[Easy-to-understand explanation! ] Reference type type conversion in Java

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 reference type conversion?

--There are upcast and downcast in reference type conversion. --Upcast treats an instance of a subclass as a superclass type object. --Downcast treats an instance of a superclass as a subclass type object. --Downcast is performed using the cast operator" () ".

3. Basic writing

Upcast

public class main class name{
    public static void main(String[] args) {
        //Instance generation
Subclass name Variable name 1=new subclass name();
        //Upcast
Superclass name variable name 2=Variable name 1;
    }
}

Downcast

public class main class name{
    public static void main(String[] args) {
        //Instance generation
Superclass name variable name 1=new subclass name();
        //Upcast
Subclass name Variable name 2= (Subclass name)Variable name 1;
    }
}

--Basic reference type conversion is described as above.

4. 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 and Hello in the name in the same procedure as in 6.3, and click the Finish button. Enter Test1 in the package, GoodMorning in the name, and Hello in the superclass in the same way as in 8.3, and click the Done button. Enter Test1 in the package, GoodEvening in the name, and GoodMorning in the superclass in the same way as in 9.3, and click the Finish button.
6. Success if Test1.java, Hello.java, GoodMorning.java, GoodEvening.java are created.

5. Description example

--Reference type conversion is used to method call and method definition range that a superclass or subclass has.

Test1.java

package Test1;
public class Test1 {
    public static void main(String[] args) {
        //Instance generation
        GoodEvening ge1 = new GoodEvening("A");
        ge1.showGoodMorning();

        //Upcast
        Hello hello = ge1;
        hello.showHello();

        //Downcast
        GoodEvening ge2 = (GoodEvening) hello;
        ge2.showGoodEvening();
        //((GoodEvening) hello).showGoodEvening();
    }
}

Hello.java

package Test1;
public class Hello{
	//Variable definition
	String name;

	//constructor
	public Hello(String name) {
		this.name = name;
	}

	//Display greetings
	void showHello() {
		System.out.println(name + "Hey,.");
	}
}

GoodMorning.java

package Test1;
public class GoodMorning extends Hello {
	//constructor
	public GoodMorning(String name) {
		super(name);
	}

	//Display greetings
	void showGoodMorning() {
		System.out.println(name + "Good morning, Mr.");
	}
}

GoodEvening.java

package Test1;
public class GoodEvening extends GoodMorning {
	//constructor
	public GoodEvening(String name) {
		super(name);
	}

	//Display greetings
	void showGoodEvening() {
		System.out.println(name + "Good evening, Mr.");
	}
}

Copy the above sentence, specify S-JIS as the character code, save the file name as Test1.java, Hello.java, GoodMorning.java, GoodEvening.java, and save it. When executed, it will be like this. ↓ ↓

important point

--Since the actual state of after upcast is subclass reduction, subclass has priority if the same method exists in the superclass.

Test1.java

package Test1;
public class Test1 {
    public static void main(String[] args) {
        //Instance generation
        TestB b = new TestB();
        b.view();//B is displayed

        //Upcast
        TestA a = (TestA) b;
        a.view();//B is displayed
    }
}
class TestA {
    public void view() {
        //Superclass display
        System.out.println("A");
    }
}
class TestB extends TestA{
    public void view() {
        //Subclass display
        System.out.println("B");
    }
}

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

6. Related

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