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I tried the java language using netbeans 12 with jdk version 8, but it always got an error in the output [duplicate]

I am trying to use Console class to get input from user but a null object is returned when I call System.console(). Do I have to change anything before using System.console?
Console co=System.console();
System.out.println(co);
try{
String s=co.readLine();
}

Using Console to read input (usable only outside of an IDE):
System.out.print("Enter something:");
String input = System.console().readLine();
Another way (works everywhere):
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class Test {
public static void main(String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.print("Enter String");
String s = br.readLine();
System.out.print("Enter Integer:");
try {
int i = Integer.parseInt(br.readLine());
} catch(NumberFormatException nfe) {
System.err.println("Invalid Format!");
}
}
}
System.console() returns null in an IDE.
So if you really need to use System.console(), read this solution from McDowell.

Scanner in = new Scanner(System.in);
int i = in.nextInt();
String s = in.next();

There are few ways to read input string from your console/keyboard. The following sample code shows how to read a string from the console/keyboard by using Java.
public class ConsoleReadingDemo {
public static void main(String[] args) {
// ====
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
System.out.print("Please enter user name : ");
String username = null;
try {
username = reader.readLine();
} catch (IOException e) {
e.printStackTrace();
}
System.out.println("You entered : " + username);
// ===== In Java 5, Java.util,Scanner is used for this purpose.
Scanner in = new Scanner(System.in);
System.out.print("Please enter user name : ");
username = in.nextLine();
System.out.println("You entered : " + username);
// ====== Java 6
Console console = System.console();
username = console.readLine("Please enter user name : ");
System.out.println("You entered : " + username);
}
}
The last part of code used java.io.Console class. you can not get Console instance from System.console() when running the demo code through Eclipse. Because eclipse runs your application as a background process and not as a top-level process with a system console.

It will depend on your environment. If you're running a Swing UI via javaw for example, then there isn't a console to display. If you're running within an IDE, it will very much depend on the specific IDE's handling of console IO.
From the command line, it should be fine though. Sample:
import java.io.Console;
public class Test {
public static void main(String[] args) throws Exception {
Console console = System.console();
if (console == null) {
System.out.println("Unable to fetch console");
return;
}
String line = console.readLine();
console.printf("I saw this line: %s", line);
}
}
Run this just with java:
> javac Test.java
> java Test
Foo <---- entered by the user
I saw this line: Foo <---- program output
Another option is to use System.in, which you may want to wrap in a BufferedReader to read lines, or use Scanner (again wrapping System.in).

Found some good answer here regarding reading from console, here another way use 'Scanner' to read from console:
import java.util.Scanner;
String data;
Scanner scanInput = new Scanner(System.in);
data= scanInput.nextLine();
scanInput.close();
System.out.println(data);

Try this. hope this will help.
String cls0;
String cls1;
Scanner in = new Scanner(System.in);
System.out.println("Enter a string");
cls0 = in.nextLine();
System.out.println("Enter a string");
cls1 = in.nextLine();

The following takes athspk's answer and makes it into one that loops continually until the user types "exit". I've also written a followup answer where I've taken this code and made it testable.
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class LoopingConsoleInputExample {
public static final String EXIT_COMMAND = "exit";
public static void main(final String[] args) throws IOException {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter some text, or '" + EXIT_COMMAND + "' to quit");
while (true) {
System.out.print("> ");
String input = br.readLine();
System.out.println(input);
if (input.length() == EXIT_COMMAND.length() && input.toLowerCase().equals(EXIT_COMMAND)) {
System.out.println("Exiting.");
return;
}
System.out.println("...response goes here...");
}
}
}
Example output:
Enter some text, or 'exit' to quit
> one
one
...response goes here...
> two
two
...response goes here...
> three
three
...response goes here...
> exit
exit
Exiting.

I wrote the Text-IO library, which can deal with the problem of System.console() being null when running an application from within an IDE.
It introduces an abstraction layer similar to the one proposed by McDowell.
If System.console() returns null, the library switches to a Swing-based console.
In addition, Text-IO has a series of useful features:
supports reading values with various data types.
allows masking the input when reading sensitive data.
allows selecting a value from a list.
allows specifying constraints on the input values (format patterns, value ranges, length constraints etc.).
Usage example:
TextIO textIO = TextIoFactory.getTextIO();
String user = textIO.newStringInputReader()
.withDefaultValue("admin")
.read("Username");
String password = textIO.newStringInputReader()
.withMinLength(6)
.withInputMasking(true)
.read("Password");
int age = textIO.newIntInputReader()
.withMinVal(13)
.read("Age");
Month month = textIO.newEnumInputReader(Month.class)
.read("What month were you born in?");
textIO.getTextTerminal().println("User " + user + " is " + age + " years old, " +
"was born in " + month + " and has the password " + password + ".");
In this image you can see the above code running in a Swing-based console.

