Develop Reference

How to correctly use try-catch-finally blocks in Java?

What would be the proper way to use the finally block to close out of the file that I was in : event.dat. If you could help me makes sense of this, I'd greatly appreciate it. I've literally spent 4 hours moving things around, playing with the code and searching for answers online, but to no avail. Code works great without that portion, but I need to know how it works for future instances. Thank you and have a good day!
I am having issues with this block:
finally{
fstream.close();
}
Which is located in the following code:
String answer;
String letter;
String inType = "";
double inAmount = 0;
double amount;
description();
GironEventClass newInput = new GironEventClass();
try{
File infile = new File("event.dat");
Scanner fstream = new Scanner(infile);
System.out.println("File Contents ");
while(fstream.hasNext())
{
inAmount = fstream.nextInt();
inType = fstream.next();
try{
newInput.donations(inType, inAmount);
}
catch(IllegalArgumentException a){
System.out.println("Just caught an illegal argument exception. ");
}
finally{
fstream.close();
}
}
System.out.println("Total Sales: " + newInput.getSale());
System.out.println("Donations: " + newInput.getDonated());
System.out.println("Expenses: " + newInput.getExpenses());
}
catch (FileNotFoundException e){
System.out.println("\nEvent.dat could not be opened. ");
}
do{
System.out.print("Are there any more items to add that were not in the text file? (Type 'Y' or 'N')");
answer = keyboard.next();
if (("Y".equals(answer)) || ("y".equals(answer)))
{
letter = inLetter();
amount = inAmount();
newInput.donations(letter, amount);
}
}while (("Y".equals(answer)) || ("y".equals(answer)));
newInput.display();
}
public static String inLetter(){
Scanner keyboard = new Scanner(System.in);
String result;
String resultTwo;
System.out.println("T = Tiket Sales");
System.out.println("D = Donations");
System.out.println("E = Expenses");
System.out.print("Please input an identifier ");
result = keyboard.nextLine();
resultTwo = result.toUpperCase();
return resultTwo;
}
public static double inAmount(){
Scanner keyboard = new Scanner(System.in);
double result;
System.out.println("Please input an amount ");
result = keyboard.nextInt();
if(result <= 0.0){
System.out.print("Please input a positive and non-zero amount ");
result = keyboard.nextInt();
}
return result;
}
public static void description(){
System.out.println("The program will ask you what amount is being spent on what.");
System.out.println(" ex: expenses, ticket sales, and profts.");
System.out.println("This program will help determine whether the event generated or lost money.");
}

This is how the scanner should work:
while scanner has object
read them ( one object per method's call)
when objects are done
close the reader.
Your problem is that you use the close function when the while conclusion is true. So you should put it outside of the while loop

The Try block attempts to do something.
The Catch block only executes if something went wrong during the Try block.
The Finally block executes after the Try block (and Catch block, if executed) EVERY time.
Your issue is that you attempt to close the fstream inside the while loop.
while(fstream.hasNext()) <----- referencing fstream
{
inAmount = fstream.nextInt();
inType = fstream.next();
try{
newInput.donations(inType, inAmount);
}
catch(IllegalArgumentException a){
System.out.println("Just caught an illegal argument exception. ");
}
finally{
fstream.close(); <---- closing said stream
}
}
Since you've closed the stream, this while loop should only ever execute once. That being said, close the fstream outside the while loop and your program should return to normal.
You can also move the while loop inside the try block, which would work as well.

You need to declare fstream outside the try block otherwise it will be not visible in the finally block.
File infile = null;
File infile = null;
try {
infile = new File("event.dat");
fstream = new Scanner(infile);
...
} catch ( ) {
...
} finally {
// You need to handle exceptions also here
infile.close();
fstream.close();
}
You can also use the new try with resources syntax and leave to the jvm the closure of your streams.

How to ask user if he wants to start program all over again if he typed a char/string instead of a number?

