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ABAP，Java和JavaScript的local class

Local class in ABAP

Suppose I have a global class with a public method ADD with following signature. I would like to implement with a local class inside this global class.

The local class could be created by clicking button “Local Definitions/Implementions”:

Now in my global class I can just delegate the ADD implementation to the local class. Notice that even ADD method is marked as public, it is still displayed as a red light in class builder, which makes sense since this ADD method in local class is not visible to outside consumers.

Local class in ABAP in widely used in the following scenarios:

(1) ABAP unit test class
(2) The pre/post exit enhancement of class method are technically implemented via a local class in enhancement include.
For example, once you click the “Post-Exit” button below,

You will see the source code as below, the local class LCL_ZCL_JERRY_POSTEXIT is automatically generated by class builder.

CLASS LCL_ZCL_JERRY_POSTEXIT DEFINITION DEFERRED.
CLASS CL_JERRY_TOOL DEFINITION LOCAL FRIENDS LCL_ZCL_JERRY_POSTEXIT.
CLASS LCL_ZCL_JERRY_POSTEXIT DEFINITION.
PUBLIC SECTION.
CLASS-DATA OBJ TYPE REF TO LCL_ZCL_JERRY_POSTEXIT. "#EC NEEDED
DATA CORE_OBJECT TYPE REF TO CL_JERRY_TOOL . "#EC NEEDED
 INTERFACES  IPO_ZCL_JERRY_POSTEXIT.
  METHODS:
   CONSTRUCTOR IMPORTING CORE_OBJECT
     TYPE REF TO CL_JERRY_TOOL OPTIONAL.
ENDCLASS.
CLASS LCL_ZCL_JERRY_POSTEXIT IMPLEMENTATION.
METHOD CONSTRUCTOR.
  ME->CORE_OBJECT = CORE_OBJECT.
ENDMETHOD.

METHOD IPO_ZCL_JERRY_POSTEXIT~GET_QUERY_RESULT.
*"------------------------------------------------------------------------*
*" Declaration of POST-method, do not insert any comments here please!
*"
*"class-methods GET_QUERY_RESULT
*"  importing
*"    !IV_COL_WRAPPER type ref to CL_BSP_WD_COLLECTION_WRAPPER
*"  changing
*"    value(RV_RESULT) type ref to IF_BOL_ENTITY_COL . "#EC CI_VALPAR
*"------------------------------------------------------------------------*
**************  define your own post enhancement here!!! **************

ENDMETHOD.
ENDCLASS.

(3) if an event handler in a given program is not intended to be reused by other programs, it could be defined as a local class within the program it is used – in this case it is not necessary to define a global class as event handler.

There are lots of such examples in ALV examples delivered by SAP, see program BCALV_GRID_DND_TREE as example.

Inner Class in Java

The above example could simply be written in Java as well. In this example the inner class lcl_local does not follow Camel naming convention since I would like to highlight that it works exactly the same as the one in ABAP example.

public class LocalClassTest{
	public int add(int var1, int var2){
		return new lcl_local().add(var1, var2);
	}
	private class lcl_local { 
		public int add(int var1, int var2){
			return var1 + var2;
		}
	}
}

Just exactly the same as in ABAP, the local class could not be used outside the class where it is defined.

One typical usecase of inner class in Java is the efficient implementation of a thread-safe singleton pattern, see sample code below:

public class Example {
    private static class StaticHolder {
        static final MySingleton INSTANCE = new MySingleton();
    }
 
    public static MySingleton getSingleton() {
        return StaticHolder.INSTANCE;
    }
}

“Local class” in JavaScript

I use double quote in title since the function object in JavaScript are not actual “class” concept in ABAP and Java.

Below source code is a simulation of private attributes & methods in JavaScript:

function createClass(conf){
    function _injectAttribute(fn){
        var prototype = fn.prototype;
        for(var publicName in publics){
            if(!publics.hasOwnProperty(publicName))
                continue;
            if(typeof publics[publicName]=="function")
                prototype[publicName] = function(publicName){
                    return function(){
                        return publics[publicName].apply(privates, arguments);
                    }
                }(publicName);
            else 
                prototype[publicName] = publics[publicName];
            if(!privates[publicName])
                privates[publicName] = prototype[publicName];
        }
        return fn;
    }
    var publics, privates;
        publics = conf.publics;
        privates = conf.privates || new Object();

    var fn = function(fn){
    	return function(){
    		return fn.apply(privates, arguments);
    	};
    }(conf.constructor || new Function());

    return _injectAttribute(fn);
}

var MyClass = createClass({
    constructor:function(){
        console.log("constructor is called: " + this.message);
    },
    publics:{
        message:"Hello, World",
        sayJavaScript:function(){
            return this._message;
        },
        sayABAP:function(msg){
            return msg + ", " + this.ABAP();
        }
    },
    privates:{
        _message: "Hello, JavaScript",
        ABAP :function(){
            return "ABAP";
        }
    }
});

var myClassInstance = new MyClass();

console.log(myClassInstance.message);
console.log(myClassInstance.sayJavaScript());
console.log(myClassInstance.sayABAP("Hello"));
console.log(myClassInstance._message);
console.log(myClassInstance.ABAP());

Testing shows it works as expected:

Local function in ES6

In ES6 it is possible to define a class via syntax suger class:

class Developer {
    constructor(name, language) {
        this.workingLanguage = language;
        let _name = name;

        let _getName = function() { 
            return _name; 
        };
        this.getName = _getName;
    }
}

var Jerry = new Developer("Jerry", "Java");
var Ji = new Developer("Ji", "JavaScript");

console.log("Developer name: " + Jerry.getName());
console.log("Jerry's working language: " + Jerry.workingLanguage);
console.log("local function accessible? " + Jerry._getName);
console.log("Jerry's name: " + Jerry._name);

ABAP，Java和JavaScript的local class

Local class in ABAP

Inner Class in Java

“Local class” in JavaScript

Local function in ES6

Further reading