JavaFX Label doesn't set text

Question

I'm doing a Calendar with JavaFX SceneBuilder.

I declare my Label on my Main class:

public class Main extends Application {
//Calendar used
public Calendar cal;
//variables for get the number of week+current year
public static int weeks, year;
@FXML
//Text areas for the 4 weeks column
public TextArea week1,week2,week3,week4;
@FXML 
//4 control Buttons
public Button prev,next,next2,prev2, start;
@FXML
//Labels
public Label lab1;

And the method which I'm trying to set text is this one:

public void ClickStart (ActionEvent event){
    lab1=new Label();
    lab1.setText("hola");
    cal=Calendar.getInstance();
    year = cal.get(Calendar.YEAR);
    weeks= cal.get(Calendar.WEEK_OF_YEAR);
    week1.setText("Week " + Integer.toString(weeks) + " " + Integer.toString(year));
    cal.add(Calendar.WEEK_OF_YEAR, 1);
    weeks= cal.get(Calendar.WEEK_OF_YEAR);
    year = cal.get(Calendar.YEAR);
    week2.setText("Week " + Integer.toString(weeks) + " " + Integer.toString(year));
    cal.add(Calendar.WEEK_OF_YEAR, 1);
    weeks= cal.get(Calendar.WEEK_OF_YEAR);
    year = cal.get(Calendar.YEAR);
    week3.setText("Week " + Integer.toString(weeks) + " " + Integer.toString(year));
    cal.add(Calendar.WEEK_OF_YEAR, 1);
    weeks= cal.get(Calendar.WEEK_OF_YEAR);
    year = cal.get(Calendar.YEAR);
    week4.setText("Week " + Integer.toString(weeks) + " " + Integer.toString(year));
    cal.add(Calendar.WEEK_OF_YEAR, -3);
    }

I include also my .fmxl file if necessary:

