如果文本不存在或为空,为什么BasicComboBoxRenderer使用getPreferredSize()设置文本两次?

通过阅读 source code

您可以看到一条解释此类行为的评论:

  /**
  77:    * Returns preferredSize of the renderer
  78:    * 
  79:    * @return preferredSize of the renderer
  80:    */
  81:   public Dimension getPreferredSize()
  82:   {
  83:     if (this.getText() != null && ! this.getText().equals(""))
  84:       return super.getPreferredSize();
  85:     else
  86:       {
  87:         // If the combo box option's text is empty or null, it won't size
  88:         // properly (ie, it'll be way too short)... so we throw in a dummy
  89:         // space to trick the superclass's sizing methods.
  90:         String oldText = this.getText();
  91:         this.setText(" ");
  92:         Dimension d = super.getPreferredSize();
  93:         this.setText(oldText);
  94:         return d;
  95:       }
  96:   }