|
读文件
http://www.baeldung.com/java-read-file
1. Overview
In this tutorial we’ll explore different ways to?read from a File in Java; we’ll make use ofBufferedReader,?Scanner,?StreamTokenizer,?DataInputStream,?SequenceInputStream?andFileChannel.
Then,we will discuss how to read a UTF-8 encoded file and how to create String from contents of a file.
Finally,we’ll explore the new techniques to read from file in Java 7.
This article is part of??here on Baeldung.
2. Read with?BufferedReader
Let’s start with a simple way to read from file using?BufferedReader; the file itself contains:
The following code reads from the file using?BufferedReader:
Note that?readLine()?will return?null?when the end of the file is reached.
3. Read with?Scanner
Next,let’s use a?Scanner?to read from the File – the file contains:
We’ll use a simple whitespace as the delimiter:
Note that the default delimiter is the whitespace,but multiple delimiters can be used with aScanner.
4. Read with?StreamTokenizer
Next,let’s read a text file into tokens using a?StreamTokenizer.
The way the tokenizer works is – first,we need to figure out what the next token is – String or number; we do that by looking at the?tokenizer.ttype?field.
Then,we’ll read the actual token based on this type:
-
tokenizer.nval?– if the type was a number
-
tokenizer.sval?– if the type was a String
The file simply contains:
The following code reads from the file both the String and the number:
Note how the end of file token is used at the end.
5. Read with?DataInputStream
We can use?DataInputStream?to read binary or primitive data type from file.
Let’s start with the actual file itself:
The following test reads the file using a?DataInputStream:
6. Read with?SequenceInputStream
Now,let’s look at how to?concatenate two input streams into one?usingSequenceInputStream; the 2 input files will simply contain:
and:
Let’s now use a?SequenceInputStream?to read the two files and merge them into one:
7. Read with?FileChannel
If we are reading a large file,?FileChannel?can be faster than standard IO.
The contents of the file:
The following code reads data bytes from the file using?FileChannel?and?RandomAccessFile:
8. Read UTF-8 encoded file
Now,let’s see how to read a UTF-8 encoded file using?BufferedReader:
9. Read a file into a String
We can make good use of?StringBuilder?to?read the entire contents of a file into a String. Let’s start with the file:
The following code append data read from the file into a?StringBuilder?line by line:
10. Read from File using Java 7
Java 7 introduces a new way of working with files and the filesystem – let’s make use of that to read files.
10.1. Read a Small File with Java 7
The file contents:
</td>
<td class="code">
<div class="container">
<div class="line number1 index0 alt2"><code class="bash plain">Hello world
</td>
</tr></table>
The following code shows how to read small file using the new?Files?class:
</td>
<td class="code">
<div class="container">
<div class="line number1 index0 alt2"><code class="java color1">@Test
<div class="line number2 index1 alt1">
<code class="java keyword">public <code class="java keyword">void <code class="java plain">whenReadSmallFileJava7_thenCorrect()
Note that you can use the?readAllBytes()?method as well if you need binary data.
10.2. Read a Large File with Java 7
If we want to read a large file with?Files?class,we can use the?BufferedReader.
The file contents:
</td>
<td class="code">
<div class="container">
<div class="line number1 index0 alt2"><code class="bash plain">Hello world
</td>
</tr></table>
The following code reads the file using the new?Files?class and?BufferedReader:
</td>
<td class="code">
<div class="container">
<div class="line number1 index0 alt2"><code class="java color1">@Test
<div class="line number2 index1 alt1">
<code class="java keyword">public <code class="java keyword">void <code class="java plain">whenReadLargeFileJava7_thenCorrect()
11. Conclusion
As you can see,there are many possibilities of reading data from a file using plain Java. You can go for?BufferedReader?to read line by line,?Scanner?to read using different delimiters,?StreamTokenizer?to read file into tokens,?DataInputStream?to read binary data and primitive data types,?SequenceInput Stream?to link multiple files into one stream,FileChannel?to read faster from large files,etc.
写文件
1. Overview
In this tutorial we’ll explore different ways to?write to a file using Java. We’ll make use ofBufferedWriter,?PrintWriter,?FileOutputStream,?DataOutputStream,?RandomAccessFile,FileChannel?and the Java 7?Files?utility class.
We’ll also take a look at locking the file while writing and discuss some final take-aways on writing to file.
This article is part of??here on Baeldung.
2. Write with?BufferedWriter
Let’s start simple – and use?BufferedWriter?to write a?String?to a new file:
The output in the file will be:
We can then?append a?String?to the existing file:
The file will then be:
3. Write with?PrintWriter
Next – let’s see how we can use a?PrintWriter?to write formatted text to a file:
The resulting file will contain:
Note how we’re not only writing a raw String to file,but also some formatted text with theprintf?method.
We can create the writer using?FileWriter,?BufferedWriter?or even?System.out.
4. Write with?FileOutputStream
Let’s now see how we can use?FileOutputStream?to?write binary data to a file. The following code converts a?String?int bytes and writes the bytes to file using aFileOutputStream:
|
