SetSwingDefaultFont.java

public static void setSwingFontDefault(javax.swing.plaf.FontUIResource newFont) {
  java.util.Enumeration<Object> uiManagerKeys = UIManager.getDefaults().keys();
  while (uiManagerKeys.hasMoreElements()) {
    Object key = uiManagerKeys.nextElement();
    Object value = UIManager.get(key);

    if (value instanceof javax.swing.plaf.FontUIResource)
      UIManager.put(key, newFont);
  }
}

//Example call
setSwingFontDefault(new FontUIResource("Serif", Font.PLAIN, 16));
    
//Or to directly set a font for just ONE component, such as Labels
UIManager.put("Label.font",new Font("Serif",Font.ITALIC,12));