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The inner class

Basic overview

• When the definition of a class is placed in the entity of another class, that class is called an inner class, and the class in which that class resides is called an outer class
• Things that can occur in a class body: member variables, member methods, constructors, constructor blocks, static statement blocks, static variables, methods, inner classes
• Nested classes
  • Inner classes (member inner classes, local inner classes, anonymous inner classes)
  • Statically nested classes

Syntax format

Class external class class name {class inner class name {inner class body; }}Copy the code

Member inner class

• Member inner classes define inner classes in another class or interface
• Matters needing attention
  • An external class object must be created before a member inner class object can be created
  • Cannot contain static variables, static code blocks, or static methods (except static constants)
  • An external class can call an inner class private member from an object of its inner class
  • A member inner class is a separate class compiled into a separate.class file
• role
  • Member inner classes can access both external class information and parent class information, completing a multi-inheritance solution
• case
  • Outer.java

  // The use of member inner classes
  public class Outer { / / outside class
  	
  	private String str = "The str1 Outer class";
  	private String str2 = "The str2 Outer class";
  
  	/* * First: the name of the inner class cannot be the same as the name of the outer class * second: the final, access modifier can be used to modify */
  	public class Inner{ / / inner classes
  
  		private String str = "The str1 Inner class";
  		private String str2 = "The str2 Inner class";
  		
  		public void show(a) {
  			// Members of inner classes take precedence over members of outer classes
  			System.out.println(str);
  			System.out.println(str2);
  			
  			// Use "external class.this. member" to access members of an external class
  			System.out.println(Outer.this.str2);
  			System.out.println(Outer.this.str); }}}Copy the code

  • TestOuter.java

  import Inner.demo1.Outer.Inner;
  
  public class TestOuter {
  
  	public static void main(String[] args) {
  		// If you want to create an inner class, you need to create an outer class object first
  		
  		// Create an external class object
  		Outer o = new Outer();
  	
  		// Create inner class
  		Inner inner = o.new Inner(a); inner.show(); }}Copy the code

  Str1 Inner str2 Outer str2 Outer str1 Inner str2 Outer str1 Inner str2 OuterCopy the code

Local inner class

• A local inner class is an inner class defined in a method H or code block

• Matters needing attention:

  • Cannot contain static variables, static code blocks, or static methods
  • Can only be used in the method or code block that defines the class and must be defined before use
  • J local variables must be valid constants when accessing the local variables of all its methods
  • Is a separate class compiled into a separate.class file
  • Only abstract and final can be used
  • When defining a static block or method, only the static members of the external class can be accessed

• case

  • Outer.java

  // Local inner class
  public class Outer { / / outside class
  
  	private String str1 = "Str1 of class Outer";
  	private String str2 = "Str2 of class Outer";
  	
  	// Define a method of an external class
  	public void print(a) {
  		System.out.println("Print method in class Outer");
  	}
  	
  	static {
  		class Inner{}  // Local inner class
  	}
  	
  	public void method(a) {
  		
  		// Must be defined before use
  		class Inner{
  			private String str1 = "Str1 in Inner class";
  			
  			Inner class members take precedence over outer class members
  			public void visitOuter(a) {
  				System.out.println(str1);
  				System.out.println(str2);
  				print();  // Skip directly to the outer class method}}// A local inner class can only be used in the method or code block that defines it
  		// Only abstract/final can be used. It cannot have the same name as an external class
  		Inner in = newInner(); in.visitOuter(); }}Copy the code

  • TestOuter.java

  public class TestOuter {
  
  	public static void main(String[] args) {
  		
  		Outer o = newOuter(); o.method(); }}Copy the code

  Inner str1 Outer str2 Outer print methodCopy the code

Anonymous inner class

• An anonymous inner class is an inner class created without a name when instantiated directly using an interface or parent class

• Syntax format

  • Interface/superclass type reference name = new interface/superclass type (){override method;

  }

• Matters needing attention

  • You must and can only inherit a parent class or implement an interface
  • No class keyword, no class name
  • Are special local inner classes
  • Can only be used once
  • A constructor cannot be defined
  • Anonymous classes cannot be abstract classes

• Advantages and Functions

  • Anonymous inner classes can make code more compact and concise
  • Encapsulation is better
  • Anonymous inner classes can be created flexibly
  • Anonymous inner classes make a multi-inheritance solution complete

• case

  • InterDemo.java

  public interface InterDemo {
  
  	public abstract void print(a);
  }
  Copy the code

  • Outer.java

  public class Outer { / / outside class
  
  	private String str1 = "The Outer class str1";
  	private String str2 = "The Outer class str2";
  	
  	// Anonymous inner class
  	InterDemo id = new InterDemo() {
  		
  		private String str1 = "Anonymous inner class str1";
  		
  		@Override
  		public void print(a) {
  		
  			System.out.println(str1);
  			System.out.println(str2);
  			
  			// If you want to call an external class variable, the external class.this. variable
  			System.out.println(Outer.this.str1); }};public void show(a) { id.print(); }}Copy the code

  • TestOuter.java

  public class TestOuter {
  
  	public static void main(String[] args) {
  		
  		Outer o = newOuter(); o.show(); }}Copy the code

  Anonymous inner class str1, str2, str1 OuterCopy the code

Statically nested classes

• Static nested classes define nested classes in another class, on an interface, and decorated with the static keyword

• Matters needing attention

  • There is no need to generate an external class object to generate a statically nested class object
  • Only static members of an external class can be accessed directly
  • An external class can call a member of an inner class through an object of a statically nested class
  • Static member variables or static member methods can be defined

• case

  • Outer.java

  public class Outer { / / outside class
  
  	// Create two member variables, one static and one non-static
  	private String str1 = "Str1 of external class";
  	private static String str2 = "Str2 for external classes";
  	
  	// Member method, static
   	public static void show1(a) {
  		System.out.println("Show methods for external classes");
  	}
  
  	// statically nested classes
  	// Can only be defined in a member of a class, not in a method or code block
  	public static class Inner{
  		
  		private String str1 = "Str1 for nested classes";
  		// private static String str2 = "private static String str2";
  		
  		public void show(a) {
  			System.out.println(str2);
  			System.out.println(str1);
  			
  			// Non-static members of an external class cannot be accessed directly
  			// But you can access the static members of the external class directlyshow1(); }}}Copy the code

  • TestOuter.java

  import Inner.demo4.Outer.Inner;
  
  public class TestOuter {
  	public static void main(String[] args) {
  		
  		// How do I create inner classes or statically nested class objects
  		// First create an object for the external class
  		Inner Inner = newOuter.Inner(); Inner.show(); }}Copy the code

  Show method of str1 outer class of str2 nested classCopy the code

The difference between statically nested and non-statically nested classes

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