Software Engineering for Self-Directed Learners
  • Java

    Enums

    Can use Java enumerations

    You can define an enum type by using the enum keyword. Because they are constants, the names of an enum type's fields are in uppercase letters e.g., FLAG_SUCCESS.

    Defining an enumeration to represent days of a week (code to be put in the Day.java file):

    public enum Day {
        SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
        THURSDAY, FRIDAY, SATURDAY
    }
    

    Some examples of using the Day enumeration defined above:

    Day today = Day.MONDAY;
    Day[] holidays = new Day[]{Day.SATURDAY, Day.SUNDAY};
    
    switch (today) {
    case SATURDAY:
    case SUNDAY:
        System.out.println("It's the weekend");
        break;
    default:
        System.out.println("It's a week day");
    }
    

    Note that while enumerations are usually a simple set of fixed values, Java enumerations can have behaviors too, as explained in this tutorial from -- Java Tutorial

    Define an enumeration named Priority. Add the missing describe method to the code below so that it produces the output given.

    public class Main {
    
        // Add your method here
    
        public static void main(String[] args) {
            describe("Red", Priority.HIGH);
            describe("Orange", Priority.MEDIUM);
            describe("Blue", Priority.MEDIUM);
            describe("Green", Priority.LOW);
        }
    }
    

    Red indicates high priority
    Orange indicates medium priority
    Blue indicates medium priority
    Green indicates low priority
    

    Use a switch statement to select between possible values for Priority.

        public static void describe(String color, Priority p) {
            switch (p) {
                case LOW:
                    System.out.println(color + " indicates low priority");
                    break;
                // ...
            }
        }
    

    Code for the enumeration is given below:

    public enum Priority {
        HIGH, MEDIUM, LOW
    }
    

    JavaFX: Basic

    Can use JavaFX to build a simple GUI

    Adapted (with permissions) from Marco Jakob's JavaFX 8 tutorial.

    JavaFx 9 Tutorial - Part 1: Scene Builder

    Introduction

    This tutorial will teach you how to create a new JavaFX application in IntelliJ, and to use the SceneBuilder to create a layouts for your application.

    Final Screenshot

    Prerequisites

    Do remember the installation path to SceneBuilder 8 as we will need it to configure IntelliJ in a later step.

    IntelliJ Configurations

    If this is the first time using IntelliJ, you need to tell IntelliJ where to find JDK 9 and SceneBuilder.

    Configuring JDK 9

    1. On the Welcome screen, press ConfigureProject DefaultProject Structure.
    Welcome Screen

    • If you already have a project open, go to the Welcome screen by going to FileClose Project.
    1. Under Project SDK:, press New...JDK.
    2. Select the directory that you install JDK on, and press OK.
    3. Under Project language level:, select 9 - Modules, private methods in interfaces etc..
    JDK 9 under Project Structure

    1. Press OK again.

    Configuring Scene Builder

    1. On the Welcome screen, press ConfigureSettings.
    2. On the left hand side, select Languages & FrameworksJavaFX
    3. Under Path to SceneBuilder:, select the path to where SceneBuilder is installed (e.g. C:\Users\Admin\AppData\Local\SceneBuilder\SceneBuilder.exe on Windows)
    Scene Builder settings

    The JavaDocs will come in handy when writing your own JavaFX applications:

    Additionally, Oracle also has a tutorial on JavaFX if you are interested.

    Create a new JavaFX Project

    1. On the Welcome screen, press Create New Project.
    • If you already have a project, you can create a new project by going FileNewProject....
    1. On the left side, select JavaFX. Make sure that the Project SDK is set to 9 and JavaFX Application is selected.
    New Project

    1. Press Next.
    2. Give a name for the application (e.g. AddressApp), and specify a suitable location.
    3. Press Finish. If prompted to create a new directory, just press OK.

    Remove the sample package and its content. We will manually create our own package and resources in our tutorial.

