更改屬性ArrayList <Class>到ArrayList <SubClass>在子類

Question

嗯，你好，這是我的問題。

我想知道，如果給定一個abstract class Foo，有可能，不知何故，得到它的屬性ArrayList<Bar>（酒吧是抽象的也行）只包含它所使用的Foo每個子類中含有一定特定的子類。

例如，在一個具體的該class FooChildextends Foo，屬性ArrayList<Bar>將現在僅包含具體class BarChild（其extends Bar）。

更具體地說，如果我有這段代碼：

public abstract class CharactersGroup { 
    private ArrayList<Character> characters; 

    public CharactersGroup() { 
     this(new ArrayList<Character>()); 
    } 

    public CharactersGroup(ArrayList<Character> list) { 
     characters = list; 
    } 
} 

和CharactersGroup一個子類是HeroesGroup，像這樣：

public class HeroesGroup extends CharactersGroup { 
    // Here is an ArrayList<Character> characters, that I would like to "become" an ArrayList<Hero> characters 

    public HeroesGroup() { 
     this(new ArrayList<Hero>()); 
    } 

    public HeroesGroup(ArrayList<Hero> list) { 
     super(list); 
    } 
} 

一個Character將只是想：

And a Hero like：

public class Hero extends Character { 
    private int level; 

    public Hero(String n, int l) { 
     super(n); 
     level = l; 
    } 
} 

Answer 1

你可以讓你的抽象類CharactersGroup通用。

public abstract class CharactersGroup<C extends Character> { 
    private ArrayList<C> characters; 

    public CharactersGroup() { 
     this(new ArrayList<C>()); 
    } 

    public CharactersGroup(ArrayList<C> list) { 
     characters = list; 
    } 
} 

子類是這樣的：

public class HeroesGroup extends CharactersGroup<Hero> { 

    public HeroesGroup() { 
     this(new ArrayList<Hero>()); 
    } 

    public HeroesGroup(ArrayList<Hero> list) { 
     super(list); 
    } 
} 

Answer 2

使用在超通用參數：

public abstract class CharactersGroup<C extends Character> { 
    private ArrayList<C extends Character> characters; 

    public CharactersGroup() { 
     this(new ArrayList<C extends Character>()); 
    } 

    public CharactersGroup(ArrayList<C extends Character> list) { 
     characters = list; 
    } 
} 

public class HeroesGroup extends CharactersGroup<Hero> { 
    // Here is an ArrayList<Character> characters, that I would like to "become" an ArrayList<Hero> characters 

    public HeroesGroup() { 
     this(new ArrayList<Hero>()); 
    } 

    public HeroesGroup(ArrayList<Hero> list) { 
     super(list); 
    } 
}