Use System.in
http://www.java-tips.org/java-se-tips/java.util/how-to-read-input-from-console.html

Searching for string in file and returning that specific line

I am working on student registration system. I have a text file with studentname, studentnumber and the student's grade stored in every line such as:
name1,1234,7
name2,2345,8
name3,3456,3
name4,4567,10
name5,5678,6
How can I search a name and then return the whole sentence? It does not get any matches when looking for the name.
my current code look like this:
public static void retrieveUserInfo()
{
System.out.println("Please enter username"); //Enter the username you want to look for
String inputUsername = userInput.nextLine();
final Scanner scanner = new Scanner("file.txt");
while (scanner.hasNextLine()) {
final String lineFromFile = scanner.nextLine();
if(lineFromFile.contains(inputUsername)) {
// a match!
System.out.println("I found " +inputUsername+ " in file " ); // this should return the whole line, so the name, student number and grade
break;
}
else System.out.println("Nothing here");
}

The problem is with Scanner(String) constructor as it:
public Scanner(java.lang.String source)
Constructs a new Scanner that produces values scanned from the
specified string.
Parameters: source - A string to scan
it does not know anything about files, just about strings. So, the only line that this Scanner instance can give you (via nextLine() call) is file.txt.
Simple test would be:
Scanner scanner = new Scanner("any test string");
assertEquals("any test string", scanner.nextLine());
You should use other constructor of Scanner class such as:
Scanner(InputStream)
Scanner(File)
Scanner(Path)

You already have the variable that holds the whole line. Just print it like this:
while (scanner.hasNextLine()) {
final String lineFromFile = scanner.nextLine();
if(lineFromFile.contains(inputUsername)) {
// a match!
System.out.println("I found " +lineFromFile+ " in file " );
break;
}
else System.out.println("Nothing here");
}

JUnit test a method that asks for user input

I have to test someone elses method that runs endlessly but asks for an input:
public void automatize()
{
String cadena = new String();
while(true)
{
System.out.println("Executing...(Enter command)");
System.out.println("Enter Q to exit");
Scanner sc= new Scanner(new InputStreamReader(System.in));
cadena=sc.nextLine();
if(cadena.toLowerCase().equals("q"))
break;
String[] command = str.split(" ");
if(comando.length!=0)
new Extractor().run(command);
}
}
How am I supposed to test this with JUnit?
This is what I've tried to do, but, well, It doesn't actually do anything:
#Test
public void testAutomatize_q() {
ByteArrayInputStream in = new ByteArrayInputStream("Q".getBytes());
System.setIn(in);
extractor.automatize();
System.setIn(System.in);
}

You can replace System.in with you own stream by calling System.setIn(InputStream in). Input stream can be byte array:
ByteArrayInputStream in = new ByteArrayInputStream("My string".getBytes());
System.setIn(in);
// do your thing
// optionally, reset System.in to its original
System.setIn(System.in)
Different approach can be make this method more testable by passing IN and OUT as parameters:
public static int testUserInput(InputStream in,PrintStream out) {
Scanner keyboard = new Scanner(in);
out.println("Give a number between 1 and 10");
int input = keyboard.nextInt();
while (input < 1 || input > 10) {
out.println("Wrong number, try again.");
input = keyboard.nextInt();
}
return input;
}
Taken from here: JUnit testing with simulated user input

You could use a framework like Mockito in order to mock the Scanner object, and return a fix value when sc.nextLine() is called.
Here's a link of mockito http://mockito.org/, see the 'how' menu to have some examples.

Is a scan line escaping?

I've been doing a small project for class, it runs perfectly without problems but when pitted against the class's auto testers it gives back 2 No line found errors. Asking the course's staff they say it's probably because I'm trying to scan a line when none exist, but I tried printing all my scans and didn't discover anything like that.
That's all the scans I have in my code:
Scanner sc = new Scanner(System.in);
String sentence;
int choice;
System.out.println("Please enter a sentence:");
sentence = sc.nextLine();
printMenu(); // calls a function to print the menu.
// gets the require action
System.out.println("Choose option to execute:");
choice = sc.nextInt();
sc.nextLine();
(I tried with and without the last sc.nextLine)
static void replaceStr(String str)
{
String oldWord, newWord;
Scanner in = new Scanner(System.in);
// get the strings
System.out.println("String to replace: ");
oldWord = in.nextLine();
System.out.println("New String: ");
newWord = in.nextLine();
// replace
str = str.replace(oldWord, newWord);
System.out.println("The result is: " + str);
in.close();
}
static void removeNextChars(String str)
{
Scanner in = new Scanner(System.in);
String remStr; // to store the string to replace
String tmpStr = ""; //the string we are going to change.
int i; // to store the location of indexStr
// gets the index
System.out.println("Enter a string: ");
remStr = in.nextLine();
i=str.indexOf(remStr);
in.close(); // bye bye
if (i < 0)
{
System.out.println("The result is: "+str);
return;
}
// Build the new string without the unwanted chars.
/* code that builds new string */
str = tmpStr;
System.out.println("The result is: "+str);
}
Any idea how a line can leak here?

