Question

I have

public class Entity

On which works class:

public class Table<Target extends Entity>
{
    public boolean save (Target target)
    public Target load (int id)
}

Till i put this objects of Table class in Database class Map everything is ok but:

public class Database
{
    public String name;
    public Map<String, Table<? extends Entity>> tables = new HashMap <String, Table<? extends Entity>> ();
    public Context context;
    public int version;
    public Database (Context context, int version)
    {
        this.context = context;
        this.version = version;
    }
    public void add (Table<? extends Entity> table)
    {
        tables.put(table.name, table);
    }
    public Table <? extends Entity> table (String name)
    {
        return (Table<? extends Entity>) tables.get(name);
    }

}

Assume we have:

 public class Apple extends Entity

I want this code to work:

    //init      
    database = new Database(getBaseContext(), 4);
    new Table<Apple> (database, Apple.class);

    //this is where solution need occurs !
    database.table("apples").save (new Apple("green apple"));

The method save(capture#1-of ? extends Entity) in the type
Table(capture#1-of ? extends Entity)
is not applicable for the arguments (Note)

How to make the map.get method work ?

Answer 1

One thing you can do to make this more convenient is map the tables by the target class:

Map<Class<? extends Entity>, Table<? extends Entity>> tables;

public void <T extends Entity> addTable(Class<T> cls, Table<T> table) {
    tables.put(cls, table);
    // alternately make it possible to get the target class from the table
}

@SuppressWarnings("unchecked")
public <T extends Entity> Table<T> getTable(Class<T> cls) {
    return (Table<T>) tables.get(cls);
}

As the annotation implies, this isn’t at all typesafe – you’ll need to make sure you never mismatch the keys and values in tables.