Why Can\'t You Define a Static Method in a Non-Static Inner Class in Java Prior to Version 16?

Static Methods in Non-Static Inner Classes Prior to Java 16

Problem

In pre-Java 16 versions, attempting to define a static method within a non-static inner class results in a compiler error. For instance, the following code snippet will fail:

<code class="java">public class Foo {
    class Bar {
        static void method() {} // Compiler error
    }
}</code>

Explanation

Non-static inner classes are tied to instances of their enclosing class. Every instance of the inner class has a hidden reference to its enclosing class instance. This means that static methods within the inner class would have to maintain a reference to the enclosing class, which violates the принцип изоляции интерфейса.

Solution

To define static methods within an inner class, you must declare the inner class as static. A static inner class does not have an implicit reference to the instance of its enclosing class. As a result, it can define static methods:

<code class="java">public class Foo {
    static class Bar { // now static
        static void method() {}
    }
}</code>

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