    We will also have to set up the IDE further, so that warnings and errors show up correctly when working with Java 9:

    1. Go to the menu FileProject Structure.
    2. Under Project language level:, ensure that 9 - Modules, private methods in interfaces etc. is selected.

    Create the Packages

    In We will create a package for each of the component. Ensure that your Project pane is open (Alt+1). Right click on the src folder, and select NewPackage:

    • seedu.address - contains the controller classes (i.e. the part that deals with the business logic)
    • seedu.address.model - contains the model classes (i.e. the part that deals with data)
    • seedu.address.view - contains the views (i.e. the part that deals with presenting the data to the user)
    New Packages

    In subsequent tutorials, our view package will also contain some controllers that are directly related to a single view. We will call them view-controllers.

    Create the FXML Layout File

    There are two ways to create the UI:

    • Use FXML, which is an XML format.
    • Programmatically create the interface in Java.

    We will use FXML for most parts, so that we can separate the view and controller from each other. Furthermore, we are able to use the Scene Builder tool to edit our FXML file. That means we will not have to directly work with XML.

    Right click on the view package, and press NewFXML file. For the file name, type PersonOverview.fxml.

    Design with Scene Builder

    Right-click on PersonOverview.fxml and choose Open with Scene Builder. Now you should see the Scene Builder with just an AnchorPane (visible under Hierarchy on the left).

    If IntelliJ prompts for a location of the SceneBuilder executable, make sure to point to where you install SceneBuilder.

    1. Select the Anchor Pane in your Hierarchy, and adjust the size under Layout (right side). (Pref Width: 600, Pref Height: 300)

    1. Add a Split Pane (horizontal) (under Containers) by dragging it from the Library into the main area. Right-click the Split Pane in the Hierarchy view and select Fit to Parent.

    1. Drag a TableView (under Controls in Library view) into the left side of the SplitPane. Select the TableView (not a Column) and set the following layout constraints in the Inspector to the TableView. Inside an AnchorPane you can always set anchors to the four borders (see this page for more information on Layouts).

    1. Go to the menu PreviewShow Preview in Window to see whether the layout configuration is done correctly. Try resizing the window. The TableView should resize together with the window as it is anchored to the borders.

    2. Change the column text (under Properties) to "First Name" and "Last Name".

    1. Select the TableView and choose constrainted-resize for the 'Column Resize Policy'. This ensures that the columns will always fill up the entire available space.

    1. Add a Label on the right side with the text "Person Details:". Adjust the layout using anchors (Top: 5, Left: 5. Right: Blank, Bottom: Blank).

    You can use the Search Bar at the top of Library/Inspector to find the respective controls/properties.

    1. Add a GridPane on the right side. Select it, and adjust its layout using anchors (Top: 30, Left: 5, Right: 5, Bottom: Blank).

    1. Add the following labels to the cells, such that the grid is of this form:
    First Name Label
    Last Name Label
    Street Label
    City Label
    Postal Code Label
    Birthday Label

    To add a row to the GridPane, select an existing row number, right click the row number and choose "Add Row Below".

    1. Add a ButtonBar at the bottom. Add three buttons to the bar ("New...", "Edit...","Delete"). Adjust the anchors so that it stays at the bottom right (Top: Blank, Left: Blank, Right: 10, Bottom: 5).

    1. Now you should see something like the following. Use the Preview menu to test its resizing behaviour.

    1. Save the .fxml file.

    Create the Main Application

    The PersonOverview.fxml that we just created only contains the content of our entire application. We need another FXML for our root layout, which will contain a menu bar and wraps PersonOverview.fxml.

    1. Inside IntelliJ, right click on the view package, and press NewFXML file. For the file name, type RootLayout.fxml.
    2. Right-click on RootLayout.fxml and choose Open with Scene Builder.
    3. Delete the AnchorPane. We will use another pane for our root layout.

