Java overriding methods when creating new instance of a class

Question

This might be simple for seasoned java developers but I just cant seem to figure it out. I read a post from here. The code was

View v = new View(this) {
    @Override
    protected void onDraw(Canvas canvas) {
        System.out.println("large view on draw called");
        super.onDraw(canvas);
    }
};

It was an Android question. Here the user creates an instance of a view and overrides a method in a single line. Is there a name for this kind of coding?

My second doubt is, he overrides a protected method from another package. Isn't protected mean package private. I know this will work as I tried it out but I just couldn't figure out why it worked. So why is this code working?

I did try to google this and search in SO before asking but couldn't figure out an answer.

score 22 · Accepted Answer · edited May 23 '17 at 12:26

protected means (roughly) "available to sub-classes". (See this table.) Since the new View(this) { ... } creates a subclass, it is possible to override the method within it.

In this case it doesn't matter that you're in a different package. (See the protected line and second column in this table.) The fact that the method is in a subclass is sufficient to "get access" to a protected method.

Potential follow-up question: What sense does it make, if I can't call the method anyway?

All methods in Java are virtual. This means that whenever the View class performs a seemingly internal call to the onDraw method, this call will be dispatched to the overridden method.

I kinda forgot about default access and protected access. But I was not aware of anonymous class. thanks. — blessanm86, Sep 08 '11 at 12:24

Guido · Answer 2 · 2011-09-08T12:02:29.817

18

That is not exactly a kind of coding. That is a Java anonymous class. It is very common in Android and in general with event listeners and that kind of stuff.

For more details you can read this link (probably not the best one):

The anonymous inner classes is very useful in some situation. For example consider a situation where you need to create the instance of an object without creating subclass of a class and also performing additional tasks such as method overloading.

About your second question, the keyword protected means that the method is only available to subclasses, so it is possible to override the method.

edited Sep 08 '11 at 12:02

answered Sep 08 '11 at 11:56

Guido

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I was unaware of anonymous classes in java. Thanks for the link and explanation. +1 vote. – blessanm86 Sep 08 '11 at 12:25
More info I found at https://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html – Nashev May 17 '22 at 13:16

score 3 · Answer 3 · answered Sep 08 '11 at 11:58

This is an anonymous class. You are correct that you are overriding a protected method and this is perfectly normal. A protected method is visible, and can therefore be overridden, by a sub class, which is what you have created here.

Package protected is the default scope when you do not provide a scope for your variable or method. That is different to protected.

score 2 · Answer 4 · answered Sep 08 '11 at 12:09

So many answeres were there for "protected", so i am going to other one :)
@override is informing the compiler to override the base class method, and if there is no base class method of this signature then throws compilation error.

These are called annotations. You can search for annotations topic in java. You can create custom annotations as well.

Regards,
SSuman185

score 1 · Answer 5 · answered Sep 08 '11 at 12:00

1

Just like others here are already answered this is called anonymous class, and overriding protected methods is legal since protected methods are visible to child classes and classes in same package.

answered Sep 08 '11 at 12:00

Nikolay Ivanov

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Java overriding methods when creating new instance of a class

5 Answers5