Is there a way to use a try/catch statement to ask the user to enter a file, if the user enters the wrong filename, the program will ask two more times, and then exit with an exception? How could I loop? Because once the user enters the wrong filename the program throws the exception immediately.
import java.io.File;
import java.io.FileNotFoundException;
import java.util.*;
import java.util.logging.Level;
import java.util.logging.Logger;
public class Main {
static String[] words = new String[5];
public static void main(String[] args)
{
Scanner kb = new Scanner(System.in);
System.out.println("enter file name:");
String fileName = kb.next();
try {
File inFile = new File(fileName);
Scanner in = new Scanner(new File(fileName));
} catch (FileNotFoundException ex) {
Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
}
}
So you don't want it to throw any kind of error when the user enters the wrong filename, right? If so, then I think this is what you want:
for(int i = 0; i < 3; i++){
try {
File inFile = new File(fileName);
Scanner in = new Scanner(new File(fileName));
break;
} catch (FileNotFoundException ex) {
if(i == 2){
Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
System.exit(0);
}
else
continue;
}
}
if the user enters the correct file name, it will break out of the loop. If not, it checks to see if the loop is on it's third iteration. If it is, (that means the user has tried and failed twice), it prints the error and exits the program. If the loop isn't on it's third iteration, it continues with the loop and re-prompts the user.
I hope it's obvious that the FileNotFoundException is thrown by the Scanner constructor. So why use it until you're sure the file exists? You shouldn't create the Scanner object till you got the correct file!
To implement this idea use this in your try block:
//read file name from stdio
File inFile = new File(fileName);
int i = 0;
while(!inFile.exists() && i++ < 2 ){
//read file name from System.in;
inFile = new File(fileName);
}
Scanner in = new Scanner(new File(fileName));
assuming you create a boolean fileIsLoaded = false, and set that to true. You can create a loop
for(int i=0;i<2 && !fileIsLoaded; i++) {
//your try/catch goes here
}
Enclose all of the code within your current main within that loop (with the boolean created beforehand). Finally, you can check the boolean afterwards in case all tries failed.
Related
Can someone please explain to me why I'm getting the catch error ?
I am trying to read values (numbers) from the file I passed in args.
And I do not quite understand where the problem comes from.
import java.util.Scanner;// Import the Scanner class to read text files
import java.io.File;// Import the File class
import java.io.FileNotFoundException;// Import this class to handle errors
import java.io.*;
public class main extends GeneralMethods {
public static void main(String[] args) {
if (args.length <= 1 || args.length > 2) {
println("Error, usage: software must get two input files");
System.exit(1);
}
String file_name1 = args[0]; // data to insert
String file_name2 = args[1]; // data to check
File data_to_insert = new File(file_name1);
File data_to_check = new File(file_name2);
Scanner input = new Scanner(System.in); // Create a Scanner object
println("Enter hashTable size");
int hashTable_size = input.nextInt(); // Read hashTable_size from user
println("Enter num of hashing function");
int num_of_has = input.nextInt(); // Read num of hashing from user
hashTable T = new hashTable(hashTable_size);
println("hashTable before insert values\n");
T.printHashTable();
input.close();
int i = 0;
try {
input = new Scanner(data_to_insert);
String data;
while ((data = input.next()) != null) {
T.set(i, Integer.parseInt(data));
i++;
}
input.close();
} catch (Exception e) {
System.out.println("\nError: Reading, An error occurred while reading input files. Check your input type");
e.printStackTrace();
}
T.printHashTable();
}
}
this is my output
Which prints the catch error
hashTable before insert values
[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]
Error: Reading, An error occurred while reading input files. Check your input type
java.util.NoSuchElementException
at java.base/java.util.Scanner.throwFor(Scanner.java:937)
at java.base/java.util.Scanner.next(Scanner.java:1478)
at main.main(main.java:36)
[1,2,3,4,5,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]
In this line,
while ((data = input.next()) != null)
The next() method of Scanner does not return null if it has no more data, instead it throws the NoSuchElementException you are getting.
Use this instead to check for more data:
while ((input.hasNext()) {
data = input.next();
//...
}
Method hasNext() returns true or false as you would expect.
Following is my code that I am working on for a school project. It does ok up until I try to read the animal.txt file. Can someone please tell me what I am doing wrong? I am attaching my compilation error as an image. Thanks in advance.
