Java - placeholder on textfield

问题:

I'm trying to create a GUI with Swing. My problem is, I have a textfield, but I want it to have a "placeholder" (like in html). I read here and there that it can be done by overriding the paint() of the textfield.

Since my code is generated I found out that I need to use the "Custom Creation Code" to override the code that was generated.

Here is what I have put in the "Custom Creation Code" field

new javax.swing.JTextField()
{
    String test = super.getText();
    String hint = "Username";

    public void paint(Graphics g)
    {
        if ( test == null || test.length() < 1 ) {
            g.setColor( Color.red );
            g.drawString(hint, 0, 0);
        }

        g.setColor(Color.BLACK);
        super.paint(g);
    }
}

This generates the following output

javax.swing.JTextField username = new javax.swing.JTextField()
{
    String test = super.getText();
    String hint = "Username";

    public void paint(Graphics g)
    {
        if ( test == null || test.length() < 1 ) {
            g.setColor( Color.red );
            g.drawString(hint, 0, 0);
        }

        g.setColor(Color.BLACK);
        super.paint(g);
    }
};

For now I see the textField but there is nothing in it, maybe I need to add some function onto some event, but I am not sure.

I would be grateful if anyone could lend a hand.

EDIT : Here is a demo of what I want to do : http://davidwalsh.name/demo/html5-placeholder.php

回答1:

I found this on the oracle forums.

public class TextFieldWithPrompt extends JTextField{

@Override
protected void paintComponent(java.awt.Graphics g) {
    super.paintComponent(g);

    if(getText().isEmpty() && ! (FocusManager.getCurrentKeyboardFocusManager().getFocusOwner() == this)){
        Graphics2D g2 = (Graphics2D)g.create();
        g2.setBackground(Color.gray);
        g2.setFont(getFont().deriveFont(Font.ITALIC));
        g2.drawString("zip", 5, 10); //figure out x, y from font's FontMetrics and size of component.
        g2.dispose();
    }
  }

https://forums.oracle.com/forums/thread.jspa?threadID=1349874

回答2:

I use to override the text fields paint method, until I ended up with more custom text fields then I really wanted...

Then I found this prompt API which is simple to use and doesn't require you to extend any components. It also has a nice "buddy" API

This has now been included in the SwingLabs, SwingX library which makes it even eaiser to use...

回答3:

Try this.

private void txtUserNameFocusGained(java.awt.event.FocusEvent evt) {                                        
    String username = txtUserName.getText();
    if(username.equals("User Name")){
        txtUserName.setCaretPosition(0);
    }

}                                       

private void txtUserNameFocusLost(java.awt.event.FocusEvent evt) {                                      
    String username = txtUserName.getText();
    if(username.equals("")){
        txtUserName.setForeground(new java.awt.Color(86, 86, 86));
        txtUserName.setCaretPosition(0);
        txtUserName.setText("User Name");
    }
}                                     

private void txtUserNameKeyPressed(java.awt.event.KeyEvent evt) {                                       
    String username = txtUserName.getText();
    if(username.equals("User Name")){
        txtUserName.setForeground(new java.awt.Color(0, 0, 0));
        txtUserName.setText(null);
        txtUserName.setCaretPosition(0);
    }
}

Be adviced, the name of the text field is "txtUserName". You can see the output as like this.