"new Foo(){}" vs. "new Foo()" in Java

Question

For example:

Object o1 = new ArrayList<String>();
Object o2 = new ArrayList<String>(){};
Object o3 = new ArrayList<String>(){{}};

What's the difference?

I can't google out the 2nd/3rd grammar of Java, any reference?

Answer 1

The first creates an ArrayList

The second creates an anonymous subclass of ArrayList which has a specific generic type of String

The third is the same but it has a initializer block which is empty.

Note: Where ever possible, you should write the simplest and clearest code you can, esp if you are thinking about performance.

Answer 2

Object o1 = new ArrayList<String>();

Creates an ArrayList.

Object o2 = new ArrayList<String>(){};

Here you are creating an anonymous class that extends ArrayList<String> and don't override anything. So the difference it's that you are subclassing an ArrayList without overrding behaviour, never do this if you don't have a good reason.

Object o3 = new ArrayList<String>(){{}};

You are creating the same as 2 but with an empty initalizer block.

Answer 3

Object o1 = new ArrayList<String>();

Creating a new ArrayList object and assigning it to o1

Object o2 = new ArrayList<String>(){};

Creating a new instance of an anonymous class that extends ArrayList and assigning it to o2

Object o3 = new ArrayList<String>(){{}};

Creating a new instance of a (different from o2) anonymous class that extends ArrayList that has a no-op instance initializer.

Functionally the anon classes assigned to o2 and o3 are equivalent but technically they will be different classes.