Some summaries of abstract classes in Java

1. Abstract classes, like normal classes, have Constructors. There are attributes in an abstract class that can be initialized with a constructor method. When a subclass object is instantiated, the construction of the abstract class is performed before the subclass Constructs.
2. Abstract classes cannot be declared with final . Abstract classes cannot use the final definition because abstract classes must have subclass inheritance .
3. an abstract class cannot be declared with static, but an inner class in an abstract class can use the static Declaration.

Abstract classa{Static Abstract classB { public Abstract voidPrint (); }}classXextendsa.b{ public voidprint () {System.out.println ( this. GetClass ()); }} public classhello{ public Static voidMain (string[] Args)throwsException {a.b AB=NewX ();    Ab.print (); }}

Output: class X

4. Abstract classes, like normal classes, can have static methods and can be passed through the class name . method name is called in the way

Abstract class a{    publicstaticvoid  test ()    {        System.out.println (" static method in the abstract class ");}    }  public class hello{    publicstaticvoid main (string[] Args)  throws  Exception    {        a.test ();    }}

5. Sometimes abstract classes require only a specific system subclass operation, which can hide the implementation of an abstract class Externally. Such a design is common in the System class library in order to hide the subclass implementations that we do not need.

Abstract classa{ public Abstract voidPrint (); Private Static classBextendsA { public voidprint () {System.out.println ("print message test"); }    }     public StaticA getinstance () {return NewB (); }} public classhello{ public Static voidMain (string[] Args)throwsException {a a=a.getinstance ();    A.print (); }}

6. Observe the output below.

Abstract classa{ publicA () { this. Print (); }     public Abstract voidPrint ();}classBextendsa{Private intnum = 100;  publicBintNum) {        Super();  this. num =num; }     public voidprint () {System.out.println ( this. num); }} public classhello{ public Static voidMain (string[] Args)throwsException {b b=NewB (30); }}

Above code output: 0

Some summaries of abstract classes in Java