This is my code that I had to use for a question. I am supposed to look for words in the command line in Eclipse by typing java Spell a and getting all word outprinted in the console that starts with a from a local text file. My program terminates for no apparent reason:
import java.util.*;
import java.io.*;
public class Spell {
public static void main (String[] args) throws Exception {
File file = new File("C:\\Users\\Dennis\\words.txt");
Scanner fileScanner = new Scanner(file);
while (fileScanner.hasNext()) {
String word = fileScanner.next();
if(word.startsWith(args[0])) {
System.out.println(word);
}
}
}
}
If you are running this program as java Spell garfield then the args[0] will contain garfield in the file if any word will match with garfield then your print statement will be executed otherwise not.You have to provide run time argument in ecclipse for running your program and then you will be able to see the actual out put you want.
rightclick in your Class contaning main method select run as->run configuration in the tab (x)=Arguments give your argument then run and see below is the screen shot for this
In eclipse, right-click on your java file -> Run as -> Run configurations -> Tab Arguments, in programme arguments, enter a. If there is no argument entered, you should get an "ArrayIndexOutOfBoundsException" at the line
if(word.startsWith(args[0])) {
I would suggest putting a break-point on the line
String word = fileScanner.next();
and then Debug.
So that you can verify the words scanned by the programme and check if the arg[0] is what you entered.
I have run your programme with words.txt like
asdf
bsdf
csdf
dsdf
and argument c, it prints out csdf correctly.
Related
I am making simple console app. My main loop looks like this.
var s = new Scanner(System.in);
while (appShouldRun) {
var action = s.nextLine();
// proccess action
}
It works fine when I run app with empty stdin.
java -jar app.jar
But I want to have some prepared scenarios in file. Run them and then wait for next user input.
Expected behaviour:
java -jar app.jar < scenario.txt
output of commands from file
| <- cursor, waiting for user input
// scenario.txt
cmd1
cmd2
cmd3
.
.
.
Problem is that when I run program with something od std in, it process commands correctly, but then it throws
java.util.NoSuchElementException: No line found
I think it's becouse stdin is closed after all lines from file is processed.
Am I wrong?
How to fix the code to get expected behaviour?
Thanks
When you redirect standard input with <, it is connected to the input file, and not connected at all to the console.
If you want your program to receive input on the console, then you can't redirect stdin. You should take the filename as a command-line argument instead. Open it, read it, close it, and then process stdin.
OR, if you really want to, and you're on a unix/linux/macOS system, you can run it like this, using the cat command to concatenate standard input to something else:
cat scenario.txt - | java -jar app.jar
I have tried do change as little code as possible. This reads the whole file like you wanted and ends without Exception
public class Main {
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
boolean appShouldRun = true;
while (appShouldRun) {
String action = s.nextLine();
System.out.println("got: " + action);
appShouldRun = s.hasNext();
}
}
}
I finished creating a program but I was told that my program
must be a Java application that takes as a command line argument the name of the file."
I understand I can use the jar command in terminal but I don't undestand how you open the terminal and take a file name as a argument. I was wondering if someone could explain what code is required to do this.
Thanks alot.
I tried creating a basic jar file in terminal with the line "jar cvf findOptimalTransport.jar ." but the jar file does not open, I think its because the current implementation takes the users input with a scannar in the code and prints via the terminal. However, this wont work because a terminal window is not opened with this command.
It doesn't have to be a jar file. Command line arguments can be entered from the command line, when you run your application.
Let me give you an example, about how this works. Let's say you have the below simple Java application:
public class MyApplication{
public static void main(String[] arguments){
System.out.println("Hello World!");
}
}
That public static void main() is a method; and more specifically the main method of your application which is what is executed when compiled and ran.
To compile and then run it, you type in the command line/terminal:
javac MyApplication.java //this will compile it
java MyApplication //this will run the main method of MyApplication
But what is that parameter in the main method? What is String[] arguments ?
When you run your program, whatever you type after the application name is an argument, of type String and it is stored in the String array String[] arguments (or most commonly String[] args).
What this means, is that, if you execute your application like this:
java MyApplication some_file.txt // Run application with one arg.
You can access that argument like so:
public class MyApplication{
public static void main(String[] arguments){
System.out.println("Hello World!");
System.out.println("You entered: " + arguments[0]);
}
}
Output:
Hello World!
You entered: some_file.txt
Note: To run a jar file, you need to navigate to the folder that the jar file is in and from the command line you can run it by typing:
java -jar <jarname>.jar
Is there a way to read data from the command prompt? I have a java program that relies on 4 input variables from an outside source. These variables are returned to the command prompt after I run a javascript program but i need a way to pass these variables from the command prompt into my java program, any help would be greatly appreciated!
While executing java program pass the parameters and all the parameters should be separated by space.
java programName parameter1 parameter2 parameter3 parameter4
This parameters would be available in your main method argument
public static void main(String[] args){
//This args array would be containing all four values, i.e. its length would be 4 and you easily iterate values.
for(int i=0; i<args.length; i++){
System.out.println("Argument " + i + " is " + args[i]);
}
Follow the link:
Command-Line Arguments - The Java™ Tutorials : https://docs.oracle.com/javase/tutorial/essential/environment/cmdLineArgs.html
shared by #BackSlash.
It has all the content which would help you to clear all your doubts.
