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Java IO

File

Public Static Fields

To let the code run on every system, use these variables.

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// depends on the system. On Mac, it is "/"
System.out.println(File.separator);
// On Mac, it's ":", used to separate filenames in a sequence of files given as a path list
System.out.println(File.pathSeparator);

For example, if we want to get a path like “Users/ruoke”, we can create it using

Stringpath="Users"+File.separator+"ruoke";

Constructors

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// 1. Creates a new File instance by converting the given pathname string into an abstract pathname.
String filePath = "/Users/ruoke/Documents/java_web/demo-java-spring/src/main/java/com/ruoke/demojavaspring/File/test.txt";
File file1 = new File(filePath);
System.out.println(file1);

// 2. Creates a new File instance from a parent pathname string and a child pathname string.
String parent = "/Users/ruoke/Documents/java_web/demo-java-spring/src/main/java/com/ruoke/demojavaspring/File";
String child = "test.txt";
File file2 = new File(parent, child);
System.out.println(file2);

// 3. Creates a new File instance from a parent abstract pathname and a child pathname string.
File parent2 = new File(parent);
File file3 = new File(parent2, child);
System.out.println(file3);

Common Methods

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// Bytes number. Only applies to files, not directories.
System.out.println(fileName.length());
System.out.println(fileName.getAbsolutePath());

// Get a file's or a dir's name
System.out.println(fileName.getName());

// Return an absolute path of its parent
System.out.println(fileName.getParent());
String file_path = "Users/ruoke/test01.txt";

// If this file/dir exists, return false. Else, create it and return true
File file4 = new File(file_path);
boolean res = file4.createNewFile();

File file5 = new File(file_path);
boolean res = file5.createNewFile();

Byte IO

Byte IO can hanlde all binary files, like pictures, video…

FileOutputStream

Under java.io package, there’s an abstract class OutputStream Class FileOutputStream extends OutputStream.

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FileOutputStream outputStream = new FileOutputStream("/Users/ruoke/Documents/java_web/demo-java-spring/src/main/java/com/ruoke/demojavaspring/File/test.txt");
FileOutputStream(File file, boolean append)//append mode

If append=false, this constructor will create a file. If this file already exits, the newly created file will replace the older one.

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outputStream.write(65);
This will write "A" to the outputfile

String s="abc";  
byte[] bytes = s.getBytes();  
    
outputStream.write(bytes);//"abc"  
outputStream.write(bytes, 1, bytes.length-1);//"bc"  
outputStream.close();

FileInputStream