Inner Classes

Named classes defined inside other classes for encapsulation and organization

Inner classes are named classes defined inside the body of another class in a .bx file. They let you organize related helper classes alongside their parent, keeping implementation details encapsulated while still being accessible when needed.

If template classes let you define helpers inline in scripts, inner classes let you do the same thing inside your classes themselves.

class Outer {

    class Inner {
        function getValue() {
            return "inner";
        }
    }

    function getInner() {
        return new Inner();
    }

}

result = new Outer().getInner().getValue();
// Result: "inner"

📋 Table of Contents

• Defining Inner Classes

• Instantiating Inner Classes

• Multiple Inner Classes

• Nested Inner Classes

• Inheritance

• Static Members

• Accessing Outer Class Statics

• Hoisting

• External Access

• Importing Inner Classes

• Metadata

• Java Interoperability

• Naming Restrictions

📥 Defining Inner Classes

An inner class is declared using the class keyword with a name, inside the body of an outer class:

class Outer {

    class Inner {
        function getValue() {
            return "inner";
        }
    }

}

Inner classes can have all the same features as top-level classes:

• Properties

• Constructors (init())

• Functions

• Static blocks

• Annotations

• extends and implements

class Outer {

    class Helper {
        property name="label" default="helper";

        function init( label ) {
            variables.label = label;
            return this;
        }

        function getLabel() {
            return variables.label;
        }
    }

    function getHelper( label ) {
        return new Helper( label );
    }

}

helper = new Outer().getHelper( "custom" );
helper.getLabel();    // "custom"

🆕 Instantiating Inner Classes

Inner classes are instantiated using new from within the outer class:

class Outer {

    class Inner {
        function getValue() {
            return "inner";
        }
    }

    function getInner() {
        return new Inner();
    }

}

outer = new Outer();
inner = outer.getInner();
inner.getValue();    // "inner"

Inner class instances are IClassRunnable objects, just like top-level class instances.

📦 Multiple Inner Classes

A single outer class can contain multiple inner classes:

class Container {

    class Adder {
        function add( a, b ) {
            return a + b;
        }
    }

    class Multiplier {
        function multiply( a, b ) {
            return a * b;
        }
    }

    function compute( a, b ) {
        return new Multiplier().multiply( new Adder().add( a, b ), 2 );
    }

}

result = new Container().compute( 3, 4 );
// Result: 14

🪆 Nested Inner Classes

Inner classes can themselves contain inner classes, creating multiple levels of nesting:

class Outer {

    class First {

        class Second {

            function getDepth() {
                return "second";
            }

        }

        function getSecond() {
            return new Second();
        }

        function getDepth() {
            return "first";
        }

    }

    function getFirst() {
        return new First();
    }

}

outer = new Outer();
first = outer.getFirst();
first.getDepth();              // "first"
first.getSecond().getDepth();  // "second"

🔗 Inheritance

Inner classes can extend other inner classes defined in the same outer class:

class Outer {

    class Animal {
        function speak() {
            return "...";
        }
    }

    class Dog extends="Animal" {
        function speak() {
            return "Woof!";
        }
    }

    function getDog() {
        return new Dog();
    }

}

outer = new Outer();
outer.getDog().speak();    // "Woof!"

🧊 Static Members

Inner classes can have static blocks and static members:

class Outer {

    class Config {
        static {
            static.MAX = 100;
        }
    }

    function getMax() {
        return Config::MAX;
    }

}

result = new Outer().getMax();
// Result: 100

🔌 Accessing Outer Class Statics

Inner classes can access the outer class's static members using both dot notation and double-colon syntax:

class MyClass {

    static {
        static.GREETING = "hello";
    }

    class Inner {

        function getGreetingDot() {
            return MyClass.GREETING;
        }

        function getGreetingColon() {
            return MyClass::GREETING;
        }

    }

    function getInner() {
        return new Inner();
    }

}

inner = new MyClass().getInner();
inner.getGreetingDot();     // "hello"
inner.getGreetingColon();   // "hello"

The outer class can also reference its own statics by name:

class MyClass {

    static {
        static.NAME = "MyClass";
    }

    function getName() {
        return MyClass.NAME;
    }

    function getNameColon() {
        return MyClass::NAME;
    }

}

🏗️ Hoisting

Inner classes are hoisted within the class body, meaning you can reference and instantiate an inner class before its definition:

class Outer {

    function getWidget() {
        return new Widget();    // Works even though Widget is defined below
    }

    class Widget {
        function getName() {
            return "widget";
        }
    }

}

result = new Outer().getWidget().getName();
// Result: "widget"

🌍 External Access

Inner classes are compiled as sibling JVM classes with $-delimited names (e.g., OuterClass$Inner). This means they can be accessed from outside the outer class using the $ separator syntax.