    1. Add BorderPane by dragging it from the Library view into the main area.

    1. Resize the BorderPane (Pref Width: 600, Pref Height: 400)

    1. Add a MenuBar into the insert TOP slot. We will not implement menu functionality for now.

    The JavaFX Main Class

    Now, we need to create the main Java class that starts up our application with the RootLayout.fxml and adds the PersonOverview.fxml in the center.

    Right-click on your seedu.address package, and choose NewJavaFXApplication. For the class name, type MainApp.

    The generated MainApp.java class extends from Application and contains two methods. This is the basic structure that we need to start a JavaFX Application. The most important part for us is the start(Stage primaryStage) method. It is automatically called when the application is launch() from within the main() method.

    As you see, the start(...) method receives a Stage as parameter. The following graphic illustrates the structure of every JavaFX application:

    Image Source: http://www.oracle.com

    It's like a theater play: The Stage is the main container which is usually a Window with a border and the typical minimize, maximize and close buttons. Inside the Stage you add a Scene which can, of course, be switched out by another Scene. Inside the Scene the actual JavaFX nodes like AnchorPane, TextBox, etc. are added.

    See this page for more info on working with the JavaFX Scene Graph.


    Open MainApp.java and replace the code with the following:

    MainApp.java

    package seedu.address;
    
    import java.io.IOException;
    
    import javafx.application.Application;
    import javafx.fxml.FXMLLoader;
    import javafx.scene.Scene;
    import javafx.scene.layout.AnchorPane;
    import javafx.scene.layout.BorderPane;
    import javafx.stage.Stage;
    
    public class MainApp extends Application {
        private Stage primaryStage;
        private BorderPane rootLayout;
    
        @Override
        public void start(Stage primaryStage) {
            this.primaryStage = primaryStage;
            this.primaryStage.setTitle("AddressApp");
    
            initRootLayout();
    
            showPersonOverview();
        }
    
        /**
         * Initializes the root layout.
         */
        public void initRootLayout() {
            try {
                // Load root layout from fxml file.
                FXMLLoader loader = new FXMLLoader();
                loader.setLocation(MainApp.class.getResource("view/RootLayout.fxml"));
                rootLayout = loader.load();
    
                // Show the scene containing the root layout.
                Scene scene = new Scene(rootLayout);
                primaryStage.setScene(scene);
                primaryStage.show();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    
        /**
         * Shows the person overview inside the root layout.
         */
        public void showPersonOverview() {
            try {
                // Load person overview.
                FXMLLoader loader = new FXMLLoader();
                loader.setLocation(MainApp.class.getResource("view/PersonOverview.fxml"));
                AnchorPane personOverview = loader.load();
    
                // Set person overview into the center of root layout.
                rootLayout.setCenter(personOverview);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    
        /**
         * Returns the main stage.
         */
        public Stage getPrimaryStage() {
            return primaryStage;
        }
    
        public static void main(String[] args) {
            launch(args);
        }
    }
    

    The various comments should give you some hints about what's going on.

    Importing JavaFx 9

    JavaFx 9 is a separate module from Java 9. We must ensure that the modules are imported correctly.

    Notice that IntellJ will complain about the imported modules "not being in the module graph":

    To fix this:

    1. Ensure that your Project pane is open (Alt+1). Right click on the src folder, and select Newmodule-info.java:
    2. Add the following requires and exports in order to import and export the modules correctly:
    module AddressApp {
        requires javafx.graphics;
        requires javafx.fxml;
        requires javafx.controls;
    
        exports seedu.address;
    }
    

    Running the Application

    If you run the application now (right click MainApp.java and select Run MainApp.main()), you should see something like this:

    Possible Issues

    If JavaFx fails to load PersonOverview.fxml, you might get the following error message:

    javafx.fxml.LoadException:
    /.../AddressApp/out/production/AddressApp/seedu/address/view/PersonOverview.fxml:15
    

    To solve this issue, open PersonOverview.fxml normally in IntelliJ and ensure that there is no such attribute around:

    fx:controller="seedu.address.view.PersonOverview"

    JavaFx 9 tutorial - Part 2: Model and TableView

    Introduction

    In this tutorial, we will create the other parts of the application (mainly the model and the controller). We will also use ObservableList and *Property to bind our list of new model's Persons, and their individual details, to the view, with the controller as the "middleman".

