Is Java “caching” anonymous classes?

Question

Consider the following code:

for(int i = 0;i < 200;i++)
{
  ArrayList<Integer> currentList = new ArrayList<Integer>() {{
    add(i);
  }};
  // do something with currentList
}

• How will Java treat the class of currentList?
• Will it consider it a different class for each of the 200 objects?
• Will it be a performance hit even after the first object is created?
• Is it caching it somehow?

I'm just curious :)

Answer 1

ArrayList<Integer> currentList = new ArrayList<Integer>() {{
    add(i);
  }};

is creating a new instance of the anonymous class each time through your loop, it's not redefining or reloading the class every time. The class is defined once (at compile time), and loaded once (at runtime).

There is no significant performance hit from using anonymous classes.

Answer 2

The compiler is going to transform any anonymous class to a named inner class. So your code, will be transformed to something along the lines of:

class OuterClass$1 extends ArrayList<Integer> {
    OuterClass$1(int i) {
      super();
      add(i);
    }
}

for (int i = 0; i < 200; i++) {
    ArrayList<Integer> currentList = new OuterClass$1(i);
}