Java中的多态性并根据这些类创建对象

问题描述

当从这些类创建对象时，我对Java中的多态性和扩展类等的工作方式感到有些困惑.

I'm a little confused exactly about how polymorphism and extending classes etc... works in Java when creating objects from these classes.

我最近遇到了一个问题，有人在这里帮助我解决了(请参阅)作为背景.

I recently had a problem which someone on here helped me resolved (see Not able to access object sub class's variable? (Java / Android)) for background.

我试图创建一个像这样的对象:

I was attempting to create an object like so:

Quad hero = new Hero();

其中Hero是Quad()的子类；

Where Hero was a subclass of Quad();

我的Hero类中有我无法访问的变量.

I had variables in my Hero class which I wasn't above to access.

解决方案是将我的对象创建更改为:

The solution was to change my object creation to:

Hero hero = new Hero();

通过我的hero对象，我可以访问Quad和Hero类中的所有方法和变量.

doing this, through my hero object, I was able to access all methods and variables in both my Quad and Hero classes.

我现在的问题是-为什么?

My question now is - Why is this?

考虑到这一点，何时使用我的原始方法会很有用:

And with this in mind, when would it be useful to use my original method:

Quad hero = new Hero();

这对我来说很有意义，因为Hero也是四边形.我已经在Java代码示例中多次看到这种类型的声明，并且以为我理解了，但是最近的事件证明不是这样.

That one makes sense to me as Hero is also a quad. I've seen this type of declaration many times in Java code examples, and thought I understood, it but recent events have proved otherwise.

如果有人能为我解释这一点，将不胜感激-谢谢

Would be grateful if someone could explain this for me - thanks

推荐答案

假定OtherHero也是Quad的子类，具有不同的行为:

Assume OtherHero is also a subclass from Quad with different behaviour:

这可以正常工作:

Quad hero = new Hero();
// some code
hero = new OtherHero();

以下内容无法编译:

Hero hero = new Hero();
// some code
hero = new OtherHero();

这似乎没有用.现在假设您有一个方法，其返回类型为Quad.我将用伪代码编写它:

This doesn't seem to be useful. Now assume you have a method, that has Quad as return type. I'll write it in pseudocode:

Quad method() {
    if (condition)
        return new Hero();
    else
        return new OtherHero();
}

所以您实际上不知道它将返回Hero还是OtherHero

So you actually don't know if it will return a Hero or an OtherHero

现在，您可以编写:

Quad foo = method();

现在，您实际上不知道foo的确切类型，但是可以将其用作Quad.

Now, you actually don't know the exact type of foo, but you can use it as Quad.

一个流行的例子是java.util.List

您可以写:

List<Integer> list = Arrays.asList(1,2,3,4,5,6);
Collections.shuffle(list);

List是一个接口，由ArrayList，LinkedList和许多其他类实现.在不知道代码的情况下，我们不知道Arrays.asList()确切地返回了哪个类，但是我们可以将其用作List.

List is an interface, implemented by ArrayList, LinkedList and many other classes. Without knowing the code, we don't know which class exactly is returned by Arrays.asList(), but we can use it as a List.

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