为什么用document.getElementById得不到对象总是返回null呢？什么原因,该如何处理 _JavaScript

为什么用document.getElementById得不到对象总是返回null呢？什么原因

for (var d = 0; d < 4; d++){
var t_obj_current = document.getElementById( "t "+d);
if (t_obj_current != null){
t_obj_current.className= "affiche_tab_nosel ";
}
}

<table cellpadding= "0 " cellspacing= "0 " border= "0 ">
<tr>
<td width= "20px " height= "20px " align= "center " class= "affiche_tab_sel " id= "t0 "> 今 </td>
<td width= "2px "> </td>
<td width= "20px " align= "center " class= "affiche_tab_nosel " id= "t1 "> 昨 </td>
<td width= "2px "> </td>
<td width= "20px " align= "center " class= "affiche_tab_nosel " id= "t2 "> 前 </td>
<td width= "2px "> </td>
<td width= "20px " align= "center " class= "affiche_tab_nosel " id= "t3 "> ... </td>
</tr>
</table>

------解决方案--------------------
for (var d = 0; d < 4; d++){
var t_obj_current = document.getElementById( "t "+d);
if (t_obj_current != null){
t_obj_current.className= "affiche_tab_nosel ";
}
}
这段js代码应该放在下面执行。放在上面的时候页面初始化还没有完成，程序当然不知道有t1、t2这些对象了
------解决方案--------------------
呵呵，楼上的正解，我的代码全部放在下面
------解决方案--------------------
不放在下面也可以：
<html>
<head>
<script type= "text/javascript ">
function wiliner()
{
for (var d = 0; d < 4; d++){
var t_obj_current = document.getElementById( "t "+d);
alert(t_obj_current);
if (t_obj_current != null){
t_obj_current.className= "affiche_tab_nosel ";
}
}
}

</script>
</head>
<body onload= "wiliner(); ">
<table cellpadding= "0 " cellspacing= "0 " border= "0 ">
<tr>
<td width= "20px " height= "20px " align= "center " class= "affiche_tab_sel " id= "t0 "> 今 </td>
<td width= "2px "> </td>
<td width= "20px " align= "center " class= "affiche_tab_nosel " id= "t1 "> 昨 </td>
<td width= "2px "> </td>
<td width= "20px " align= "center " class= "affiche_tab_nosel " id= "t2 "> 前 </td>
<td width= "2px "> </td>
<td width= "20px " align= "center " class= "affiche_tab_nosel " id= "t3 "> ... </td>
</tr>
</table>