1. abstract class

Definition

It doesn’t create objects directly , You create classes of objects only by inheriting subclasses ( A template ) Call it abstract class . Its definition and rules are as follows :

• There doesn’t have to be an abstract method in an abstract class , But the class containing abstract method must be abstract class or interface
• Abstract classes need to use abstract Keyword declaration
• Methods in abstract classes only declare , Don’t realize
• Abstract classes must have subclasses , And subclasses must override all abstract methods .
• Abstract classes can contain Field, Method , Constructors , Initialization block , Inner class , There are six kinds of components

Be careful

abstract Cannot coexist with the following keywords

• static Class name . Abstract method name (); It doesn’t make any sense
• final After retouching , Abstract methods cannot be overridden
• private The purpose of abstract methods is to make subclasses re implement , After retouching , Subclass not visible .

public abstract class Test {

private String name;
private int age;
public Test() {

}
public Test(String name, int age) {

this.name = name;
this.age = age;
}
public String getName() {

return name;
}
public void setName(String name) {

this.name = name;
}
public int getAge() {

return age;
}
public void setAge(int age) {

this.age = age;
}
public void printName(String name) {

System.out.println("name: " + name);
}
// Abstract methods in abstract classes 
public abstract void printAge();
}

explain : There are not too many differences between abstract classes and ordinary classes , Just define one or more abstract methods when defining a class , Except that you can’t instantiate objects directly , The rest is no different from the general class .

2. Interface (interface)

An interface can be thought of as a special class , It consists of global constants and public Defined abstract methods are composed of ( The default abstract method is public jurisdiction ), Interface to use interface Keyword declaration , It produces a completely abstract class ( More abstract than abstract classes ), No specific implementation is provided .

The interface definition can be as follows :

public interface InterfaceDemo {

public final static String name = "FEEL";
public void eat();
public void sleep();
}

And for the use of interfaces , We use implements Keyword to implement the interface , This is different from inheritance .

Specific reference is as follows :

public class Test implements InterfaceDemo{

public static void main(String[] args) {

System.out.println(name);
Test test=new Test();
test.eat();
test.sleep();
}
@Override
public void eat() {

System.out.println("FEEL Eating snacks ");
}
@Override
public void sleep() {

System.out.println("FEEL At rest ");
}
}

The interface has the following features

• Methods in interfaces can have parameter lists and return types , But there can be no methodology .
• The interface can contain fields , But is implicitly declared as static and final.
• The fields in the interface are only stored in the Interface static storage area , It doesn’t belong to the interface .
• Methods in an interface can be declared as public Do not declare , Default public Handle
• When implementing an interface , You need to declare the defined method as public type
• If you want to implement all the methods in the interface , The current creation is still an interface
• Extend an interface to generate a new interface to use extends keyword , Implement interface usage implements keyword
• The method in the interface is abstract (abstract) Of , It can’t be static (static) Of , The methods in the interface are all abstract , And abstract methods don’t static, Because there is static The abstract method of cannot be overridden , That’s the only way , Interfaces make sense .

review : A class can inherit an abstract class , And implement multiple interfaces . About abstract classes and interfaces

3. The difference between abstract classes and interfaces

• An interface is a variant of an abstract class , All methods in the interface are abstract , The abstract class is just a kind of method that can be declared and not implemented .
• Interfaces can inherit more , You can’t abstract classes
• The method defined in the interface is not implementable , And the part of the abstract class can implement
• The basic data type in the interface must be static Modified , And abstract classes are not
• Interfaces cannot contain static code blocks and static methods , And abstract classes can contain

in addition , Abstract classes are more functional than interfaces , But defining abstract classes is expensive . Although the interface is a little poor in function , But it’s just a description of an action , And you can implement multiple interfaces in a class , therefore , It helps to reduce the complexity of design .