    Create the Model class

    We need a model class in order to hold information about the people in our address book. Add a new class to the model package (seedu.address.model) called Person. The Person class will encapsulate the details of an individual person such as name, address and birthday. Add the following code to the class. The JavaFX specifics will be explained after the code snippet.

    Person.java

    package seedu.address.model;
    
    import java.time.LocalDate;
    
    import javafx.beans.property.IntegerProperty;
    import javafx.beans.property.ObjectProperty;
    import javafx.beans.property.SimpleIntegerProperty;
    import javafx.beans.property.SimpleObjectProperty;
    import javafx.beans.property.SimpleStringProperty;
    import javafx.beans.property.StringProperty;
    
    /**
     * Model class for a Person.
     */
    public class Person {
    
        private final StringProperty firstName;
        private final StringProperty lastName;
        private final StringProperty street;
        private final IntegerProperty postalCode;
        private final StringProperty city;
        private final ObjectProperty< LocalDate > birthday;
    
        /**
         * Default constructor.
         */
        public Person() {
            this(null, null);
        }
    
        /**
         * Constructor with some initial data.
         */
        public Person(String firstName, String lastName) {
            this.firstName = new SimpleStringProperty(firstName);
            this.lastName = new SimpleStringProperty(lastName);
    
            // Some initial dummy data, just for convenient testing.
            this.street = new SimpleStringProperty("some street");
            this.postalCode = new SimpleIntegerProperty(1234);
            this.city = new SimpleStringProperty("some city");
            this.birthday = new SimpleObjectProperty< LocalDate >(LocalDate.of(1999, 2, 21));
        }
    
        public String getFirstName() {
            return firstName.get();
        }
    
        public void setFirstName(String firstName) {
            this.firstName.set(firstName);
        }
    
        public StringProperty firstNameProperty() {
            return firstName;
        }
    
        public String getLastName() {
            return lastName.get();
        }
    
        public void setLastName(String lastName) {
            this.lastName.set(lastName);
        }
    
        public StringProperty lastNameProperty() {
            return lastName;
        }
    
        public String getStreet() {
            return street.get();
        }
    
        public void setStreet(String street) {
            this.street.set(street);
        }
    
        public StringProperty streetProperty() {
            return street;
        }
    
        public int getPostalCode() {
            return postalCode.get();
        }
    
        public void setPostalCode(int postalCode) {
            this.postalCode.set(postalCode);
        }
    
        public IntegerProperty postalCodeProperty() {
            return postalCode;
        }
    
        public String getCity() {
            return city.get();
        }
    
        public void setCity(String city) {
            this.city.set(city);
        }
    
        public StringProperty cityProperty() {
            return city;
        }
    
        public LocalDate getBirthday() {
            return birthday.get();
        }
    
        public void setBirthday(LocalDate birthday) {
            this.birthday.set(birthday);
        }
    
        public ObjectProperty< LocalDate > birthdayProperty() {
            return birthday;
        }
    }
    

    Explanations

    • With JavaFX, it is common to use *Property for all fields of a model class. A Property allows us, for example, to automatically be notified when the lastName or any other variable is changed. This helps us keep the view in sync with the data.
    • To learn more about *Property, refer to Using JavaFX Properties and Binding
    • LocalDate, the type that we are using for birthday, is part of the new Date and Time API since JDK 8

    A List of Persons

    The main data that our application manages is simply a bunch of persons. Let's create a list of Person objects inside the MainApp class. All other controller classes will later get access to that central list inside the MainApp.

    ObservableList

    We are working with JavaFX view classes that need to be informed about any changes made to the list of persons. This is important, since otherwise the view would not be in sync with the data. For this purpose, JavaFX introduces some new Collection classes.

