[FIXED] Binding the sum of an observableList of BigDecimals to a Label

Issue

I need to get two specific fields of every object inside an ObservableList, multiply them and get the sum of all their products. I also need to keep the sum updated

For now I'm trying to get the sum of only one of those fields first (just because its easier and might help me to understand what is happening).

I have discovered:

• How to bind an ObjectBinding<BigDecimal> to a Label with a Formatter?

which is almost what I'm trying to do. However, applying the same code will throw a NullPointerException. Which I assume it's because there was more behind the scene.

Also, according to:

• Javafx How to make a Binding to a list of Properties

the observableList needs to have and extractor so it can track the updates of the required field on each element.

Here is what I tried, only ONE field first!!:

// Graphical stuff
void createExempleMenu(){

ObservableList<Itens> orc = FXCollections.observableArrayList(o -> new Observable[] {o.valueProperty()});
Label valueFinalLabel;

valueFinalLabel.textProperty().bind( // this gets NullPointerException
                Bindings.createObjectBinding(() ->{ 
                if(orc==null || orc.isEmpty())
                    return BigDecimal.ZERO; // case orc is empty
                else
                    return orc.stream().map(i->i.getValue()).reduce(BigDecimal.ZERO,BigDecimal::add);},
                        
                orc).asString()
        );
}

---

// item
class Itens {
    private ObjectProperty<BigDecimal> value;
    public BigDecimal getValue() {
        return value.get();
    }
    public void setValue(BigDecimal b){
        value.set(b);
    }
    public void valueProperty(BigDecimal b){
        return value;
    }
}

Solution

You have a few issues with the Itens class:

• You never initialize value.
• The method valueProperty(BigDecimal b) must return ObjectProperty<BigDecimal> instead of void.
• You have a typo in method getValue(), instead of returning value.get() you are returning valor.get().

class Itens {

    private final ObjectProperty<BigDecimal> value = 
            new SimpleObjectProperty<>(this, "value");

    Itens(BigDecimal value) {
        this.value.set(value);
    }

    public BigDecimal getValue() {
        return valueProperty().get();
    }
    public void setValue(BigDecimal value){
        valueProperty().set(value);
    }
    public ObjectProperty<BigDecimal> valueProperty(){
        return value;
    }

}

Also, you never initialize the Label.

Finally, you can improve your Bindings:

• You can create a StringBinding directly instead of creating an ObjectBinding and then creating StringBinding.
• orc is never null, so you don't need to check for null.
• You don't need to check for orc.isEmpty() because when you reduce the stream you are supplying an identity `BigDecimal.ZERO

Label valueFinalLabel = new Label();

ObservableList<Itens> orc = FXCollections.observableArrayList(
        obs -> new Observable[] {obs.valueProperty()});

valueFinalLabel.textProperty().bind(
        Bindings.createStringBinding(() -> orc.stream()
                .filter(Objects::nonNull)
                .map(Itens::getValue)
                .reduce(BigDecimal.ZERO, BigDecimal::add).toString(), orc));

Test:

valueFinalLabel.textProperty().addListener((obs, oldVal, newVal) -> 
        System.out.println(String.format("Label change from %s to %s", 
                oldVal, newVal)));

Itens i1 = new Itens(BigDecimal.valueOf(1));
Itens i2 = new Itens(BigDecimal.valueOf(2));
Itens i3 = new Itens(BigDecimal.valueOf(3));

orc.add(i1);
orc.add(i2);
orc.add(i3);

i1.setValue(BigDecimal.valueOf(4));

Output:

Label change from 0 to 1
Label change from 1 to 3
Label change from 3 to 6
Label change from 6 to 9

Answered By - Oboe
Answer Checked By - Marie Seifert (JavaFixing Admin)