Instantiating via $ Syntax

// Using fully qualified path with $ separator
widget = new path.to.OuterClass$Widget( "my-widget" );
widget.getName();    // "my-widget"

Accessing Statics via $ Syntax

// Access inner class static field
result = path.to.OuterClass$Widget::WIDGET_TYPE;

Nested Inner Classes via Chained $

// Three levels deep: Outer$First$Second
second = new path.to.OuterClass$First$Second();
second.getDepth();    // "second"

📥 Importing Inner Classes

Inner classes can be imported using the $ separator syntax:

import path.to.OuterClass$Widget;

widget = new Widget( "imported-widget" );
widget.getName();    // "imported-widget"

Importing with Alias

import path.to.OuterClass$Widget as MyWidget;

widget = new MyWidget( "aliased-widget" );
widget.getName();    // "aliased-widget"

Accessing Statics After Import

import path.to.OuterClass$Widget;

result = Widget::WIDGET_TYPE;
// Result: "gadget"

Referencing Inner Classes via Outer Class

You can also reference inner classes statically via the outer class name using dot or double-colon syntax:

import path.to.OuterClass;

// Dot notation
widgetClass = OuterClass.Widget;

// Double-colon notation
widgetClass = OuterClass::Widget;

// Instantiate via the class reference
widget = new widgetClass( "test" );

Using createObject()

widget = createObject( "component", "path.to.OuterClass$Widget" );

📊 Metadata

Inner classes expose metadata through $bx (ClassMeta) and getMetadata().

$bx Metadata

outer = new OuterClass();

// Get ClassMeta object
meta = outer.$bx;

// Get inner class map
inners = meta.getInnerBoxClasses();
// Returns Map<Key, Class<?>> with entries like { "Inner": OuterClass$Inner }

// Get enclosing class (null for top-level)
enclosing = outer.getInner().$bx.getEnclosingBoxClass();

getMetadata() Struct

outer = new OuterClass();
meta = getMetadata( outer );

// Inner classes struct
meta.innerClasses;
// { "Widget": "path.to.OuterClass$Widget", "Util": "path.to.OuterClass$Util" }

// Enclosing class (empty for top-level)
meta.enclosingClass;    // ""

For an inner class instance:

widget = new OuterClass$Widget( "test" );
meta = getMetadata( widget );

meta.name;              // "path.to.OuterClass$Widget"
meta.simpleName;        // "Widget"
meta.fullname;          // "path.to.OuterClass$Widget"
meta.enclosingClass;    // "path.to.OuterClass"
meta.innerClasses;      // {} (empty if no nested inner classes)
meta.type;              // "Class"
meta.properties;        // Array of property metadata
meta.functions;         // Array of function metadata

getClassMetadata()

meta = getClassMetadata( "path.to.OuterClass$Widget" );
meta.name;    // "path.to.OuterClass$Widget"

isInstanceOf() with Inner Classes

widget = new OuterClass$Widget( "test" );

isInstanceOf( widget, "Widget" );                          // true
isInstanceOf( widget, "path.to.OuterClass$Widget" );       // true
isInstanceOf( widget, "OuterClass" );                      // false (not the parent)

☕ Java Interoperability

Inner classes can implement Java interfaces, making them useful for Java interop patterns:

class implements="java:java.lang.Iterable" {

    property Array items;

    function init( array items = [] ) {
        variables.items = arguments.items;
        return this;
    }

    function iterator() {
        return new BoxIterator( variables.items );
    }

    // Inner class implementing java.util.Iterator
    class BoxIterator implements="java:java.util.Iterator" {

        property name="data";
        property name="position";

        function init( array data ) {
            variables.data = arguments.data;
            variables.position = 0;
            return this;
        }

        boolean function hasNext() {
            return variables.position < variables.data.len();
        }

        function next() {
            if ( !hasNext() ) {
                throw( type = "java.util.NoSuchElementException", message = "No more elements" );
            }
            variables.position++;
            return variables.data[ variables.position ];
        }

    }

}

// Use from Java or BoxLang
myList = new MyIterable( [ "a", "b", "c" ] );
iter = myList.iterator();
while ( iter.hasNext() ) {
    println( iter.next() );
}

⚠️ Naming Restrictions

Outer Class Name is Reserved

When a class has inner classes, the outer class name becomes a reserved variable name. You cannot assign to it or use it as a function argument:

// ERROR: Cannot assign to outer class name
class MyClass {
    class Inner {}

    function doStuff() {
        MyClass = "oops";    // Throws BoxRuntimeException
    }
}

// ERROR: Cannot use outer class name as argument
class MyClass {
    class Inner {}

    function doStuff( MyClass ) {    // Throws BoxRuntimeException
        return MyClass;
    }
}

Inner Class Cannot Match File Name

An inner class cannot have the same name as the enclosing file's class name:

// File: MyFile.bx
class {

    class MyFile {    // ERROR: Cannot match file name
        function getValue() {
            return "inner";
        }
    }

}

Inner Class Names Cannot Be Assigned as Statics

You cannot overwrite an inner class name via static assignment:

import path.to.OuterClass;

OuterClass::Inner = "oops";    // Throws BoxRuntimeException
OuterClass.Inner = "oops";     // Throws BoxRuntimeException

JVM Compilation: Inner classes are compiled as sibling JVM classes with $-delimited names (e.g., OuterClass$Inner). This is consistent with how Java handles inner classes and enables seamless Java interoperability.

Tip: Inner classes are ideal for helper classes, iterator implementations, builder patterns, and any situation where a class is tightly coupled to its parent and unlikely to be used independently.