[JAVA] Factory Method Pattern

Factory Method Pattern In cases where you cannot create an instance with new, such as when the package is different, use the Factory Method Pattern to get an instance
The factory abstract class defines the skeleton to instantiate A subclass of the factory abstract class creates an actual instance user receives an instance through the factory abstract class

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This page has the following structure If you just want to check the code, fly second -Case where you cannot create an instance with new ・ Confirmation of Factory Method Pattern

Case where new cannot be instantiated

Check with the following class structure

package name	class	Access modifier	Explanation
Default	user(Main.class)	public	Check the operation
implement	sam.class	Default	nothing special

`java:package.implement.sam`


package implement;

class sam{}

`user(Main.class)`


import implement.*;
public static void main(String[] args){
  sam s = new sam();
}

result

`python`


sam cannot be resolved to a type

Because the access modifier of sam.class is Default It can only be accessed from the class that belongs to the package

Therefore, sam.class is completely independent from user and cannot be touched. For example, use the Factory Method Pattern in such cases

Confirmation of Factory Method Pattern

Check with the following configuration

package name	Access modifier	class	import	Explanation
framework	public	abstract samFramework.class	None	Abstract class of objects used by user
framework	public	abstract factoryFramwork.class	None	Abstract class that defines instantiation
implement	Default	sam.class	import framework.samFramework	user is sam()Use an instance of
implement	public	factory.class	import framework.samFramework import framework.factoryFramework	sam.Create an instance of class
Default	public	user(Main.class)	import framework.samFramework import framework.factoryFramework import implement.factory

User Includes the meaning that other developers use this pattern

samFramework.class and factoryFramework.class I haven't imported anything Also, user (Main.class) cannot import sam (). Unable to get sam instance with new sam ()

Let's make a class

`samFramework.class`


package framework;

public abstract class samFramework{
  public abstract void show();
}

`factoryFramework.class`


package framework;

public abstract class factoryFramework{
  public abstract samFramework newInstance();
}

`sam.class`


package implement;
import framework.samFramework;

class sam extends samFramework {
  public void show(){System.out.println("sam");}
}

`factory.class`


package implement;
import framework.factoryFramework;
import framework.samFramework;

public class factory extends factoryFramework {
  public samFramework newInstance(){return new sam();}
}

`user(Main.class)`


import framework.factoryFramework;
import framework.samFramework;
import implement.factory;

class Main {
public static void main(String[] args){
  factoryFramework factory = new factory();
  samFramework sam = factory.newInstance();
  sam.show();
}}