[input error image1
package finalproject;
//enabling java programs
import java.util.Scanner;
import javax.swing.JOptionPane;
import java.io.FileInputStream;
import java.io.IOException;
public class Monitoring {
public static void choseAnimal() throws IOException{
FileInputStream file = null;
Scanner inputFile = null;
System.out.println("Here is your list of animals");
file = new FileInputStream("\\src\\finalproject\\animals.txt");
inputFile = new Scanner(file);
while(inputFile.hasNext())
{
String line = inputFile.nextLine();
System.out.println(line);
}
}
public static void choseHabit(){
System.out.println("Here is your list of habits");
}
public static void main(String[] args) throws IOException{
String mainOption = ""; //user import for choosing animal, habit or exit
String exitSwitch = "n"; // variable to allow exit of system
Scanner scnr = new Scanner(System.in); // setup to allow user imput
System.out.println("Welcome to the Zoo");
System.out.println("What would you like to monitor?");
System.out.println("An animal, habit or exit the system?");
mainOption = scnr.next();
System.out.println("you chose " + mainOption);
if (mainOption.equals("exit")){
exitSwitch = "y";
System.out.println(exitSwitch);
}
if (exitSwitch.equals( "n")){
System.out.println("Great, let's get started");
}
if (mainOption.equals("animal")){
choseAnimal();
}
if (mainOption.equals("habit")) {
choseHabit();
}
else {
System.out.println("Good bye");
}
}
}
\\src\\finalproject\\animals.txt suggests that the file is an embedded resource.
First, you should never reference src in you code, it won't exist once the program is built and package.
Secondly, you need to use Class#getResource or Class#getResourceAsStream in order to read.
Something more like...
//file = new FileInputStream("\\src\\finalproject\\animals.txt");
//inputFile = new Scanner(file);
try (Scanner inputFile = new Scanner(Monitoring.class.getResourceAsStream("/finalproject/animals.txt"), StandardCharsets.UTF_8.name()) {
//...
} catch (IOException exp) {
exp.printStackTrace();
}
for example
Now, this assumes that file animals.txt exists in the finalproject package
The error message clearly shows that it can't find the file. This means there's two possibilities:
File does not exist in the directory you want
Directory you want is not the directory you have.
I would start by creating a File object looking at "." (current directory) to and printing that to see what directory it looks by default. You may need to hard code the file path, depending on what netbeans is using for a default directory.
I am trying to write a program that inputs a text file through the command line and then prints out the number of words in the text file. I've spent around 5 hours on this already. I'm taking an intro class using java.
Here is my code:
import java.util.*;
import java.io.*;
import java.nio.*;
public class WordCounter
{
private static Scanner input;
public static void main(String[] args)
{
if (0 < args.length) {
String filename = args[0];
File file = new File(filename);
}
openFile();
readRecords();
closeFile();
}
public static void openFile()
{
try
{
input = new Scanner(new File(file));
}
catch (IOException ioException)
{
System.err.println("Cannot open file.");
System.exit(1);
}
}
public static void readRecords()
{
int total = 0;
while (input.hasNext()) // while there is more to read
{
total += 1;
}
System.out.printf("The total number of word without duplication is: %d", total);
}
public static void closeFile()
{
if (input != null)
input.close();
}
}
Each way I've tried I get a different error and the most consistent one is "cannot find symbol" for the file argument in
input = new Scanner(new File(file));
I'm also still not entirely sure what the difference between java.io and java.nio is so I have tried using objects from both. I'm sure this is an obvious problem I just can't see it. I've read a lot of similar posts on here and that is where some of my code is from.
