The difference between inheriting super and combining Java inheritance and C++

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Preface

Java inheritance is one of the three main features. Let me take a note here. Because there is a little difference from C++ inheritance, I sorted it out for the convenience of future learning. I hope you can point out the wrong places.

1. The basic concept of inheritance (is-a)

The purpose is for code reuse or class reuse. This is different from C++. The parent class can also be called a super class. The keyword is =>extends (the meaning of extension)

2. Code sample

Note: Unlike C++, inheritance in Java can only be single inheritance, that is, a subclass can only have one parent class, and a single parent class can have multiple subclasses.
Parent people

public class people {
    
    
    public int age;
    public String name;
}

Subclass student, there is an additional school

public class student extends people{
    
    
    public String school;
}

Main method

public class Main {
    
    
    public static void main(String[] args) {
    
    
        student s1=new student();

    }
}

As you can see, creating an instance of a subclass can also access the parent class, because my attributes are all public. If it is a private parent class, you can't see it.

1.Super and construction method

Every class has a constructor. If you do not explicitly create a constructor, the compiler will automatically generate a constructor with no parameters for this class.

When there is a construction method in the parent class, and the construction method has parameters, the compiler will no longer automatically generate a no-argument construction method. At this time, if you create a subclass instance, you need to explicitly call the construction of the parent class. Method ( super ), and pass the parameters, otherwise the parent class instance cannot be created, and a compilation error will occur.

Note: When creating a subclass instance, the parent class instance will be constructed first

public class student extends people{
    
    
    public String school;

    public student(int age, String name, String school) {
    
    
        super(age, name);
        this.school = school;
    }
}

2. Special knowledge points

When the subclass and the parent class have the same attributes, use the super keyword to get the attributes of the parent class;

public class student extends people{
    
    
    public String school;
    public int age=10;
    public void func(){
    
    
        System.out.println(age);
        System.out.println(super.age);
    }
 }

If the inheritance is too complicated, the maintenance cost will be very high. In order to prevent a class from being inherited, you can use the final keyword;

final class people {
    
    
    public int age=100;
    public String name;
 }

At this time, the student class cannot be inherited

3. Combination (has-a)

Composition is also for code reuse, which is also an important attribute of object-oriented. It can be understood that there are many other classes in a class

public class Homehvae {
    
    
    public Cat cat=new Cat();
    public Dog dog=new Dog();
    public student stu=new student();
}

There are cats, dogs and a student at home.