我有一个位于assets 文件夹中的大文本文件，并且我有一个按钮可以成功读取文件的下一行。但是，如果用户单击另一个按钮，我想阅读上一行。
将整个文件读入内存不是一种选择。文件行没有编号。
【问题讨论】：
InputStream is = getResources().getAssets().open("abc.txt");
String result= convertStreamToString(is);
public static String convertStreamToString(InputStream is)
throws IOException {
Writer writer = new StringWriter();
char[] buffer = new char[2048];
try {
Reader reader = new BufferedReader(new InputStreamReader(is,
"UTF-8"));
int n;
while ((n = reader.read(buffer)) != -1) {
writer.write(buffer, 0, n);
}
} finally {
is.close();
}
String text = writer.toString();
return text;
}
【讨论】：
StringBuilder 可能比上面示例中使用的StringWriter 更好，否则它应该可以正常工作。
如果您只需要跟踪前一行，您可以执行以下操作，在每次迭代中跟踪前一行（我假设您使用的是阅读器；对于本示例，BufferedReader）：
String previous = null, line; // null means no previous line
while (line = yourReader.readLine()) {
// Do whatever with line
// If you need the previous line, use:
if (yourCondition) {
if (previous != null) {
// Do whatever with previous
} else {
// No previous line
}
}
previous = line;
}
如果您需要跟踪多个前一行，则可能必须将其扩展为一个数组，但如果您的文件很大，您将在内存中保留大量内存——就像您将阅读整个文件，一旦你到达最后一行。
在 Java 或 Android 中没有简单的方法来读取上一行，只有下一行（因为在文件 I/O 中向前比向后更容易）。
我能想到的另一种选择是保留一个线标记（从 0 开始），并在您通过线前进时增加它。然后，要倒退，您必须再次逐行读取文件，直到到达该行减一。如果您需要倒退，请转到该新行减一，依此类推。这很可能是一项繁重的操作，但会满足您的需求。
编辑：如果以上都不起作用，还有a method to read in a file backwards，您可以在其中通过向前迭代找到上一行。只是一个替代的想法，但绝对不是一个容易实现的想法。
【讨论】：
public class LoadFromAltLoc extends Activity {
//a handle to the application's resources
private Resources resources;
//a string to output the contents of the files to LogCat
private String output;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
//get the application's resources
resources = getResources();
try
{
//Load the file from the raw folder - don't forget to OMIT the extension
output = LoadFile("from_raw_folder", true);
//output to LogCat
Log.i("test", output);
}
catch (IOException e)
{
//display an error toast message
Toast toast = Toast.makeText(this, "File: not found!", Toast.LENGTH_LONG);
toast.show();
}
try
{
//Load the file from assets folder - don't forget to INCLUDE the extension
output = LoadFile("from_assets_folder.pdf", false);
//output to LogCat
Log.i("test", output);
}
catch (IOException e)
{
//display an error toast message
Toast toast = Toast.makeText(this, "File: not found!", Toast.LENGTH_LONG);
toast.show();
}
}
//load file from apps res/raw folder or Assets folder
public String LoadFile(String fileName, boolean loadFromRawFolder) throws IOException
{
//Create a InputStream to read the file into
InputStream iS;
if (loadFromRawFolder)
{
//get the resource id from the file name
int rID = resources.getIdentifier("fortyonepost.com.lfas:raw/"+fileName, null, null);
//get the file as a stream
iS = resources.openRawResource(rID);
}
else
{
//get the file as a stream
iS = resources.getAssets().open(fileName);
}
//create a buffer that has the same size as the InputStream
byte[] buffer = new byte[iS.available()];
//read the text file as a stream, into the buffer
iS.read(buffer);
//create a output stream to write the buffer into
ByteArrayOutputStream oS = new ByteArrayOutputStream();
//write this buffer to the output stream
oS.write(buffer);
//Close the Input and Output streams
oS.close();
iS.close();
//return the output stream as a String
return oS.toString();
}
}
【讨论】：