I've gotten the program to compile before but then it freezes in command prompt.
java.nio is the New and improved version of java.io. You can use either for this task. I tested the following code in the command line and it seems to work fine. The "cannot find symbol" error message is resolved in the try block. I think you were confusing the compiler by instantiating a File object named file twice. As #dammina answered, you do need to add the input.next(); to the while loop for the Scanner to proceed to the next word.
import java.io.File;
import java.io.IOException;
import java.util.Scanner;
public class WordCounter {
private static Scanner input;
public static void main(String[] args) {
if(args.length == 0) {
System.out.println("File name not specified.");
System.exit(1);
}
try {
File file = new File(args[0]);
input = new Scanner(file);
} catch (IOException ioException) {
System.err.println("Cannot open file.");
System.exit(1);
}
int total = 0;
while (input.hasNext()) {
total += 1;
input.next();
}
System.out.printf("The total number of words without duplication is: %d", total);
input.close();
}
}
Your code is almost correct. Thing is in the while loop you have specified the terminating condition as follows,
while (input.hasNext()) // while there is more to read
However as you are just increment the count without moving to the next word the count just increases by always counting the first word. To make it work just add input.next() into the loop to move to next word in each iteration.
while (input.hasNext()) // while there is more to read
{
total += 1;
input.next();
}
I want to be able to remove blank lines from a text file, for example:
Average Monthly Disposable Salary
1
Switzerland
$6,301.73
2014
2
Luxembourg
$4,479.80
2014
3
Zambia
$4,330.98
2014
--To This:
Average Monthly Disposable Salary
1
Switzerland
$6,301.73
2014
2
Luxembourg
$4,479.80
2014
3
Zambia
$4,330.98
2014
All of the code I have is below:
public class Driver {
public static void main(String[] args)
throws Exception {
Scanner file = new Scanner(new File("src/data.txt"));
PrintWriter write = new PrintWriter("src/data.txt");
while(file.hasNext()) {
if (file.next().equals("")) {
continue;
} else {
write.write(file.next());
}
}
print.close();
file.close();
}
}
The problem is that the text file is empty once I go back and look at the file again.
Im not sure why this is acting this way since they all seem to be blank characters, \n showing line breaks
Your code was almost correct, but there were a few bugs:
You must use .nextLine() instead of .next()
You must write to a different file while reading the original one
Your print.close(); should be write.close();
You forgot to add a new line after each line written
You don't need the continue; instruction, since it's redundant.
public static void main(String[] args) {
Scanner file;
PrintWriter writer;
try {
file = new Scanner(new File("src/data.txt"));
writer = new PrintWriter("src/data2.txt");
while (file.hasNext()) {
String line = file.nextLine();
if (!line.isEmpty()) {
writer.write(line);
writer.write("\n");
}
}
file.close();
writer.close();
} catch (FileNotFoundException ex) {
Logger.getLogger(Test.class.getName()).log(Level.SEVERE, null, ex);
}
}
If you want to keep the original name, you can do something like:
File file1 = new File("src/data.txt");
File file2 = new File("src/data2.txt");
file1.delete();
file2.renameTo(file1);
Try org.apache.commons.io and Iterator
try
{
String name = "src/data.txt";
List<String> lines = FileUtils.readLines(new File(name));
Iterator<String> i = lines.iterator();
while (i.hasNext())
{
String line = i.next();
if (line.trim().isEmpty())
i.remove();
}
FileUtils.writeLines(new File(name), lines);
}
catch (IOException e)
{
e.printStackTrace();
}
You could copy to a temporary file and rename it.
String name = "src/data.txt";
try(BufferedWriter bw = new BufferedWriter(name+".tmp)) {
Files.lines(Paths.get(name))
.filter(v -> !v.trim().isEmpty())
.forEach(bw::println);
}
new File(name+".tmp").renameTo(new File(name));
This piece of code solved this problem for me
package linedeleter;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Scanner;
public class LineDeleter {
public static void main(String[] args) throws FileNotFoundException, IOException {
File oldFile = new File("src/data.txt"); //Declares file variable for location of file
Scanner deleter = new Scanner(oldFile); //Delcares scanner to read file
String nonBlankData = ""; //Empty string to store nonblankdata
while (deleter.hasNextLine()) { //while there are still lines to be read
String currentLine = deleter.nextLine(); //Scanner gets the currentline, stories it as a string
if (!currentLine.isBlank()) { //If the line isn't blank
nonBlankData += currentLine + System.lineSeparator(); //adds it to nonblankdata
}
}
PrintWriter writer = new PrintWriter(new FileWriter("src/data.txt"));
//PrintWriter and FileWriter are declared,
//this part of the code is when the updated file is made,
//so it should always be at the end when the other parts of the
//program have finished reading the file
writer.print(nonBlankData); //print the nonBlankData to the file
writer.close(); //Close the writer
}
}
As mentioned in the comments, of the code block, your sample had the print writer declared after your scanner meaning that the program had already overwritten your current file of the same name. Therefore there was no code for your scanner to read and thus, the program gave you a blank file
the
System.lineSeparator()
Just adds an extra space, this doesn't stop the program from continuing to write on that space, however, so it's all good
Please bear with me here as I'm new to the site.
below is a program that I've written for my programming in Java class, and while most of it has gone well so far, I can't seem to get rid of a specific bug.
When the program reaches the third if block (choice == 3) it doesn't let the user enter any data, and if the line
"outputStream = openOutputTextFile(newerFileName);"
is present in the if block then a FileNotFoundException occurs. After tinkering around with my code for a while I've found that the error is being thrown because the program cannot find the inputStream anymore. Although I've checked and have found that the program can still find, read, and write to the file that is throwing the error.
I'm thinking that since the error only occurs when I put the outputStream in, and is being thrown by the inputStream, then it probably has something to do with file streams. I just don't know what exactly
Does anyone have any ideas on how I could solve this issue?
public class FileProgram {
public static PrintWriter openOutputTextFile(String fileName)
throws FileNotFoundException {
PrintWriter toFile = new PrintWriter(fileName);
return toFile;
}
public static Scanner readFile(String fileName)
throws FileNotFoundException {
Scanner inputStream = new Scanner(new File(fileName));
return inputStream;
}
public static void main(String args[]) throws FileNotFoundException {
ArrayList<String>fileReader = new ArrayList<String>(10);
PrintWriter outputStream = null;
Scanner inputStream = null;
Scanner keyboard = new Scanner(System.in);
try {
System.out.println("Enter the name of the text file you want to copy.");
String oldFileName = keyboard.nextLine();
inputStream = readFile(oldFileName);
while(inputStream.hasNextLine()) {
String currentLine = inputStream.nextLine();
fileReader.add(currentLine);
}
System.out.println("All data has been collected. Enter the name for the new text file");
String newFileName = keyboard.nextLine();
outputStream = openOutputTextFile(newFileName);
File userFile = new File(newFileName);
if(userFile.exists())
{
System.out.println("The name you entered matches a file that already exists.");
System.out.println("Here are your options to fix this issue.");
System.out.println("Option 1: Shut down the program.");
System.out.println("Option 2: Overwrite the old file with the new empty one.");
System.out.println("Option 3: Enter a different name for the new file.");
System.out.println("Enter the number for the option that you want.");
int choice = keyboard.nextInt();
if(choice == 1) {
System.exit(0);
} else if(choice == 2) {
outputStream = new PrintWriter(newFileName);
} **else if(choice == 3) {
System.out.println("Enter a different name.");
String newerFileName = keyboard.nextLine();
outputStream = openOutputTextFile(newerFileName);
}**
}
for(int i = 0; i < fileReader.size(); i++) {
String currentLine = fileReader.get(i);
outputStream.println(currentLine);
//System.out.println(currentLine);
}
System.out.println("The old file has been copied line-by-line to the new file.");
}
catch(FileNotFoundException e) {
System.out.println("File not found");
System.out.println("Shutting program down.");
System.exit(0);
}
finally {
outputStream.close();
inputStream.close();
}
}
}
You are having trouble getting a line of input from your Scanner object after calling .nextInt(). In response to the numeric choice, the user enters an integer followed by a newline.
This line reads the integer from the input buffer:
int choice = keyboard.nextInt();
However, there's still a newline in the input buffer right after the number. Thus when you call .nextLine():
String oldFileName = keyboard.nextLine();
You get an empty line. You cannot create a file with an empty string for a file name, so a FileNotFoundException is thrown (this is per spec, see the other answer).
One solution is to consistently use .nextLine(), getting a line at a time from the input buffer. When you need an integer, simply parse the string manually:
int choice = Integer.parseInt( keyboard.nextLine() );
By the way, in debugging this sort of issue it's very useful to get into the habit of adding some printout statements to see what's going on:
public static PrintWriter openOutputTextFile(String fileName)
throws FileNotFoundException {
System.out.println( "Trying to create file: '" + fileName + "'" );
PrintWriter toFile = new PrintWriter(fileName);
return toFile;
}
There are more advanced debugging techniques, but this one is extremely simple, and using it is a lot more effective than using nothing at all.