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.VBox?>

<GridPane xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/2.2" fx:controller="application.Main">
  <children>
    <GridPane>
      <children>
        <GridPane>
          <children>
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Monday" wrapText="true" GridPane.columnIndex="1" GridPane.rowIndex="0" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Tuesday" wrapText="true" GridPane.columnIndex="2" GridPane.rowIndex="0" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Wednesday" wrapText="true" GridPane.columnIndex="3" GridPane.rowIndex="0" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Thursday" wrapText="true" GridPane.columnIndex="4" GridPane.rowIndex="0" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Friday" wrapText="true" GridPane.columnIndex="5" GridPane.rowIndex="0" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Saturday" wrapText="true" GridPane.columnIndex="6" GridPane.rowIndex="0" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Sunday" wrapText="true" GridPane.columnIndex="7" GridPane.rowIndex="0" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Monday" wrapText="true" GridPane.columnIndex="1" GridPane.rowIndex="9" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Tuesday" wrapText="true" GridPane.columnIndex="2" GridPane.rowIndex="9" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Wednesday" wrapText="true" GridPane.columnIndex="3" GridPane.rowIndex="9" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Thursday" wrapText="true" GridPane.columnIndex="4" GridPane.rowIndex="9" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Friday" wrapText="true" GridPane.columnIndex="5" GridPane.rowIndex="9" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Saturday" wrapText="true" GridPane.columnIndex="6" GridPane.rowIndex="9" />
            <TextArea editable="false" mouseTransparent="false" prefWidth="200.0" text="Sunday" wrapText="true" GridPane.columnIndex="7" GridPane.rowIndex="9" />
            <Button id="prev" fx:id="prev2" mnemonicParsing="false" onAction="#ClickMinus" prefHeight="30.0" prefWidth="70.0" text="prev" GridPane.columnIndex="8" GridPane.rowIndex="0" />
            <Button fx:id="next" mnemonicParsing="false" onAction="#ClickPlus" prefHeight="29.999900000002526" prefWidth="70.00009999999747" text="next" GridPane.columnIndex="0" GridPane.rowIndex="9" />
            <Button fx:id="next2" mnemonicParsing="false" onAction="#ClickPlus" prefHeight="30.0" prefWidth="70.0" text="next" GridPane.columnIndex="8" GridPane.rowIndex="9" />
            <Button fx:id="prev" mnemonicParsing="false" onAction="#ClickMinus" prefHeight="30.0" prefWidth="70.0" text="prev" GridPane.columnIndex="0" GridPane.rowIndex="0" />
            <TextArea fx:id="week1" prefWidth="200.0" text="Week x Year x" wrapText="true" GridPane.columnIndex="0" GridPane.rowIndex="2" />
            <TextArea fx:id="week2" prefWidth="200.0" text="Week x Year x" wrapText="true" GridPane.columnIndex="0" GridPane.rowIndex="4" />
            <TextArea fx:id="week4" prefWidth="200.0" text="Week x Year x" wrapText="true" GridPane.columnIndex="0" GridPane.rowIndex="8" />
            <Label  id= "lab1" fx:id="lab1" prefHeight="40.000099999997474" prefWidth="150.0" text="" GridPane.columnIndex="1" GridPane.rowIndex="1" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="2" GridPane.rowIndex="1" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="3" GridPane.rowIndex="1" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="4" GridPane.rowIndex="1" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="5" GridPane.rowIndex="1" />
            <Label minHeight="14.0" prefHeight="14.0" prefWidth="150.0" text="Label" GridPane.columnIndex="1" GridPane.rowIndex="3" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="2" GridPane.rowIndex="3" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="3" GridPane.rowIndex="3" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="2" GridPane.rowIndex="5" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="1" GridPane.rowIndex="5" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="3" GridPane.rowIndex="5" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="4" GridPane.rowIndex="5" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="4" GridPane.rowIndex="3" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="5" GridPane.rowIndex="3" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="5" GridPane.rowIndex="5" />
            <Label prefHeight="533.0" prefWidth="70.0" text="Label" GridPane.columnIndex="1" GridPane.rowIndex="7" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="2" GridPane.rowIndex="7" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="6" GridPane.rowIndex="1" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="7" GridPane.rowIndex="1" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="6" GridPane.rowIndex="3" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="3" GridPane.rowIndex="7" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="4" GridPane.rowIndex="7" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="5" GridPane.rowIndex="7" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="6" GridPane.rowIndex="5" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="6" GridPane.rowIndex="7" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="7" GridPane.rowIndex="7" />
            <Label prefHeight="44.0" prefWidth="87.0" text="Label" GridPane.columnIndex="7" GridPane.rowIndex="5" />
            <Label prefHeight="44.0" prefWidth="139.0" text="Label" GridPane.columnIndex="7" GridPane.rowIndex="3" />
            <TextArea fx:id="week3" prefHeight="100.00009999999747" prefWidth="70.0" text="Week x Year x" wrapText="true" GridPane.columnIndex="0" GridPane.rowIndex="6" />
            <Button fx:id="start" mnemonicParsing="false" onAction="#ClickStart" prefHeight="30.0" prefWidth="70.0" text="Start" GridPane.columnIndex="0" GridPane.rowIndex="1" />
          </children>
          <columnConstraints>
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="70.0" />
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="150.0" />
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="150.0" />
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="150.0" />
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="150.0" />
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="150.0" />
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="150.0" />
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="150.0" />
            <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" prefWidth="70.0" />
          </columnConstraints>
          <rowConstraints>
            <RowConstraints minHeight="10.0" prefHeight="40.0" vgrow="SOMETIMES" />
            <RowConstraints minHeight="10.0" prefHeight="40.0" vgrow="SOMETIMES" />
            <RowConstraints minHeight="10.0" prefHeight="100.0" vgrow="SOMETIMES" />
            <RowConstraints minHeight="10.0" prefHeight="40.0" vgrow="SOMETIMES" />
            <RowConstraints minHeight="10.0" prefHeight="100.0" vgrow="SOMETIMES" />
            <RowConstraints minHeight="10.0" prefHeight="40.0" vgrow="SOMETIMES" />
            <RowConstraints minHeight="10.0" prefHeight="100.0" vgrow="SOMETIMES" />
            <RowConstraints minHeight="10.0" prefHeight="40.0" vgrow="SOMETIMES" />
            <RowConstraints minHeight="10.0" prefHeight="100.0" vgrow="SOMETIMES" />
            <RowConstraints minHeight="10.0" prefHeight="40.0" vgrow="SOMETIMES" />
          </rowConstraints>
        </GridPane>
      </children>
      <columnConstraints>
        <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" />
      </columnConstraints>
      <rowConstraints>
        <RowConstraints minHeight="10.0" vgrow="SOMETIMES" />
      </rowConstraints>
    </GridPane>
  </children>
  <columnConstraints>
    <ColumnConstraints hgrow="SOMETIMES" minWidth="10.0" />
  </columnConstraints>
  <rowConstraints>
    <RowConstraints minHeight="10.0" vgrow="SOMETIMES" />
  </rowConstraints>
</GridPane>

The problem is that when I run the application and I press my Start button, the label doesn't change to the text given(By default is on 'empty').

Answer 1

Since you already have the solution in the comments, I'll just give you a short explanation of why deleting

lab1 = new Label(); 

solved the problem.

When you declared lab1 in your fxml file you told the JavaFX FXMLLoader to instantiate thisLabel for you. So essentially it did the new Label() call for you. It then passes a reference to the Label that it created to the variable lab1 of your class Main. When you changed lab1 to reference your newly created label in lab1 = new Label(); you overwrote the reference to the label that is in your UI. That new label is not yet added to any Scene and is not shown, so when you change its text, the change is not visible.