抽象类InputStream的实例

Question

InputStream class defined as-

 public abstract class InputStream extends Object 

Then how System class can contain "in" object of InputStream class ?

Answer 1

This is because in is not an object, its a reference.

Its a reference to an InputStream OR a sub-class of an InputStream. As its abstract, it can only be a sub-class.

Answer 2

The in field defined by the System class doesn't need to reference a concrete implementation of InputStream -- as you've worked out, it can't -- it just needs to reference something that extends InputStream .

On Linux at least, in references a BufferedInputStream that itself wraps a FileInputStream. Other implementations may vary and that's the point: the use of an abstract class like InputStream allows the implementing class to vary and potentially be changed from one version of Java to the next whilst keeping calling code happy.

Answer 3

An InputStream is the abstract, but the concrete class (the one actually referenced by System.in ) can be any subclass of InputStream, including an anonymous class .

Some subclasses listed in the javadoc for InputStream include:

• AudioInputStream
• ByteArrayInputStream
• FileInputStream
• FilterInputStream
• InputStream (CORBA)
• ObjectInputStream
• PipedInputStream
• SequenceInputStream
• StringBufferInputStream

Executing this code to find out which subclass System.in actual is:

System.out.println(System.in.getClass());

Yields this answer:

class java.io.BufferedInputStream