Java inner class test.

ExternalClass.java

package innerClassTest;

public class ExternalClass {
    String str = "external";
    
    class InnerClass {
        String str = "inner";
    }
    
    InnerClass createInner(){
        return new InnerClass();
    }
    
    static class InnerStaticClass {
        String str = "innerStatic";
    }
}

Main.java

package innerClassTest;

import innerClassTest.ExternalClass.InnerClass;
import innerClassTest.ExternalClass.InnerStaticClass;

public class Main {
    public static void main(String[] args) {
        ExternalClass external = new ExternalClass();
        System.out.println(external.str);
        
        // ---------------------------------------------------------------------- 
        // It is necessary to create an external instance to access an inner class.
        // When an instance of an external class is made, an instance of an inner class is NOT made automatically.
        // ----------------------------------------------------------------------
        InnerClass inner = external.createInner();
        System.out.println(inner.str);
        
        // A static inner class is same as a top level (external) class.
        InnerStaticClass innerStatic = new InnerStaticClass();
        System.out.println(innerStatic.str);        
    }
}