What is java abstract class

In the Java language, when the abstract keyword is used to modify a class, the class is called an abstract class. An abstract class is a collection of public attributes of all its subclasses and a class that contains one or more abstract methods.

Abstract class can be seen as a further abstraction of the class. In the object-oriented field, abstract classes are mainly used for type hiding.

Abstract method (Recommended learning: java course)

Abstract classes can contain general methods and abstract methods . The definition of an abstract method is different from that of a general method. An abstract method is directly followed by a semicolon after the method header, while a general method contains a method body enclosed in curly braces. All abstract methods must exist in abstract classes.

Methods defined in abstract classes are called abstract methods. These methods only have the declaration of the method header, and a semicolon is used to replace the definition of the method body, that is, only the interface form of the member method is defined without the concrete operate.

Only the redefinition of abstract member methods by a derived class can truly implement operations related to the derived class.

After each subclass inherits the abstract method of the parent class, it redefines it with different statements and method bodies, forming several subclasses with the same name, the same return value, the same parameter list, and the same purpose but There are certain differences in the specific implementation methods.

The purpose of defining abstract methods in abstract classes is to implement an interface, that is, all subclasses present a method with the same name to the outside world.

Use abstract class to define abstract classes in Java language. The following example:

/* 文件名 : Employee.java */public abstract class Employee{
   private String name;   private String address;   private int number;   public Employee(String name, String address, int number)
   {
      System.out.println("Constructing an Employee");      this.name = name;      this.address = address;      this.number = number;   }
   public double computePay()
   {
     System.out.println("Inside Employee computePay");     return 0.0;   }
   public void mailCheck()
   {
      System.out.println("Mailing a check to " + this.name
       + " " + this.address);   }
   public String toString()
   {
      return name + " " + address + " " + number;   }
   public String getName()
   {
      return name;   }
   public String getAddress()
   {
      return address;   }
   public void setAddress(String newAddress)
   {
      address = newAddress;   }
   public int getNumber()
   {
     return number;   }}

Notice that the Employee class is no different. Although the class is an abstract class, it still has 3 member variables, 7 member methods and 1 constructor.

But it needs to inherit and override abstract methods and cannot be instantiated directly.

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