定义字符串常量时出错

Question

给定：

public static final String XML_POLICY =
            "<?xml version="1.0"?>"
            +"<!DOCTYPE cross-domain-policy SYSTEM "/xml/dtds/cross-domain-policy.dtd">"
            + "<cross-domain-policy>"
            + "<allow-access-from domain="*" to-ports="*" />"
            + "</cross-domain-policy>";

上出现编译错误。

"<?xml version="1.0"?>"

它说';‘是预期的。

等等

+"<!DOCTYPE cross-domain-policy SYSTEM "/xml/dtds/cross-domain-policy.dtd">"

上面写着“找不到symbol，symbol: class xml and class dtds”

可能的原因是什么？IDE是Netbeans 6.8 Beta。

Answer 1

您需要转义双引号-而且不只是第一行。doctype和allow-access-from行也需要它。

// Reformatted slightly to avoid scrolling :)
public static final String XML_POLICY =
  "<?xml version=\"1.0\"?>"
  + "<!DOCTYPE cross-domain-policy SYSTEM \"/xml/dtds/cross-domain-policy.dtd\">"
  + "<cross-domain-policy>"
  + "<allow-access-from domain=\"*\" to-ports=\"*\" />"
  + "</cross-domain-policy>";

另一种方法是在XML中使用单引号，这在大多数情况下都很好，并使代码更易于阅读：

public static final String XML_POLICY =
    "<?xml version='1.0'?>"
    +"<!DOCTYPE cross-domain-policy SYSTEM '/xml/dtds/cross-domain-policy.dtd'>"
    + "<cross-domain-policy>"
    + "<allow-access-from domain='*' to-ports='*' />"
    + "</cross-domain-policy>";

注意，这并不是特定于Netbeans或XML的。双引号用于指示字符串的开始和结束，这就是为什么如果您想要中间有一个字符串，就需要对其进行转义。例如：

String x = "I said, \"Hello.\" The child laughed.";

Java中还有各种各样的其他转义序列--更多细节请参见section 3.10.6 of the language specification。

Answer 2

您必须转义字符串中的“

public static final String XML_POLICY =     
        "<?xml version=\"1.0\"?>" + // etc etc

Answer 3

read the section on escape sequences