Java通用类型实例化

Question

There are many questions like this, but none of them seem to answer my question specifically.

How do you instantiate a new T?

I have a generic method, I need to return a new instance of the type in the type parameter. Here is my code...

class MyClass {

  public static MyClass fromInputStream( InputStream input ) throws IOException {

    // do some stuff, and return a new MyClass.

  }
}

Then in a seperate class I have a generic method like so...

class SomeOtherClass {

  public <T extends MyClass>download(URL url) throws IOException {

    URLConnection conn = url.openConnection();

    return T.fromInputStream( conn.getInputStream() );

  }
}

I also tried the following...

class SomeOtherClass {

  public <T extends MyClass>download(URL url) throws IOException {

    URLConnection conn = url.openConnection();

    return new T( conn.getInputStream() ); // Note my MyClass constructor takes an InputStream...

  }
}

But neither permutation of the above will compile! The error is:

File: {...}/SomeOtherClass.java
Error: Cannot find symbol
symbol : class fromInputStream
location : class MyClass

Any suggestions would be appreciated!

Answer 1

I think a common approach is to require the class of type T to be passed in like so:

class SomeOtherClass {

  public <T extends MyClass> T download(Class<T> clazz, URL url) throws IOException {

    URLConnection conn = url.openConnection();

    return clazz.getConstructor(InputStream.class).newInstance(conn.getInputStream() ); // Note my MyClass constructor takes an InputStream...

  }
}

Answer 2

Other than passing in a Class object and using reflection like in johncarl's answer , you could use a generic factory:

public abstract class InputStreamFactory<T> {

    public T make(InputStream inputStream) throws IOException;
}

And revise download :

public <T extends MyClass> T download(URL url, InputStreamFactory<? extends T> factory) throws IOException {

    URLConnection conn = url.openConnection();

    return factory.make(conn.getInputStream());
}

Each MyClass derivation could provide its own factory implementation:

public class MySubClass extends MyClass {

    public static final InputStreamFactory<MySubClass> FACTORY =
            new InputStreamFactory<MySubClass>() {
                @Override
                public MySubClass make(InputStream inputStream) throws IOException {
                    return new MySubClass(inputStream); //assuming this constructor exists
                }
            };
}

And the caller could reference it:

MySubClass downloaded = new SomeOtherClass().download(url, MySubClass.FACTORY);

Answer 3

You no need to use parameter here to call method. Because you have static method, will be enough to access fromInputStream method directly from MyClass, I mean:

return MyClass.fromInputStream( conn.getInputStream() );

Hope it will help you