The content from the link is quoted below:
Displaying Command-Line Arguments passed by user from command-line to a Java program
The following example displays each of its command-line arguments on a
line by itself:
public class DisplayCommandLineParameters {
public static void main (String[] args) {
for (String s: args) {
System.out.println(s);
}
}
}
To compile the program: From the Command Prompt, navigate to the directory containing your .java file, say C:\test, by typing the cd
command below.
C:\Users\username>cd c:\test
C:\test>
Assuming the file, say DisplayCommandLineParameters.java, is in the
current working directory, type the javac command below to compile it.
C:\test>javac DisplayCommandLineParameters.java
C:\test>
If everything went well, you should see no error messages.
To run the program: The following example shows how a user might run the class.
C:\test>java DisplayCommandLineParameters Hello Java World
Output:
Hello
Java
World
Note that the application displays each word — Hello, Java and World —
on a line by itself. This is because the space character separates
command-line arguments.
To have Hello, Java and World interpreted as a single argument, the
user would join them by enclosing them within quotation marks.
C:\test>java DisplayCommandLineParameters "Hello Java World"
Output: Hello Java World
I do have some serious troubles understanding the console in java. I am running Eclipse, and I wanted to write a small program which prompts a few text messages to the console and receives a few strings as input arguments from it. Problem is: When I run my program, it opens the command line window properly, but my outputs are only printed on the Eclipse-Console.
In some way, I do understand why this is the case. The Command Line Windows expects commands, and not just some kind of a string or something. But how do i manage to output my Strings into the Command Line Window and read Strings from it, and not just commands.
Or am I doing it the wrong way? Do I have to open another "Console" where all my messages will be prompted and from which i can read strings a user wrote?
This is the code i use to open a command line window on start:
public static void main(String args[]) throws Exception {
Process process = new ProcessBuilder(new String[] { "cmd", "/C",
"start", "cmd" }).start();
System.out.println(process.waitFor());
Edit: I did still not manage to get this to working. Somehow, when I compiled the program, and I run it, it properly opens a command window, but no messages are posted there. Seems like "System.out.println("xxx") does not have any effect on this window.
There's no "console" specified by your program, but an stdin, stdout and stderr for input, output and error output. When you run your program from windows, these streams are bound to a command window, and if you run it in eclipse, they will be associated to the eclipse console. To give a more obscure example, ff you were running it through ssh, the streams would be associated to ssh, and ssh associated to your command window, and so on.
So, you're not doing anything wrong, you just need to run the program from the command line if you want stdout and stdin to come from that command window.
How do you open a command window, by the way?
You might want to read through this page:
http://pages.cs.wisc.edu/~hasti/cs368/JavaTutorial/NOTES/JavaIO_Scanner.html
Basically what you need to is create the input stream, tell the user to input something, and then get the input. E.g.
private static Scanner newScanner = new Scanner(System.in);
public static void main(String[] args) {
System.out.println("Put your input here: ");
String inputValue = newScanner.nextLine();
System.out.println(inputValue);
}
Just remember to import the scanner library!
The static method main, which receives an array of strings. The array should have two elements: the path where the files are located (at index 0), and the name of the files to process (at index 1). For example, if the name was “Walmart” then the program should use “Walmart.cmd” (from which it will read commands) and “Walmart.pro” (from which it will read/write products).
I don't want anyone to write the code for me because this is something I need to learn. However I've been reading this through and the wording is confusing. If someone could help me understand what it wants from me through pseudo-code or an algorithm it would be greatly appreciated.
Where I'm confused is how to initialize arg[0] and arg[1] and exactly
what they are being initialized to.
The main method's String array input argument consists of whatever String arguments you pass to the program's main method when you run the program. For example, here is a simple program that loops over args and prints a nice message with each argument's index and value on a separate line:
package com.example;
public class MainExample {
public static void main(String[] args) {
for (int i = 0; i < args.length; i++) {
System.out.printf("args[%d]=%s\n", i, args[i]);
}
}
}
Once you've compiled the program, you can run it on the command-line and pass it some arguments:
java -cp . com.example.MainExample eh? be sea 1 2 3 "multiple words"
Output:
args[0]=eh?
args[1]=be
args[2]=sea
args[3]=1
args[4]=2
args[5]=3
args[6]=multiple words
So lets explain to you
Create a class Inventory : if you don't know how to create a class google it just as is
The static method main: Every executable class in java (at least from the console) has the main method you should google java main method and propably in the same place you find it you will see the default arguments that it receives
When you learn about the default arguments of method main you will undertand about the 'args' that has to be on it
You will have t study the class String google it "java String class"
You will have to study the class File google it "java File class"
At the end everything else would be just logic and I beleave you have learned some at this point.
public class Inventory { // class inventory
public static void main(String[] args) // main method
{
if(args.length==2){ // check if args contains two elements
String filePath = args[0];
String fileName = args[1];
filePath+= System.getProperty("file.separator")+fileName;
File fileCMD = new File(filePath+".cmd");
//fileCMD.createNewFile();
File filePRO =new File(filePath+".pro");
//filePRO.createNewFile();
}
else {
//write the code to print the message Usage: java Inventory Incorrect number of parameters for a while and exit the program.
}
}
This is what I've understood. Basically you have to write a program to create two files, one called fileName.cmd and the other fileName.pro. You have to construct the path of the files using the arguments (input parameters of the main method) and system's file separator. If the arguments don't have two elements you have to print the 'invalid' message. That's it.
Where I'm confused is how to initialize arg[0] and arg[1] and exactly
what they are being initialized to.
You have to use command line to pass the arguments and launch the program , something like the following code in cmd or terminal:
java inventory thePath theFileName
That's how it get initialized.