Objectives
- Perform basic file output
- Perform basic file input
Basic file I/O notes
- import java.io.*; // for I/O
- import java.util.*; // for Scanner class
- add "throws IOException" to methods that use file I/O (or use try/catch blocks)
- Scanner infile = new Scanner(new FileReader(filename)); // to read a file
- String s1 = infile.nextLine(); // reads a String from infile
- int n1 = infile.nextInt(); // reads an integer from infile
- double d1 = infile.nextDouble(); // reads a double from infile
- PrintWriter outfile = new PrintWriter(filename); // to write a file
- outfile.println(n); // writes the value of n to outfile
Sample code
- Write items to file and then read them: import java.io.*; import java.util.*; public class TestFile1 { public static void main(String[] args) throws IOException { PrintWriter outfile = new PrintWriter("stuff.txt"); int year = 2017; double bodyTemp = 98.6; outfile.println("CIS 160"); outfile.println("Kishwaukee College"); outfile.println(year); outfile.println(bodyTemp); outfile.close(); Scanner infile = new Scanner(new FileReader("stuff.txt")); System.out.println(infile.nextLine()); System.out.println(infile.nextLine()); System.out.println(infile.nextInt()); System.out.println(infile.nextDouble()); infile.close(); } }
- Read from file until done: import java.io.*; import java.util.*; public class TestFile2 { public static void main(String[] args) throws IOException { PrintWriter outfile = new PrintWriter("stuff.txt"); for (int i=1; i<=10; i++) { outfile.println(i*2); } outfile.close(); Scanner infile = new Scanner(new FileReader("stuff.txt")); while (infile.hasNext()) { System.out.println(infile.nextInt()); } infile.close(); } }
- Read using BufferedReader: import java.io.*; import java.util.*; public class TestFile3 { public static void main(String[] args) throws IOException { PrintWriter outfile = new PrintWriter("stuff.txt"); outfile.println("Kishwaukee College"); for (int i=1; i<=10; i++) { outfile.println(i*2); } outfile.println("CIS 160"); outfile.close(); BufferedReader infile = new BufferedReader(new FileReader("stuff.txt")); // All input treated as a String String lineIn; while ((lineIn = infile.readLine()) != null) { System.out.println(lineIn); // Note: Need Integer.parseInt(lineIn) to treat input as integer } infile.close(); } }
Sample programs
- see FileOutput.java
- see FileInput.java
- see FileOutput2.java: modified to output more data
- see FileInput2.java: uses Scanner object to read file
- see BasicFileTest.java
- see BasicFileTest2.java
Resources
Note: The David J. Eck text goes into much more detail and covers much more than we need for this course.