[Easy-to-understand explanation! ] How to use Java inheritance [Override explanation]

1. Prior knowledge

-[Latest] How to build Java environment on Ubuntu

-[Even beginners can do it! ] How to create a Java environment on Windows 10 (JDK14.0.1)

-[Easy-to-understand explanation! ] How to use Java instance

-[Even beginners can do it! ] How to install Eclipse on Windows 10 (Java environment construction)

As prior knowledge, the contents of the above link are required.

2. What is inheritance?

--Inheritance is to define a class with extensions and changes based on the class already defined. --The class that is the source of inheritance is called " superclass ". --The class newly defined by inheritance is called " subclass ". --Define subclass with" subclass name extends superclass name". --In subclass, describe only the difference from the superclass. --Java allows only superclass to be defined for one class due to language specifications. --You can create a subclass by furtherinheriting thesubclass. --All classes are [java.lang.Object class](https://docs.oracle.com/javase/jp/7/api/java/lang) defined in the Java Standard Class Library (Java API). /Object.html) is inherited. --Classes with the final modifier cannot inherit. --Subclasses cannot directly access the private variableof the superclass. --In subclass instantiation, thesuperclass constructor is processed and then the subclass constructoris processed. --You can arbitrarily call thesuperclass constructor by writing super () at the beginning of the subclass constructor. --You can redefine a method defined in a superclass in a subclass by using override`.

3. What is an override?

--Override means to redefine a method defined in a superclass in a subclass. --Override is used when you want to perform different processing with the same method name / argument as the superclass method. --A basic description example of override is as follows.

Super class

class superclass name{

    //Superclass method
Method name(){
        //processing
    }
}

Subclass

class subclass name extends superclass name{

    //override
    @Override
Method name(){
        //processing
    }
}

4. Basic writing of inheritance

Test class

package package name;
public class main class name{
    public static void main(String[] args) {
        //Instance generation
Subclass name variable name=new subclass name();
        //Put a value in setter
Variable name.set instance variable name(Actual argument);
        //Get the value entered by getter
        System.out.println(Variable name.getインスタンスVariable name());
    }
}

Super class

package package name;
class superclass name{
    //Definition of instance variables
private type name variable name;

    //Constructor (executed when instantiating)
Super class name(Type name argument){
Initialization process, etc.
    }
    // setter
void set instance variable name(Type name Argument name){
        this.Variable name=Argument name;
    }
    // getter
Type name get instance variable name(){
return variable name;
    }
}

Subclass

package package name;
class subclass name extends superclass name{
    //Definition of instance variables
private type name variable name;

    //Constructor (executed when instantiating)
Subclass name(Type name argument){
        super(argument);
Initialization process, etc.
    }
    // setter
void set instance variable name(Type name Argument name){
        this.Variable name=Argument name;
    }
    // getter
Type name get instance variable name(){
return variable name;
    }
}

--Basic encapsulation is described as above.

5. Preparation

1. Start Eclipse and select [File (F)]-> [New (N)]-> [Java Project].
2. Enter Test1 for the project name and click the Done button.
3. Select [File (F)] → [New (N)] → [Class].
4. Enter Test1 for the package and name and click the Done button.
5. Confirm that Test1.java has been created. Enter Test1 in the package, enter TestSuper in the name, and click the Finish button in the same way as in 6.3.
6. Confirm that TestSuper.java has been created. Enter Test1 in the package and TestSub in the name in the same procedure as in 8.3, and click the Finish button.
7. Success if Test1.java, TestSuper.java, and TestSub.java are created.

6. Description example

Test1.java

package Test1;
public class Test1 {
	public static void main(String[] args) {
        //Instance generation
		TestSuper ts1 = new TestSuper("A");
		TestSub ts2 = new TestSub("B","Gyoza");

		//View instance
		ts1.showName();
		System.out.println();
		ts2.showName();
    }
}

TestSuper.java

package Test1;
public class TestSuper {
	//Definition of instance variables
    private String name;

    //Constructor (executed when instantiating)
    public TestSuper(String name) {
		this.name = name;
	}
    // setter
	public void setName(String name) {
		this.name = name;
	}
	// getter
	public String getName() {
		return name;
	}
	public void showName() {
		System.out.println("name:"+name);
	}
}

TestSub.java

package Test1;
public class TestSub extends TestSuper {
	//Definition of instance variables
    private String food;

 //Constructor (executed when instantiating)
	public TestSub(String name,String food) {
		super(name);
		this.food = food;
	}
	// setter
	public void setFood(String food) {
		this.food = food;
	}
    // getter
	public String getFood() {
		return food;
	}
	@Override
	public void showName() {
		super.showName();
		showFood();
	}
	public void showFood() {
		System.out.println("Favorite food:"+food);
	}
}

--Copy the above sentence, specify S-JIS as the character code, save the file name as Test1.java, TestSuper.java, TestSuper.java, and execute it. .. ↓ ↓

7. Related

-[Useful to remember !!!] Easy creation of constructor and getter / setter in Eclipse -[Even beginners can do it! ] How to write Javadoc -[Easy-to-understand explanation! ] How to use Java encapsulation -[Easy-to-understand explanation! ] How to use Java overload