Launch parameters

A program started from a terminal can be given command-line arguments: words after the class's name, as in java Main --name=Noor --candles=30. They arrive in main's parameter, String[] args, which every main has had since module 1. Here args would hold two Strings, --name=Noor and --candles=30. Programs use them for settings that change from one run to the next: a name, a size, a file to open.

From main to start

A JavaFX program hands its arguments on to the application: launch(Main.class, args). After the class, launch takes any number of Strings (its parameter is written String... args): an array of them, as here, Strings written one after another, as in launch(Main.class, "--name=Noor", "--candles=30"), or none at all, as in launch(Main.class).

In start, getParameters(), a method that Main inherits from Application, returns them as a Parameters object with three methods:

An argument is named when it starts with two dashes, then a key that begins with a letter (or _), then =: --name=Noor is the key name with the value Noor. -name=Noor (one dash), --quiet (no =) and --3d=yes (the key starts with a digit) are unnamed. A key given twice keeps its last value.

A party has a banner that shows whose birthday it is. To try it without a terminal, this main gives launch the arguments itself:

import java.util.Map;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.layout.VBox;
import javafx.scene.text.Font;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage stage) {
        Map<String, String> named = getParameters().getNamed();
        Label banner = new Label("Happy birthday, " + named.get("name") + "!");
        banner.setFont(Font.font("Serif", 28));
        Label cake = new Label("Candles: " + named.get("candles"));
        System.out.println(getParameters().getRaw());
        System.out.println(getParameters().getUnnamed());

        stage.setScene(new Scene(new VBox(banner, cake)));
        stage.setTitle("Party");
        stage.show();
    }

    public static void main(String[] args) {
        launch(Main.class, "--name=Noor", "--candles=30", "balloons");
    }
}

Window "Party"
  VBox
    Label "Happy birthday, Noor!", font Serif 28
    Label "Candles: 30"

[--name=Noor, --candles=30, balloons]
[balloons]

getNamed() is a Map, so get("name") finds the value by its key (module 31). The values are Strings, even 30: for a number to calculate with, convert it, as with input, Integer.valueOf(named.get("candles")).

A default for a missing argument

get returns null for a key that isn't there. Started without arguments, the banner would say Happy birthday, null!. getOrDefault (module 31) gives a value of your own instead. This version passes on main's own arguments, as a real program does, and on this page it runs without any:

import java.util.Map;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.layout.VBox;
import javafx.scene.text.Font;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage stage) {
        Map<String, String> named = getParameters().getNamed();
        Label banner = new Label("Happy birthday, " + named.getOrDefault("name", "friend") + "!");
        banner.setFont(Font.font("Serif", 28));
        Label cake = new Label("Candles: " + named.getOrDefault("candles", "plenty"));

        stage.setScene(new Scene(new VBox(banner, cake)));
        stage.setTitle("Party");
        stage.show();
    }

    public static void main(String[] args) {
        launch(Main.class, args);
    }
}

Window "Party"
  VBox
    Label "Happy birthday, friend!", font Serif 28
    Label "Candles: plenty"

The application in a class of its own

The class with main doesn't have to be the application. Main can stay a plain class whose main starts another class, one that extends Application. Here main also gives only one of the two arguments:

import javafx.application.Application;

public class Main {
    public static void main(String[] args) {
        Application.launch(PartyBanner.class, "--name=Kai");
    }
}

import java.util.Map;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.layout.VBox;
import javafx.scene.text.Font;
import javafx.stage.Stage;

public class PartyBanner extends Application {

    @Override
    public void start(Stage stage) {
        Map<String, String> named = getParameters().getNamed();
        Label banner = new Label("Happy birthday, " + named.getOrDefault("name", "friend") + "!");
        banner.setFont(Font.font("Serif", 28));
        Label cake = new Label("Candles: " + named.getOrDefault("candles", "plenty"));

        stage.setScene(new Scene(new VBox(banner, cake)));
        stage.setTitle("Party");
        stage.show();
    }
}

Window "Party"
  VBox
    Label "Happy birthday, Kai!", font Serif 28
    Label "Candles: plenty"

• launch is a static method of Application. Inside a class that extends Application it can be called by its name alone; anywhere else it's called with the class's name, Application.launch(...), as Math.max(...) is.
• JavaFX makes the PartyBanner object itself, so PartyBanner must be public, with a constructor that takes no parameters, as module 48 said of Main.

Arguments here and on your computer

• On your own computer, the arguments go after the class's name in the command that starts the program, or in your editor's settings for running it.
• In Java Arena, the checks of a challenge give the arguments, and the task card shows them under "Command-line arguments". "Run with my input" starts your program without arguments, so it shows what your program does when they're missing.