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This article mainly explains "what internal classes and internal interfaces Java has". The content of the explanation is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn what internal classes and internal interfaces Java has.

one。 Inner class

An inner class defines a class in a class. We can think of a class in a class as a property of the class, which can be static or non-static. Internal classes can also be defined in the methods of the class, plus anonymous classes, there are a total of 5 internal classes.

1. Static inner class

The static inner class defines a class of static inside the class, as shown in the following code:

Public class StaticInnerClass {

Static class Inner {

Void print () {

Log.info ("Inner class is:" + this)

}

}

Public static void main (String [] args) {

StaticInnerClass.Inner inner = new StaticInnerClass.Inner ()

Inner.print ()

}

}

Note: in the static inner class, because the static variable can be accessed directly based on the class name, we use new StaticInnerClass.Inner () to instantiate the inner class.

two。 Non-static inner class

Non-static inner classes can also be non-static even classes defined in class, as shown in the following code:

Public class InnerClass {

Class Inner {

Void print () {

Log.info ("Inner class is:" + this)

}

}

Public static void main (String [] args) {

InnerClass.Inner inner = new InnerClass () .new Inner ()

Inner.print ()

}

}

Note: for a non-static inner class to access a class's variables, you need to instantiate the outer inner class, and then instantiate the inner class: new InnerClass (). New Inner (). (note: two new are required)

3. Static method inner class

The inner class of a static method is to define a class in a static method, which is actually equivalent to a variable in the method, which, of course, cannot be static. The following code is shown:

Public class StaticMethodInnerClass {

Private static String x = "static x"

Public static void print () {

Class MyInner {

Public void printOuter () {

Log.info ("x is" + x)

}

}

MyInner I = new MyInner ()

I.printOuter ()

}

Public static void main (String [] args) {

StaticMethodInnerClass.print ()

}

}

Note: classes in methods cannot be instantiated externally.

4. Inner class of non-static method

The same non-static method can also define an inner class, as shown in the following code:

Public class MethodInnerClass {

Private String x = "non static x"

Public void print () {

Class MyInner {

Public void printOuter () {

Log.info ("x is" + x)

}

}

MyInner I = new MyInner ()

I.printOuter ()

}

Public static void main (String [] args) {

New MethodInnerClass () .print ()

}

}

Note: here you need to instantiate the external class before you can continue to call.

5. Anonymous inner class

Anonymous classes are classes that are instantiated directly when needed. Anonymous classes are common, for example, when building SortedSet, you can pass in a custom Comparator, we can implement it with anonymous classes, or you can use lambda expressions directly. The following code is shown:

Public class AnonymousClass {

Public static void main (String [] args) {

SortedSet sortedSet1 = new ConcurrentSkipListSet (new Comparator () {

@ Override

Public int compare (Object o1, Object O2) {

Return 0

}

});

SortedSet sortedSet2 = new ConcurrentSkipListSet ((o1, O2)-> 0)

}

}

two。 Internal interface

Inner Interface refers to the interface defined in the interface. Our most common is the Entry in Map:

Public interface Map {

Interface Entry {

K getKey ()

}}

The inner interface here must be static, because the interface cannot be instantiated, so in order to access the interface in the interface, it must be defined as static. If not specified, the default is static.

Let's take a look at an implementation of this inner interface, as shown in the following code:

Public class MapImpl implements Map.Entry {

@ Override

Public Object getKey () {

Return 0

}

@ Override

Public Object getValue () {

Return null

}

@ Override

Public Object setValue (Object value) {

Return null

}

} Thank you for your reading. the above is the content of "what internal classes and internal interfaces does Java have?" after the study of this article, I believe you have a deeper understanding of the problem of what internal classes and internal interfaces Java has, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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