    Among all these collections, we need the ObservableList. To create a new ObservableList, add the following code at the beginning of the MainApp class. We'll also add a constructor that creates some sample data and a public getter method:

    MainApp.java

    // ... AFTER THE OTHER VARIABLES ...
    
    /**
    * The data as an observable list of {@link Person}.
    */
    private ObservableList< Person > personData = FXCollections.observableArrayList();
    
        /**
         * Constructor
         */
        public MainApp() {
            // Add some sample data
            personData.add(new Person("Hans", "Muster"));
            personData.add(new Person("Ruth", "Mueller"));
            personData.add(new Person("Heinz", "Kurz"));
            personData.add(new Person("Cornelia", "Meier"));
            personData.add(new Person("Werner", "Meyer"));
            personData.add(new Person("Lydia", "Kunz"));
            personData.add(new Person("Anna", "Best"));
            personData.add(new Person("Stefan", "Meier"));
            personData.add(new Person("Martin", "Mueller"));
        }
    
        /**
         * Returns the data as an observable list of {@link Person}.
         */
        public ObservableList< Person > getPersonData() {
            return personData;
        }
    
        // ... THE REST OF THE CLASS ...
    

    The PersonOverviewController

    We have our model and view. Let's get our data into our table. We'll need a controller for our PersonOverview.fxml to act as a "middleman" for the model and view.

    Create a normal Java class inside the view package called PersonOverviewController.

    Note: We must put the class inside the same package as PersonOverview.fxml, otherwise the SceneBuilder won't be able to find it.

    We will add some instance variables that give us access to the table and the labels inside the view. The fields and some methods have a special @FXML annotation. This is necessary in order for the .fxml file to have access to private fields and private methods. After we have everything set up in the .fxml file, the application will automatically fill the variables when the .fxml file is loaded.

    So let's add the following code:

    Note: Remember to always use the javafx imports, NOT awt or swing.

    PersonOverviewController.java

    package seedu.address.view;
    
    import javafx.fxml.FXML;
    import javafx.scene.control.Label;
    import javafx.scene.control.TableColumn;
    import javafx.scene.control.TableView;
    import seedu.address.MainApp;
    import seedu.address.model.Person;
    
    public class PersonOverviewController {
        @FXML
        private TableView< Person> personTable;
        @FXML
        private TableColumn< Person, String> firstNameColumn;
        @FXML
        private TableColumn< Person, String> lastNameColumn;
    
        @FXML
        private Label firstNameLabel;
        @FXML
        private Label lastNameLabel;
        @FXML
        private Label streetLabel;
        @FXML
        private Label postalCodeLabel;
        @FXML
        private Label cityLabel;
        @FXML
        private Label birthdayLabel;
    
        // Reference to the main application.
        private MainApp mainApp;
    
        /**
         * The constructor. It is called before the initialize() method.
         */
        public PersonOverviewController() {
        }
    
        /**
         * Initializes the controller class. This method is automatically called
         * after the fxml file has been loaded.
         */
        @FXML
        private void initialize() {
            // Initialize the person table with the two columns.
            firstNameColumn.setCellValueFactory(cellData -> cellData.getValue().firstNameProperty());
            lastNameColumn.setCellValueFactory(cellData -> cellData.getValue().lastNameProperty());
        }
    
        /**
         * Is called by the main application to give a reference back to itself.
         */
        public void setMainApp(MainApp mainApp) {
            this.mainApp = mainApp;
    
            // Add observable list data to the table
            personTable.setItems(mainApp.getPersonData());
        }
    }
    

    Explanations

    • All fields and methods where the .fxml file needs access must be annotated with @FXML.
      • Actually, only if they are private, but it's better to have them private and mark them with the annotation!
    • The initialize() method is automatically called after the .fxml file has been loaded. At this time, all the FXML fields should have been initialized already.
    • The setCellValueFactory(...) that we set on the table colums are used to determine which field inside the Person objects should be used for the particular column. The arrow -> indicates that we're using a Java 8 feature called Lambdas. (Another option would be to use a PropertyValueFactory, but this is not type-safe).