I'm new here and new to Java, started like 1-2 weeks ago and I wanted to make an app that is applying pythagora's theory. All done, but now I'm stuck in a place where I want the user to be asked if he wants to try again, start again, if he enters a character or string instead of a number.
What am I doing wrong? This is my code, I've added comments too so it would be easier in case you don't understand what I was trying to achieve.
Thanks in advance!
package pitagoracalculator;
import java.util.Scanner;
public class PitagoraCalculator {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
double nr1;
double nr2;
double ipot;
boolean raspuns;
String raspuns1;
do{
try{
//user is asked for first number
System.out.print("Introduceti primul numar: ");
nr1 = input.nextDouble();
//user is asked for second number
System.out.print("Introduceti al 2-lea numar: ");
nr2 = input.nextDouble();
//result of calculation
ipot = (nr1*nr1)+(nr2*nr2);
System.out.println("Rezultatul este: "+ipot+"^2");
//in case user inserts a string/char instead of a double => error
} catch (Exception e){
System.err.println("Nu ati introdus un numar.");
break;
}
//user is asked if he wants to do another calculation
System.out.println("Doriti sa faceti un alt calcul? (da/nu)");
raspuns1 = input.next();
//if his answer is yes, raspuns = true, else raspuns = false
if(raspuns1.equalsIgnoreCase("da"))
raspuns = true;
else{
System.out.println("La revedere!");
raspuns = false;
}
//checks if the answer was true or false
}while(raspuns == true);
}
}

Replace
//in case user inserts a string/char instead of a double => error
} catch (Exception e){
System.err.println("Nu ati introdus un numar.");
break;
}
with
//in case user inserts a string/char instead of a double => error
} catch (Exception e){
System.err.println("Nu ati introdus un numar.");
//clear pending input.
if (input.hasNext()) {
input.next();
}
continue;
}
"break" exits the loop, "continue" will continue with the next cycle of the loop.
You also have to initialize the raspuns variable with true
boolean raspuns = true;

You can call method in exceptional case.For this you should make a method in same class and put your all stuff in that method use if-else loop..

Why do I get a "No line found" Exception in this code?

The following method causes a No line found Exception:
private static String getRebaseAnswer(boolean isFirst, boolean isLast) {
System.out.println("Would you like to (c)ontinue, (s)kip this commit, or"
+ " change this commit's (m)essage?");
Scanner in = new Scanner(System.in);
String answer;
while (true) {
answer = in.nextLine(); // <--- This Line
if (answer.equals("c") || answer.equals("m")) {
in.close();
return answer;
} else if (answer.equals("s") && !isFirst && !isLast) {
in.close();
return answer;
} else {
System.out.println("Would you like to (c)ontinue, (s)kip this commit, or"
+ " change this commit's (m)essage?");
}
}
}
I am calling the method in this method:
...
String answer;
Scanner in;
currHead = branchHeads.get(arg);
while (toRebase != null) {
System.out.println("Currently replaying:");
toRebase.getNode().printInfo();
answer = getRebaseAnswer(isFirst, toRebase.getParent() == null); // <--- This Line
...
What is causing the error?? Shouldn't the scanner wait for me to input a line before continuing the getRebaseAnswer method? A different method in my code has the exact same structure as the above method and encounters no problems. I've checked multiple other posts about this problem but their suggestions are all not pertinent to this problem or do not solve it.
This method runs with no problems:
private static boolean handleDangerous() {
System.out.println("Warning: The command you entered may alter the files in your"
+ " working directory. Uncommitted changes may be lost. Are you sure you"
+ " want to continue? (yes/no)");
Scanner in = new Scanner(System.in);
String answer;
while (true) {
answer = in.nextLine();
if (answer.equals("yes")) {
in.close();
return true;
} else if (answer.equals("no")) {
in.close();
return false;
} else {
System.out.println("Not a valid answer, please enter (yes/no).");
}
}
}

When you create a scanner connected to System.in and close it, you also close System.in. Therefore, subsequent attempts to read from System.in will result in the exception you observe.
The way is avoid this is to create the Scanner only once, and never close it until your program is finished. This Scanner should be passed to whichever function needs to read from System.in.

don't close scanner otherwise Stream will also be closed.
in.close();
remove this line from current location and put it in main method at the end so after all the operation stream will be closed..
You might be calling some other method which have already closed the stream and then you are calling this method.

