类型不匹配：无法从 StringBuilder 转换为 String

此方法返回给定 URL 的源。

private static String getUrlSource(String url) {
    try {
        URL localUrl = null;
        localUrl = new URL(url);
        URLConnection conn = localUrl.openConnection();
        BufferedReader reader = new BufferedReader(
            new InputStreamReader(conn.getInputStream()));
        String line = "";
        String html;
        StringBuilder ma = new StringBuilder();
        while ((line = reader.readLine()) != null) {
            ma.append(line);
        }
        return ma;
    } catch (Exception e) {
        Log.e("ERR",e.getMessage());
    }
}

它给了我这个错误：

Type mismatch: cannot convert from StringBuilder to String

还有两个选择：

1. Change the return type to StringBuilder.但我希望它返回一个字符串。
2. Change type of ma to String.更改字符串后没有append()方法。

Just use

return ma.toString();

代替

return ma;

ma.toString()返回 StringBuilder 的字符串表示形式。

See StringBuilder#toString() http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/StringBuilder.html#toString()欲了解详情

正如 Valeri Atamaniouk 在评论中建议的那样，您还应该在catch块，否则你会得到一个编译器错误missing return statement，所以编辑

} catch (Exception e) {
    Log.e("ERR",e.getMessage());
}

to

} catch (Exception e) {
    Log.e("ERR",e.getMessage());
    return null; //or maybe return another string
}

将是一个好主意。

EDIT

正如 Esailija 所建议的，我们在这段代码中有三个反模式

} catch (Exception e) {           //You should catch the specific exception
    Log.e("ERR",e.getMessage());  //Don't log the exception, throw it and let the caller handle it
    return null;                  //Don't return null if it is unnecessary
}

所以我认为最好做这样的事情：

private static String getUrlSource(String url) throws MalformedURLException, IOException {
    URL localUrl = null;
    localUrl = new URL(url);
    URLConnection conn = localUrl.openConnection();
    BufferedReader reader = new BufferedReader(
            new InputStreamReader(conn.getInputStream()));
    String line = "";
    String html;
    StringBuilder ma = new StringBuilder();
    while ((line = reader.readLine()) != null) {
        ma.append(line);
    }
    return ma.toString();
}

然后，当你调用它时：

try {
    String urlSource = getUrlSource("http://www.google.com");
    //process your url source
} catch (MalformedURLException ex) {
    //your url is wrong, do some stuff here
} catch (IOException ex) {
    //I/O operations were interrupted, do some stuff here
}

检查以下链接以获取有关 Java 反模式的更多详细信息：

• Java 反模式 http://www.odi.ch/prog/design/newbies.php
• 编程反模式 http://www.javacodegeeks.com/2011/10/programming-antipatterns.html
• Java 应用程序中的反模式简介 http://www.devx.com/Java/Article/29162