Here is the problem. You are using in.close(); at multiple places(last statement in replaceStr method and around the middle in removeNextChars method). When you close the scnaner using close() method, it closes your InputStream (System.in) as well. That InputStream can't be reopened with-in your program.
public void close() throws IOException --> Closes this input stream and releases any system resources associated with this stream. The general contract of close is that it closes the input stream. A closed stream cannot perform input operations and **cannot be reopened.**
Any read attempts after the scanner close will result into exception NoSuchElementException.
Please close your scanner only once, when your program is done.
EDIT: Scanner Closing/usage:
In yout main function:
Scanner sc = new Scanner(System.in);
....
.....
replaceStr(Scanner sc, String str);
.....
....
removeNextChars(Scanner sc ,String str);
....
....
//In the end
sc.close();
static void replaceStr(Scanner in, String str){
//All the code without scanner instantiation and closing
...
}
static void removeNextChars(Scanner in, String str){
//All the code without scanner instantiation and closing
...
}
You should be all good.

taking integer input in java

I am actually new to java programming and am finding it difficult to take integer input and storing it in variables...i would like it if someone could tell me how to do it or provide with an example like adding two numbers given by the user..

Here's my entry, complete with fairly robust error handling and resource management:
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
/**
* Simple demonstration of a reader
*
* #author jasonmp85
*
*/
public class ReaderClass {
/**
* Reads two integers from standard in and prints their sum
*
* #param args
* unused
*/
public static void main(String[] args) {
// System.in is standard in. It's an InputStream, which means
// the methods on it all deal with reading bytes. We want
// to read characters, so we'll wrap it in an
// InputStreamReader, which can read characters into a buffer
InputStreamReader isReader = new InputStreamReader(System.in);
// but even that's not good enough. BufferedReader will
// buffer the input so we can read line-by-line, freeing
// us from manually getting each character and having
// to deal with things like backspace, etc.
// It wraps our InputStreamReader
BufferedReader reader = new BufferedReader(isReader);
try {
System.out.println("Please enter a number:");
int firstInt = readInt(reader);
System.out.println("Please enter a second number:");
int secondInt = readInt(reader);
// printf uses a format string to print values
System.out.printf("%d + %d = %d",
firstInt, secondInt, firstInt + secondInt);
} catch (IOException ioe) {
// IOException is thrown if a reader error occurs
System.err.println("An error occurred reading from the reader, "
+ ioe);
// exit with a non-zero status to signal failure
System.exit(-1);
} finally {
try {
// the finally block gives us a place to ensure that
// we clean up all our resources, namely our reader
reader.close();
} catch (IOException ioe) {
// but even that might throw an error
System.err.println("An error occurred closing the reader, "
+ ioe);
System.exit(-1);
}
}
}
private static int readInt(BufferedReader reader) throws IOException {
while (true) {
try {
// Integer.parseInt turns a string into an int
return Integer.parseInt(reader.readLine());
} catch (NumberFormatException nfe) {
// but it throws an exception if the String doesn't look
// like any integer it recognizes
System.out.println("That's not a number! Try again.");
}
}
}
}

java.util.Scanner is the best choice for this task.
From the documentation:
For example, this code allows a user to read a number from System.in:
Scanner sc = new Scanner(System.in);
int i = sc.nextInt();
Two lines are all that you need to read an int. Do not underestimate how powerful Scanner is, though. For example, the following code will keep prompting for a number until one is given:
Scanner sc = new Scanner(System.in);
System.out.println("Please enter a number: ");
while (!sc.hasNextInt()) {
System.out.println("A number, please?");
sc.next(); // discard next token, which isn't a valid int
}
int num = sc.nextInt();
System.out.println("Thank you! I received " + num);
That's all you have to write, and thanks to hasNextInt() you won't have to worry about any Integer.parseInt and NumberFormatException at all.
See also
Java Tutorials/Essential Classes/Basic I/O/Scanning and Formatting
Related questions
How do I keep a scanner from throwing exceptions when the wrong type is entered? (java)
How to use Scanner to accept only valid int as input
Other examples
A Scanner can use as its source, among other things, a java.io.File, or a plain String.
Here's an example of using Scanner to tokenize a String and parse into numbers all at once:
Scanner sc = new Scanner("1,2,3,4").useDelimiter(",");
int sum = 0;
while (sc.hasNextInt()) {
sum += sc.nextInt();
}
System.out.println("Sum is " + sum); // prints "Sum is 10"
Here's a slightly more advanced use, using regular expressions:
Scanner sc = new Scanner("OhMyGoodnessHowAreYou?").useDelimiter("(?=[A-Z])");
while (sc.hasNext()) {
System.out.println(sc.next());
} // prints "Oh", "My", "Goodness", "How", "Are", "You?"
As you can see, Scanner is quite powerful! You should prefer it to StringTokenizer, which is now a legacy class.
See also
Java Tutorials/Essential Classes/Regular expressions
regular-expressions.info/Tutorial
Related questions
Scanner vs. StringTokenizer vs. String.Split

you mean input from user
Scanner s = new Scanner(System.in);
System.out.print("Enter a number: ");
int number = s.nextInt();
//process the number

If you are talking about those parameters from the console input, or any other String parameters, use static Integer#parseInt() method to transform them to Integer.