4.6 Using Text Files

1. Exam Points

• Read data from a file. (File, Scanner, nextXXX() methods)
• Read data using loops. ( hasNext() method + nextXXX() method)
• Use the split method of the String class to split the content read.
• Note: use nextXXX() method only once in the loop body.

2. Knowledge Points

(1) Read data from file

• A file is storage for data that persists(持久化) when the program is not running.
• In a program, we can retrieve data from a file.
• To read data from a file, you can use the File and Scanner classes
• Example 1: read data from a file
  

  
  // the classes used must be imported using import statements.
  import java.io.File;
  import java.io.IOException;
  import java.util.Scanner;
  
  public class Demo{
  
      // add the 'throws IOException' statement in the header of the method
      public static void main(String[] args) throws IOException {
  
          // create a File object and specify the name of the file to read
          File file = new File("file1.txt");
          // create a Scanner object, and specify the File object to use
          Scanner sc = new Scanner(file);
  
          // read an integer value from the file
          int data1 = sc.nextInt();
  
          // read a double value from the file		
          double data2 = sc.nextDouble();
  
          // read a boolean value from the file
          boolean data3 = sc.nextBoolean();
  
          // read a String value from the file
          String data4=sc.next();
  
          // read the rest of the current line from the file
          String data5=sc.nextLine();
  
          // close the Scanner object once reading is done
          sc.close();
  
      }
  }
  

• Example 2: read data from a file using loops
  

  File file = new File("file2.txt");
  Scanner sc = new Scanner(file);
  
  while (sc.hasNext()) { // check if there is next item to read
      String str = sc.next(); // get the next String
      String[] strs = str.split("_"); // split the String using '_'
      System.out.println(strs[0] + " " + strs[1]); // print each part of the String
  }
  

• Note
  
  • the types of data retrieved must match the types of data in the file.
  • Example, if you read integer, boolean, String values one by one, then the values must be in the order of integer, boolean, String in the file.
  • The File and IOException classes are from the java.io package.
  • Import statements must be used to make these classes available for use in the program.

(2) Methods of the Scanner class

• int nextInt() : returns the next int read from the file.
• double nextDouble() : returns the next double read from the file.
• boolean nextBoolean() : returns the next boolean value read from the file.
• String next() : returns the next String value read from the file.
• String nextLine() : returns the rest content in the current line.
• boolean hasNext() : returns true if there is a next item to read in the file, or false otherwise.
• void close() : close the scanner.

(3) Methods of the String class

• String[] split(String x) : returns a String array where each element is a substring of this String, which has been split around matches of the given expression x.
• Example:
  

  String text = "apple,orange,pear,banana,avocado";
  
  // the String text is split into five Strings using ',' as a separator
  String[] fruits = text.split(",");
  
  // after splitting, the fruits array has five elements.
  

3. Exercises

• Practice
• Solution