Use Package Scope with Inner Classes

Consider the following class definition:

public class Foo {
    private int mValue;

    public void run() {
        Inner in = new Inner();
        mValue = 27;
        in.stuff();
    }

    private void doStuff(int value) {
        System.out.println("Value is " + value);
    }

    private class Inner {
        void stuff() {
            Foo.this.doStuff(Foo.this.mValue);
        }
    }
}

The key things to note here are that we define an inner class (Foo$Inner) that directly accesses a private method and a private instance field in the outer class. This is legal, and the code prints "Value is 27" as expected.

The problem is that Foo$Inner is technically (behind the scenes) a totally separate class, which makes direct access to Foo's private members illegal. To bridge that gap, the compiler generates a couple of synthetic methods:

/*package*/ static int Foo.access$100(Foo foo) {
    return foo.mValue;
}
/*package*/ static void Foo.access$200(Foo foo, int value) {
    foo.doStuff(value);
}

The inner-class code calls these static methods whenever it needs to access the "mValue" field or invoke the "doStuff" method in the outer class. What this means is that the code above really boils down to a case where you're accessing member fields through accessor methods instead of directly. Earlier we talked about how accessors are slower than direct field accesses, so this is an example of a certain language idiom resulting in an "invisible" performance hit.

We can avoid this problem by declaring fields and methods accessed by inner classes to have package scope, rather than private scope. This runs faster and removes the overhead of the generated methods. (Unfortunately it also means the fields could be accessed directly by other classes in the same package, which runs counter to the standard OO practice of making all fields private. Once again, if you're designing a public API you might want to carefully consider using this optimization.)