[Java 기초문법] Generic usage case

[Java 기초문법] by Professional Java Developer Career Starter: Java Foundations @ Udemy

 

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In general, generics allow us to kind of constrain what types of data we can associate with a class. We can use generics on methods and things that return data types as well. And also they can protect us from certain types of runtime errors because if you're not using generics, but you're working with code that is expecting to work with certain data types.

 

Respository class 생성 

package datastore;

public class Repository {
}

 

records holders 생성 

private List<String> records = new ArrayList<>();

 

repository pattern 

very common name 

 

List<String> findAll(){
    return records;
}

main method with actual action regarding repository 

String findByID(long id){
    return records.get(Long.valueOf(id).intValue());
}

public static void main(String[] args) {
    Repository repo = new Repository();
    repo.save("house");
    repo.save("car");
}

 

이때 list generic보다 한단계 더 generic 타입을 사용하는 방법으로서 T를 사용할 수 있는데 이로써 데이터 타입을 지정해주지 않고 자유도를 높일 수 있다. 

 

 

package datastore;

import java.util.ArrayList;
import java.util.List;

public class Repository<T> {
    private List<T> records = new ArrayList<>();

    List<T> findAll(){
        return records;
    }

    T save(T record){
        records.add(record);
        return record;
    }

    T findByID(long id){
        return records.get(Long.valueOf(id).intValue());
    }

    public static void main(String[] args) {
        Repository repo = new Repository();
        repo.save("house");
        repo.save("car");
    }
}

 

Here even though main method is not modified with type T right down there, it is allowed to instatntiate it still without any generic mentioning anywhere here.

 

 

Let's make another repo calss of a person. 

 

 

with this can create another instance of a repository now, but this one will work for person instances and call it pRepo.

 

Repository<Person> pRepo = new Repository();
pRepo.save("house");

 

and here if I try to save string, occurs error saying that required type is Person. 

 

 

public static void main(String[] args) {
    Repository<String> repo = new Repository();
    repo.save("house");
    repo.save("boat");

    Repository<Person> pRepo = new Repository();
    pRepo.save(new Person("Jake", "Johnson"));
    pRepo.save(new Person("Mary", "Johnson"));
    pRepo.save(new Person("Jerry", "Johnson"));

    System.out.println(pRepo.findAll());
}

return : 

That we didn't really have to change the underlying class of repo other than to just make it generic.