Creating instance of static abstract class

Question

I know that abstract classes cannot be instantiated. But I have a doubt in the code below. This code is a part of android bitmap fun demo ( http://commondatastorage.googleapis.com/androiddevelopers/shareables/training/BitmapFun.zip).

// ImageWorkerAdapter class is nested in another abstract class ImageWorker
public static abstract class ImageWorkerAdapter
{
public abstract Object getItem(int num);
public abstract int getSize();
}

//this snippet is seen in Images.java
public final static ImageWorkerAdapter imageWorkerUrlsAdapter = new ImageWorkerAdapter() { 
@Override
public Object getItem(int num) {
return Images.imageUrls[num];
}

I cannot understand how it is possible to create an instance of an abstract class. Please help me to understand this code.

@Nambari: no, they can't. See http://docs.oracle.com/javase/6/docs/api/java/lang/Class.html#newInstance%28%29 — JB Nizet, Dec 09 '12 at 13:46
@Nambari: I have an exact reference: the javadoc I linked to, which says: "Throws: InstantiationException - if this Class represents an abstract class" — JB Nizet, Dec 09 '12 at 13:50
@VysakhPrem: No, no I am not saying it won't work. It should work. But the reason what I have in my mind is different. JB Nizet clarified that. — kosa, Dec 09 '12 at 14:14

score 6 · Answer 1 · answered Dec 09 '12 at 13:44

6

This code represents the initialization of an anonymous class extending ImageWorkerAdapter abstract class:

new ImageWorkerAdapter() { 
    @Override
    public Object getItem(int num) {
    return Images.imageUrls[num];
}

Indeed, the anonymous implementation is defined between the curly braces.

As being an anonymous class, it's perfectly valid to rely on an abstract class or interface.

answered Dec 09 '12 at 13:44

Mik378

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At the first sight, I was confused. Thank you for the explanation. – Vysakh Prem Dec 09 '12 at 14:42

score 4 · Accepted Answer · edited May 23 '17 at 11:48

4

The code you show does not show an abstract class being instantiated.

It shows an anonymous class being instantiated. In this case, the anonymous class extends an abstract class. You can also implement interfaces in anonymous classes, Runnable being a very common example.

edited May 23 '17 at 11:48

Community

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answered Dec 09 '12 at 13:45

Pete Kirkham

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score 1 · Answer 3 · answered Dec 09 '12 at 13:46

1

Your code does not instantiate an abstract class, but defines new anonymous abstract class which among others extends and implement (override) other abstract class.

answered Dec 09 '12 at 13:46

Michael A

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Creating instance of static abstract class

3 Answers3