如何删除GridPane的前一个元素（按钮）并在javaFx中添加其他元素？

Question

在javaFx中，我有类别GridPane，其中我在初始化时动态地加载了按钮。
See Image contains one Category GridPane and another Items GridPane

现在，当我点击任何类别按钮时，例如＆＃39;类别2＆＃39;然后它将加载Items GridPane中类别2的所有项目（按钮）。

然后，当我点击另一个类别时，它不会删除Items GridPane的旧元素。 Items GridPane的旧元素保持不变。

那么如何删除Items GridPane的旧元素并添加新Category的新元素。

public class GenerateBillController implements Initializable {

    int m1=0,n1=0,m2=0,n2=0,z1=0,z2=0,y1=0,y2=0;

    @FXML
    GridPane gpCategory;

    @FXML
    GridPane gpItems;

    @Override
    public void initialize(URL arg0, ResourceBundle arg1) {
    try {
        cat=lm.getCategory();
        list = FXCollections.observableArrayList();
        for(int i=0;i<cat.length;i++)
        {
            list.add(cat[i]);
        }
        while(z1<16){  //add 16 buttons to Category GridPane
            addCatButton();
            z1++;
        }


    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    }


    private void addCatButton() {
    // TODO Auto-generated method stub

    final Button temp = new Button("Button " + y1);
    final int numButton= y1;
    temp.setId("" + y1);
    temp.setText(cat[z1]);
    temp.setOnAction(new EventHandler<ActionEvent>() {
    @Override
    public void handle(ActionEvent e) {
        System.out.println("id("+temp.getId()+")="+numButton+"->"+temp.getText());
    try {
        //gpItems.getChildren().removeAll(list);
        items=lm.getItems(temp.getText());
        for(int i=0;i<items.length;i++)
        {
            list2.add(items[i]);
        }
        while(z2<9)
        {
            addItemButton();
            z2++;
        }
    } catch (SQLException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    }
}

private void addItemButton() {
    // TODO Auto-generated method stub
    final Button temp1=new Button("Button "+y2);
    final int nb=y2;
    temp1.setId(""+y2);
    temp1.setText(items[z2]);
    temp1.setOnAction(new EventHandler<ActionEvent>() {
    @Override
    public void handle(ActionEvent arg0) {
        // TODO Auto-generated method stub
        System.out.println("id("+temp1.getId()+")= "+nb+"-> " +temp1.getText());
        try {
            String p=lm.getPrice(temp1.getText());
            if(p=="")
            {

            }
            else
            {
            }
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

});
gpItems.add(temp1, m2, n2);

if(m2==2)
{
    m2=-1;
    n2++;
}
m2++;   
y2++;
}
});
gpCategory.add(temp, m1, n1);
if(m1==3)
{
    m1=-1;
    n1++;
}
m1++;   
y1++;
}