方法通用返回Java类

Question

I am building generic methods for queries to the database with Hibernate, the following method consults the information of an entity by primary key.

@PersistenceContext(unitName = "UnitPers")
private EntityManager entity;

public Object findByPk(Class clazz, Object pk) throws Exception {
    Object obj = null;
    try {
        obj = entity().find(clazz, pk);
    } catch (Exception e) {
        throw e;
    }
    return obj;
}

This method works correctly, but I do not want to make a cast (object to myTypeEntity) when making the call.

private myTypeEntity entity;    
entity = (myTypeEntity) findByPk(myTypeEntity.class, new Bigdecimal("1"));

//GET and SET Methods

I want to avoid doing the cast when calling this method, bearing in mind that it is one of the purposes of the generic, but I have not managed to do it, any ideas? or what I intend to do is not possible.

Answer 1

You can use generics instead of using Object:

public <T> T findByPk(Class<T> clazz, Object pk) throws Exception {
    T obj = null;
    try {
        obj = entity().find(clazz, pk);
    } catch (Exception e) {
        throw e;
    }
    return obj;
}

And then it should work without doing the cast

private myTypeEntity entity;    
entity = findByPk(myTypeEntity.class, new Bigdecimal("1"));

Answer 2

By introducing a generic type variable on the method level you can define what type should be returned, this can be seen in @cpr4t3s' answer.

You can also shorten his snippet to the following:

public <T> T findByPk(Class<T> clazz, Object pk) throws Exception {
    return entity().find(clazz, pk);
}

Because:

1. You're just throwing the exception up in the catch -block
2. You're doing nothing, but return the obj -variable