Question

在Java 7中，对于给定的代码：

final Integer i=9;
final int x=5;

switch(x){
case 1:
case i://compilation error is thrown here 
}

这背后的原因是什么？

Answer 1

Integer i = 9;

这个i现在是对Integer对象的引用，它不是在Java中打开的有效类型。

以下是有效的变量类型，您可以启用

Convertible ints - int，byte，short，char
枚举
字符串常量 - Java 7中添加的支持

除了这些有效值之外，您不能只打开任何其他对象

Answer 2

在许多语言中， switch语句表示对一组编译时常量的持续时间查找。

javac编译器将switch语句转换为非常有效的字节码表示，对于该优化，编译时常量是必要的。

Answer 3

这就是我寻找的原因：

诸如Integer之类的包装器对象不能在case语句中使用，因为它们不是编译时常量（因为装箱和装箱在运行时发生）。因此，您只能使用编译时常量的原语也必须是最终的。

Answer 4

1）switch case不支持任何Object类型，更多支持原始变量作为常量和文字。

2）从Java 7开始，它支持字符串。

<?
    $con = mysql_connect('localhost','root','');
    mysql_select_db('social_net', $con) or die('error in(application/view/chatting.php) at line 3');
?>
<!DOCTYPE html>
<html lang="en">
  <head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="description" content="">
<meta name="author" content="">
<link rel="shortcut icon" href="facivon.ico">

<title>Facebook like chat</title>
<link href="style.css" rel="stylesheet">
<style>

.chat_box{
    position:fixed;
    right:20px;
    bottom:0px;
    width:250px;
}
.chat_body{
    background:white;
    height:400px;
    overflow-y:scroll;
    border-right: 1px solid rgba(0, 0, 0, 0.28);
    border-left: 1px solid rgba(0, 0, 0, 0.28);
    padding:5px 0px;
}

.chat_head,.msg_head{
    background:#f39c12;
    color:white;
    padding:15px;
    -moz-user-select:none;
    user-select:none;
    font-weight:bold;
    cursor:pointer;
    border-radius:5px 5px 0px 0px;
}

.msg_box{
    position:fixed;
    bottom:-5px;
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    border-left: 1px solid rgba(0, 0, 0, 0.28);
    width:250px;
    background:white;
    border-radius:5px 5px 0px 0px;
}

.msg_head{
    background:#3498db;
    -moz-user-select:none;
    user-select:none;
}

.msg_body{
    background:white;
    height:200px;
    font-size:12px;
    padding:15px;
    overflow:auto;
    overflow-x: hidden;
}
.msg_input{
    width:100%;
    border: 1px solid white;
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    -webkit-box-sizing: border-box; /* Safari/Chrome, other WebKit */
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}

.close{
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}
.minimize{
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}

.user{
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}
.user:hover{
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}
.user-0{
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    border-radius:4px;
}
.user-1{
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    background:#2ecc71;
    height:7px;
    width:7px;
    left:10px;
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    border-radius:4px;
}

.msg_a{
    position:relative;
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.msg_a:before{
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.msg_b{
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    padding:10px;
    min-height:15px;
    margin-bottom:5px;
    position:relative;
    margin-left:10px;
    border-radius:5px;
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}
.msg_b:after{
    content:"";
    position:absolute;
    width:0px;
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    border: 10px solid;
    border-color: transparent transparent transparent #EEF2E7;
    right:-20px;
    top:7px;
}
#finfrndslink
{
    display:block;
    padding:.6em;
    margin:0px auto;
    width:94%;
    margin-top: 10em;
    background: rgb(54, 42, 42) none repeat scroll 0% 0%;
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}
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  <body>
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    $user_name = $this->session->userdata('user');
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  <div class="chat_box">
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    if($count!=0)
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        $i=1;
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                <div class="user" id="u<?=$i?>" style="text-transform:capitalize"><span class="user-<?=$row['logged_in'];?>"></span><?=$row['f_name'].' '.$row['l_name']?></div>
            <?
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        <?
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                            $('<div class="msg_b">'+msg+'</div>').insertBefore('#msg_push<?=$i?>');
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                    <div class="msg_head" id="mh<?=$i?>">
                    <font style="text-transform:capitalize"><?=$row1['f_name'].' '.$row1['l_name']?></font>
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                    <div class="msg_wrap" id="mw<?=$i?>">
                        <div class="msg_body" id="msg_body<?=$i?>">
                            <div class="msg_a">This is from A   </div>
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                            <div class="msg_push" id="msg_push<?=$i?>"></div>
                        </div>
                    <div class="msg_footer"><textarea class="msg_input" id="ti<?=$i?>" placeholder="write your message here" rows="2"></textarea></div>
                    </div>
                </div>
            <?
            $i++;
        }
    ?>
  </div>
</body>
</html>

为什么我们不能将包装类对象作为常量？

4 个答案: