Nested classes in Java, see also https://blog.gypsyengineer.com/en/security/whats-new-security-features-in-java-11.html

Outer.java

package com.gypsyengineer.innerclass.field;
 
public class Outer {
 
    private int secret = 10;
 
    public void check() {
        if (secret < 0) {
            System.out.println("Oops");
        } else {
            System.out.println("Okay");
        }
    }
 
    public class Inner {
 
        public void go() {
            secret = 42;
        }
    }
 
}