Java bufferreader crashes on ctrl z

I'm making a game which plays until the user enters quit in the command line.
The user can enter different commands like get and go, with the get command the user can say what to get like, get baseball bat. What I do in my code is split the command.
everything is working fine but I have found a bug which I can't solve. If I enter "get" and press space and then ctrl+z it gets in a while loop which never ends.
It only happens with ctrl+z (1 time with ctrl c but after that 1 time not anymore)
private void run()
{
while (! quitCommand)
{
String input = null;
try
{
input = null;
System.out.println("Input "+ input);
System.out.println("Give a command.");
BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
input = is.readLine();
handleCommand(input);
// As long as the command isn’t to quit:
// get the next input line and handle it. (With handleCommand.)
}
catch (Exception e)
{
System.out.println("Something went wrong we are sorry try again.");
e.printStackTrace();
}
}
}
/**
* #param userInput (This is the entire input string from the user.)
*
* (Tell others to) Perform the task which belongs to the given
* command.
*/
private void handleCommand(String userInput)
{
// Split the user input string.
if (userInput != null) // user input can not be empty
{
String[] delenTekst = userInput.split(" ");
// The first word is a command. The rest is extra information
String command = delenTekst[0];
String extra = "";
for (int i = 1; i < delenTekst.length; i ++)
{
if (i == 1)
{
extra = extra + delenTekst[i];
}
else
{
extra = extra +" " + delenTekst[i];
}
}
switch (command)
{
// Check if the command is to travel between rooms. If so, handle
case "go"
:
this.checkRoomTravel(extra);
break;
// If there isn't any room travel, then check all other command
case "get"
:
System.out.println("Looking for " +extra );
this.handleGetCommand(extra);
break;
case "quit"
:
quitCommand = true;
break;
default
:
System.out.println("Command is not known try help for information");
break;
}
}
else
{
userInput = "help";
}
}
I'm new to java so it can be something really simple.
On the top of my script I have a private boolean quitCommand = false; which is to check if the user entered quit.

Ctrl+Z closes the Console and therefore your readLine() returns null as pretended to indicate that end of file was reached. So all you need to do, is to check for null returned by readLine() and handle this as you handle the "quit".
I've changed your code (just to test my thesis) and also stream lined a few things, e.g. you dont need to recreate a BufferedReader every time you read a line.
private boolean quitCommand = false;
private void runIt() {
BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
String input = null;
while(!quitCommand) {
try {
System.out.print("Give a command: ");
input = is.readLine();
// As long as the command isn’t to quit:
if(input == null || "quit".equals(input.trim())) quitCommand = true;
if(quitCommand) break;
// get the next input line and handle it. (With handleCommand.)
String[] words = input.trim().split("\\s+");
// ** This is the original handleCommand line **
System.out.println(input + ":" + Arrays.toString(words));
}
catch (Exception e) {
System.out.println("Something went wrong we are sorry try again.");
e.printStackTrace();
}
}
}
BTW: To split the input into words I'd use the regular expression as shown in my code. This works also if the user enters tabs or multiple spaces.

On DOS/Windows Ctrl+Z means end of input. This causes readLine() to return null no matter how many times you call it. This is likely to cause your code to fail as you don't appear to check for it. I suspect you are getting a NullPointerException which you are pretending didn't happen and trying again, endlessly.

Java reprompt for user input with try... catch

So if a user puts in a postfix value like say 453-* , my method EvalPostFix() does the work, but when the user inputs something invalid like 43*+ or any invalid string want the program to repromt the user for input dont know how to implement with try catch..
'
String in;
while(true){
System.out.println("Please enter the numbers first followed by the operators, make sure to have one less operator than of numbers");
try {
in = getString();
int result = EvalPostFix(in);
System.out.println(result);
} catch (IOException e) {
// TODO Auto-generated catch block
String s = "Not a valid postfix string";
e.toString();
in = getString();
}
}
'

Looking at your code I think you just need to get rid of the in = getString(); in the catch block and add an break at the end of the try block.
I don't recommend using a while(true) or an IOException for what you are doing though, but that should get your code working.

Use a flag:
boolean flag = false;
while(!flag)
{
//make the loop break, if no exception caught
flag = true;
try{
}
catch{
//make the loop repeat
flag = false;
}
}
this should repeat the prompt every time you catch an exception. you can also use this to validate input.
how the flag is oriented depends on your preference. I like to flag true when an error occured ;)
this will also break your while loop, as soon as you get a valid input.

Something like this is can be used to get an input of desired specifications
public static void userMove() {
System.out.println("Where would you like to move? (R, L, U, D)\n");
Scanner input = new Scanner(System.in) ;
while (true){
String userInput = input.next() ;
if(userInput.length()>1){
System.out.println("Please input a valid direction");
}else{
break ;
}
}
}