Question

此方法返回给定URL的来源。

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：

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

Answer 1

只需使用

return ma.toString();

而不是

return ma;

ma.toString()返回StringBuilder的字符串表示形式。

有关详细信息，请参阅StringBuilder#toString()

正如Valeri Atamaniouk在评论中建议的那样，你也应该在catch块中返回一些东西，否则你会得到missing return statement的编译错误，所以编辑

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

到

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

这是一个好主意。

修改

正如Esailija所说，我们在此代码中有三种反模式

} 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 }

所以我觉得做这样的事情会更好：

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，当你打电话给它时：

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 }

检查这些链接以获取有关Java反模式的更多详细信息：

Java Anti-Patterns

Programming Anti-Patterns

An Introduction to Antipatterns in Java Applications

Answer 2

尝试

返回ma.toString（）; 因为您不能直接将stringbuilder变量存储到字符串变量中。

类型不匹配：无法从StringBuilder转换为String

2 个答案: