Example of forcing alignment in a FlowPane.

FlowSpacer.java

package com.isp.stackoverflow;

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.FlowPane;
import javafx.stage.Stage;

/**
 * Example of forcing alignment in a FlowPane.
 *
 * @author okr
 * @date 19.01.2016
 *
 */
public class FlowSpacer extends Application
{

  @Override
  public void start( final Stage primaryStage )
  {
    final Button btn1 = new Button( "One" );
    final Button btn2 = new Button( "Two" );
    final Button btn3 = new Button( "Three" );
    final Button btn4 = new Button( "Four" );
    final Label label = new Label( "My Label" );
    final FlowPane root = new FlowPane( btn1, btn2, label, btn3, btn4 );
    final Scene scene = new Scene( root );

    scene.widthProperty().addListener( ( observable, oldWidth, newWidth ) ->
    {
      final double spacerMargin = newWidth.doubleValue()
          - scene.getRoot().getChildrenUnmodifiable().stream().mapToDouble( node -> node.getLayoutBounds().getWidth() ).sum();
      FlowPane.clearConstraints( btn3 );
      FlowPane.setMargin( btn3, new Insets( 0, 0, 0, spacerMargin ) );
    } );

    primaryStage.setScene( scene );
    primaryStage.show();
  }

  /**
   * @param args ignored.
   */
  public static void main( final String[] args )
  {
    launch( args );
  }
}