Interface and abstract class / method learning demo

Implement abstract methods

abstract class ： Abstract classes simply cannot be used new Method to instantiate the class , That is, classes without concrete instance objects , Abstract classes are somewhat similar to “ Templates ” The role of , The purpose is to create and modify new classes according to their format , Objects cannot be created directly by abstract classes , New subclasses can only be derived from abstract classes , Then create objects from their subclasses , When a class is declared as an abstract class , You need to add a modifier in front of this class abstract, Member methods in abstract classes can include general methods and abstract methods

Abstract method ： Abstract method is based on abstract The method of decoration , This method only declares the returned data type , Method name and required parameters , There is no method body , That is to say, the abstract method only needs to be declared and does not need to be declared in advance , When a method is abstract , This means that the method must be overridden by the method of the subclass , Otherwise, the method of its subclass is still abstract Of , And this subclass must also be abstract , That is to say abstract.

abstract class
understand ：
1, When a class has no specific information to describe an object , Is defined as an abstract class .
characteristic ：
1, Abstract class cannot be instantiated .
2, There are constructors （ Classes have constructors ）
3, There can be no abstract methods in an abstract class , The class of the abstract method must be an abstract class

application ： Model all the implementations that the parent class cannot determine , Instead, the subclass provides the class of the object to be implemented

Abstract method
1, The class of the abstract method must be an abstract class
2, Format ： There is no method body {}
3, If a subclass inherits an abstract class , And rewrite all the abstract methods , Then this class is an entity class .
4, If you don't rewrite all abstract methods , Then this class must be an abstract class
5, Different objects have different processing methods , Then it makes no sense to define concrete in the parent class

Limit ：
1,abstract Can't be used to modify properties , Constructors ,private,final,static
2, Constructors cannot be rewritten
3, Subclasses cannot override private The method of decoration
4,final Decorated cannot be rewritten
5,static Decorated direct call , If you use abstract There is no point in modifying it as abstract

Constructors , It's a special method . It is mainly used to initialize objects when creating objects , That is, to assign initial values to object member variables , Total and new Operators are used together in statements that create objects . A particular class can have multiple constructors , They can be distinguished according to the number of parameters or the type of parameters That is, overloading the constructor .

Interface （ english ：Interface）, stay JAVA Programming language is an abstract type , Is a collection of abstract methods , The interface usually uses interface To declare . A class inherits an interface by way of , So as to inherit the abstract methods of the interface .

Interfaces are not classes , Interfaces are written in a similar way to classes , But they belong to different concepts . Class describes the properties and methods of an object . The interface contains the methods that the class will implement .

Unless the class that implements the interface is an abstract class , Otherwise, the class will define all methods in the interface .

Interface cannot be instantiated , But it can be realized . A class that implements an interface , All methods described in the interface must be implemented , Otherwise, it must be declared as an abstract class . in addition , stay Java in , The interface type can be used to declare a variable , They can be a null pointer , Or be bound to an object implemented by this interface .

Please check the details rookie

describe
Known abstract classes Base It defines calculate Method , The calculation process of this method depends on sum() and avg(), The latter two methods are abstract methods . It is required to define Base Subclasses of Sub class , And implement the abstract method of the parent class , bring main The operation logic in the function is executed correctly .
Input description ：
Two integers
Output description ：
The sum of two integers divided by the average of the two integers （ The average value is int type , The decimal problem is not considered ）

public static void main(String[] args) {
    
        // Sub You need to define subclasses 
        Base base = new Sub();

        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNextInt()) {
    
            int x = scanner.nextInt();
            int y = scanner.nextInt();
            base.setX(x);
            base.setY(y);
            System.out.println(base.calculate());
        }
    }

}

abstract class Base {
    

    private int x;
    private int y;

    public int getX() {
    
        return x;
    }

    public void setX(int x) {
    
        this.x = x;
    }

    public int getY() {
    
        return y;
    }

    public void setY(int y) {
    
        this.y = y;
    }

    public int calculate() {
    
        if (avg() == 0) {
    
            return 0;
        } else {
    
            return sum() / avg();
        }
    }

    /** *  return x and y And  */
    public abstract int sum();

    /** *  return x and y Average value  */
    public abstract int avg();

}

class Sub extends Base {
    

    //write your code here......
    

}

Analysis of the answer ：

Because the member variable of the parent class is private Type of , Therefore, subclasses can only be accessed through the functions of the parent class . Parent class calculate Method is called sum and avg Method , But these two methods are abstract methods , There is no definition in the parent class , We need to complete it in subclasses .

So we override in subclasses sum and avg Method , Calls a method of the parent class getX and getY Calculate the sum and mean value after obtaining the variable value .

 public static void main(String[] args) {
    
        // Sub You need to define subclasses 
        Base base = new Sub();

        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNextInt()) {
    
            int x = scanner.nextInt();
            int y = scanner.nextInt();
            base.setX(x);
            base.setY(y);
            System.out.println(base.calculate());
        }
    }

}

abstract class Base {
    

    private int x;
    private int y;

    public int getX() {
    
        return x;
    }

    public void setX(int x) {
    
        this.x = x;
    }

    public int getY() {
    
        return y;
    }

    public void setY(int y) {
    
        this.y = y;
    }

    public int calculate() {
    
        if (avg() == 0) {
    
            return 0;
        } else {
    
            return sum() / avg();
        }
    }

    /** *  return x and y And  */
    public abstract int sum();

    /** *  return x and y Average value  */
    public abstract int avg();

}

class Sub extends Base {
    
    @Override
    public int sum() {
    
        System.out.println(" and "+ (getX()+getY()));
        return getX()+getY();

    }

    @Override
    public int avg() {
    
        System.out.println(" Average "+ (getX()+getY())/2);
   // return (getX()+getY())/2;
        return sum()/2;
    }

 

The result is

5
6
 Average 5
 and 11
 and 11
 Average 5
 and 11

2

Implementation interface

describe
Known interface Comparator, The interior defines max function , Used to return the maximum of two integers . Please define the implementation class of this interface , bring main The comparison logic in the method can be executed correctly , The name of the implementation class is required to be ComparatorImpl.
Input description ：
Two integers
Output description ：
The maximum of two integers

public static void main(String[] args) {
    
        Comparator comparator = new ComparatorImpl();

        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNextInt()) {
    
            int x = scanner.nextInt();
            int y = scanner.nextInt();
            System.out.println(comparator.max(x, y));
        }
    }

}

interface Comparator {
    
    /** *  Returns the maximum of two integers  */
    int max(int x, int y);
}

Analysis of the answer ：

ComparatorImpl Implementation class override max Method

  public static void main(String[] args) {
    
        Comparator comparator = new ComparatorImpl();

        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNextInt()) {
    
            int x = scanner.nextInt();
            int y = scanner.nextInt();
            System.out.println(comparator.max(x, y));
        }
    }

}

interface Comparator {
    
    /** *  Returns the maximum of two integers  */
    int max(int x, int y);
}



class ComparatorImpl implements Comparator{
    


    @Override
    public int max(int x, int y) {
    
        // Three mesh operation 
        return x>y?x:y;
    }

The result of the operation is

5
6
6