    Note:

    We're only using StringProperty values for our table columns in this example. When you want to use IntegerProperty or DoubleProperty, the setCellValueFactory(...) must have an additional asObject():

    myIntegerColumn.setCellValueFactory(cellData ->
        cellData.getValue().myIntegerProperty().asObject());
    

    This is necessary because of a bad design decision of JavaFX (see this discussion for more details).

    Connecting MainApp with the PersonOverviewController

    The setMainApp(...) method must be called by the MainApp class. This gives us a way to access the MainApp object and get the list of Persons and other things. Add the following three lines to showPersonOverview() the method:

    MainApp.java - additional lines to add to showPersonOverview() method

    // (.... root layout statement goes here ....)
    
    // Give the controller access to the main app.
        PersonOverviewController controller = loader.getController();
        controller.setMainApp(this);
    
    // (.... catch statement goes here ....)
    

    Your showPersonOverview() method in MainApp should now look like this:

    MainApp.java - new showPersonOverview() method

    /**
    * Shows the person overview inside the root layout.
    */
    public void showPersonOverview() {
        try {
            // Load person overview.
            FXMLLoader loader = new FXMLLoader();
            loader.setLocation(MainApp.class.getResource("view/PersonOverview.fxml"));
            AnchorPane personOverview = loader.load();
    
            // Set person overview into the center of root layout.
            rootLayout.setCenter(personOverview);
    
            // Give the controller access to the main app.
            PersonOverviewController controller = loader.getController();
            controller.setMainApp(this);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    

    Hook the View to the controller

    We're almost there! But one thing is missing: We haven't told our PersonOverview.fxml file which controller to use, and which element should match to which field inside the controller.

    1. Open PersonOverview.fxml with the SceneBuilder.
    2. Open the Controller group on the left side (just below Hierarchy), and select the seedu.address.view.PersonOverviewController as the controller class.

    1. Select the TableView in the Hierarchy group.
    2. In the Inspector view, under the Code group, set 'fx:id' to personTable.

    1. Do the same for the table columns. Select firstNameColumn and lastNameColumn for the 'fx:id' respectively.
    2. For each label in the second column of the grid pane, choose the corresponding 'fx:id'.

    1. Save the .fxml file.

    Opening up the PersonOverviewController to JavaFx

    If you try and run the application now, you will encounter the following error:

    javafx.fxml.LoadException: ...
    
    ...
    
    Caused by: java.lang.IllegalAccessException: class javafx.fxml.FXMLLoader$ValueElement (in module javafx.fxml) cannot access class seedu.address.view.PersonOverviewController (in module AddressApp) because module AddressApp does not export seedu.address.view to module javafx.fxml
    

    This is because JavaFx is unable to access our PersonOverviewController class.

    To fix this, add this line of code to src/module-info.java:

    module AddressApp {
        ... 
    
        opens seedu.address.view;
    }
    

    The file should now look something like this:

    module-info.java

    module AddressApp {
        requires javafx.graphics;
        requires javafx.fxml;
        requires javafx.controls;
    
        exports seedu.address;
        opens seedu.address.view;
    }
    

    Start the Application

    When you start your application now, you should see something like this:

    Congratulations! The application now shows the list of Persons in the view!

    You may notice that selecting a person in the TableView does nothing to the labels at the right side. That is because the user interaction portion has not been programmed yet, which we will cover in the next part of the tutorial.

    After going through the two parts above, you should be familiar with building basic JavaFX GUIs using IntelliJ. You can continue with the original tutorial (which is written for Eclipse), with the following links:

    Streams: Basic

    Can use Java8 streams

    Java 8 introduced a number of new features (e.g. Lambdas, Streams) that are not trivial to learn but also extremely useful to know.

    Here is an overview of new Java 8 features . (written by Benjamin Winterberg)

    Tutorials:

     


     

    A video tutorial by well-known Java coach Venkat Subramaniam