在Java中使用抽象类

Question

Java Documentation says NumberFormat is an abstract class. But the following line of code is confusing me.

 NumberFormat nf = NumberFormat.getInstance();

How can we use this NumberFormat.getInstance() when NumberFormat is an abstract class? Can anyone help me in understanding this please?

Answer 1

NumberFormat.getInstance() is a static factory method , which can return an instance of a concrete subclass of the abstract NumberFormat class:

It's something like this:

abstract class Instrument {
    public static Instrument getInstance() {
        return new Guitar();
    }
}

class Guitar extends Instrument { }

So, when you call:

Instrument.getInstance();

You would get an instance of Guitar , which is a concrete subclass.

Answer 2

getInstance()方法返回NumberFormat的子类-语言环境的默认值。

Answer 3

You can call NumberFormat.getInstance() because it's a static method. You don't need an instance of a class to call a static method on that class.

NumberFormat.getInstance() can declare a return type of NumberFormat because it will actually return an instance of a concrete subclass - but that's compatible with NumberFormat in the same way that this works:

String x = "test";
Object y = x;

If you print out NumberFormat.getInstance().getClass() you'll see which actual concrete subclass is being used - but you shouldn't depend on it returning any particular class; the point is that you're just meant to use methods specified on NumberFormat itself.

Answer 4

默认情况下， NumberFormat.getInstance()返回非抽象的DecimalFormat实现。