<!-- https://abap2ui5.github.io/docs/get_started/hello_world -->

# Hello World

Copy this class into your system:

```abap
CLASS zcl_app_hello_world DEFINITION PUBLIC.
  PUBLIC SECTION.
    INTERFACES z2ui5_if_app.

  PROTECTED SECTION.
  PRIVATE SECTION.
ENDCLASS.

CLASS zcl_app_hello_world IMPLEMENTATION.
  METHOD z2ui5_if_app~main.
    client->message_box_display( `Hello World` ).
  ENDMETHOD.
ENDCLASS.
```

Open the abap2UI5 startup page in your browser — the page the
[Quickstart](/get_started/quickstart#_3-first-launch) ends on — enter the class
name `ZCL_APP_HELLO_WORLD` in the input field, and launch it.

That is a complete abap2UI5 app: one class, one method, no frontend project and no OData service.

## Starting an App by URL

The startup page is a convenience around a URL parameter you will use from now
on. `?app_start=<class>` appended to your service URL starts that app directly:

```
https://<host>:<port>/sap/bc/<your_service>?app_start=zcl_app_hello_world
```

That is the form a browser bookmark, a Launchpad tile and every sample
catalog use — the sample pages all say "start it with `?app_start=<class>`"
and mean exactly this.

If launching reports `The app 'ZCL_APP_HELLO_WORLD' does not exist in the
system.`, the framework could not instantiate the class: a typo in the name, or
the class is not activated yet. Anything that goes wrong *after* this point —
an app that renders empty, a binding that does not update, an error view on a
roundtrip — is cataloged with symptom, cause and fix in
[Common Failures](/cookbook/troubleshooting/common_failures).

::: tip **Naming**
Name your own apps in your customer namespace (`Z...`/`Y...`). The `Z2UI5_` prefix is reserved for the framework and its samples.
:::

## A Real Screen, an Event and Data Exchange

A message box is not an app. This second class is still one class, and it has
everything a real one has — a view, data traveling both ways, and an event:

```abap
CLASS zcl_app_hello_screen DEFINITION PUBLIC.
  PUBLIC SECTION.
    INTERFACES z2ui5_if_app.
    DATA name TYPE string.

  PROTECTED SECTION.
  PRIVATE SECTION.
ENDCLASS.

CLASS zcl_app_hello_screen IMPLEMENTATION.
  METHOD z2ui5_if_app~main.

    IF client->check_on_navigated( ).

      name = `World`.

      DATA(view) = z2ui5_cl_ui5_view_builder=>factory(
          )->ele( n = `View` ns = `mvc`
              )->a( n = `xmlns`     v = `sap.m`
              )->a( n = `xmlns:mvc` v = `sap.ui.core.mvc`

              )->ele( `Shell`
                  )->ele( `Page`
                      )->a( n = `title` v = `abap2UI5 - Hello World`

                      )->tag( `Input`
                          )->a( n = `value` v = client->_bind( name )
                      )->tag( `Button`
                          )->a( n = `text`  v = `Say Hello`
                          )->a( n = `press` v = client->_event( `SAY_HELLO` ) ).

      client->view_display( view->stringify( ) ).

    ELSEIF client->check_on_event( `SAY_HELLO` ).

      client->message_toast_display( |Hello { name }!| ).

    ENDIF.

  ENDMETHOD.
ENDCLASS.
```

Type a name, press the button, and the toast greets it. Three things are
happening, and none of them needed any code you had to write yourself:

- the **view** is a UI5 XML view, built in ABAP with `z2ui5_cl_ui5_view_builder`
- `client->_bind( name )` ties the input to the public attribute, so what the
  user types is in `name` on the next roundtrip
- `client->_event( )` sends the button press back to `main`, and
  `check_on_event( )` catches it

## Jump into the Code

Press `Ctrl+F12` in any running app to open the **Developer Tools** — tabs for
the app's source code, the rendered view XML, the model data, the
request/response pair and the error log. It is the first place to look when
something does not render the way you expected:

![Developer Tools opened with Ctrl+F12 showing code, view, and model](/get_started/image-2.webp)

## Next: Build Something

Want all of that explained rather than just shown? The
[Walkthrough](/tutorials/walkthrough/) takes these two classes apart and grows them
into a complete app, one concept per step — views, events, binding, lists,
tables, a selection screen and popups. Every step is a complete class you can
run in the browser.

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## Working Samples

Complete apps from the [sample catalog](https://github.com/abap2UI5/samples/blob/main/SAMPLES.md)
that use what this page describes. Each is a single class — pull the repository with
[abapGit](https://abapgit.org) and start it with `?app_start=<class>`.

| Sample | Class |
|---|---|
| Basics I — Hello World, the Smallest App | [`Z2UI5_CL_SMP_APP_493`](https://github.com/abap2UI5/samples/blob/main/src/01/z2ui5_cl_smp_app_493.clas.abap) |

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