a parameter of the method methodInFirstLevel shadows the variable of the inner class FirstLevel. Consequently, when you use the variable x in the method methodInFirstLevel , it refers to the method parameter.
To refer to the member variable of the inner class FirstLevel , use the keyword this to represent the enclosing scope:. Refer to member variables that enclose larger scopes by the class name to which they belong. For example, the following statement accesses the member variable of the class ShadowTest from the method methodInFirstLevel :. Serialization of inner classes, including local and anonymous classes, is strongly discouraged.
When the Java compiler compiles certain constructs, such as inner classes, it creates synthetic constructs ; these are classes, methods, fields, and other constructs that do not have a corresponding construct in the source code. They enable you to logically group classes that are only used in one place, thus this increases the use of encapsulation , and creates more readable and maintainable code.
The scope of a nested class is bounded by the scope of its enclosing class. Thus in above example, class NestedClass does not exist independently of class OuterClass.
A nested class has access to the members, including private members, of the class in which it is nested. However, the reverse is not true i. A nested class is also a member of its enclosing class. As a member of its enclosing class, a nested class can be declared private , public , protected , or package private default. Nested classes are divided into two categories: static nested class : Nested classes that are declared static are called static nested classes.
Syntax: Attention reader! Get hold of all the important Java Foundation and Collections concepts with the Fundamentals of Java and Java Collections Course at a student-friendly price and become industry ready. In this case, creating the event handler for the button as an inner class would be best practice as the inner class would not be utilized anywhere else other than with the specific button within the GUI class.
One purpose of inner classes is to attach listeners. For example, suppose you have a JMenuItem. You can make it quit your app as shown in this code:. You may also want a class to have access to outer class state variables which is entirely subservient to that class.
For example, consider writing a simple color calculator. It might have a text area into which you type a hex code. When you hit enter, you want a JPanel to display the color. Here is a crude outline of what you might do. If you find that there is enough code which could be better done by class as class provides us to specify stats and behavior with fields and methods and you don't want this class needs to be used outside of enclosing class.
Here the inner class is hidden from the outside world. Inner class can access the private member of enclosing class which provides us encapsulation. Let me give example.. Suppose you want to set the gear to cycle and you have a business rule like there are only up to 6 gears.
So you can create Inner Class Cycle which would have a method to set the gear. That method has some validation which are checked before setting gear. This is a style question. Anything that can be done with an inner class can also be done as a as series of external classes.
Inner classes are especially useful for classes that are lightweight or tightly bound to the enclosing class. For example, a comparator is frequently both these things. It needs intimate knowledge of the implementation of the class, and may only be a few lines long.
It may be an ideal candidate as an internal class. The inner class used for grouping classes logic, for example, if you have class B and this class used only at class A, So it is better to put class B as an inner class at class A, as this will give readability and reusability for your code.
Lets say you have a type and its a class, called OuterClass, in a package called "com. Most applications of nested classes see it being applied on basis of design decisions. What that means is, every case of a nested class can be replaced with other designs. But having said that, it is also true that we can also replace the inheritance pattern with composition pattern and it is gaining momentum lately although an inheritance pattern is definitely better when the dependencies between the classes is so much so that composing the dependencies entirely would be ugly.
These kind of design decision cases are numerous and all of the answers above list several such cases. So it would not be wrong to think that this feature was introduced more as a new pattern than as a feature or functionality. Conceptually inner classes can be used to represent types in the universe that would not exist without that parent type.
In other words, with a language that allows inner classes, the types are all 'type definers'. A type can then be considered something that explicitly or implicitly defines new types. For example, imagine we have a universe where "Food" can be applied to anything.
Even itself. Food is a fundamental concept in our universe. We introduce a subclass of Food called Meat. Without that concept, there is no such thing as "Meat Eater". So we can note 'can' define a nested type "Meat. Eater" which could implement an IEater interface and define animals as being a containment structure of lists of different IEaters.
This same philosophy applies neatly to more abstract and technically useful arrangements such as Mementos in the Memento Design Pattern , a configuration object defined as a nested class, and other type-specific behaviours or structures. I would just consider that this is just a feature of language. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
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