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Why does initializing a string in an if statement seem different than in a switch statement? [duplicate]

This question already has answers here:
Why do I get a "variable might not have been initialized" compiler error in my switch block?
(2 answers)
Closed 4 years ago.
I'm learning Java and I'm making simple programs to find the season that a month is in, based off some book examples. These two classes demonstrate two ways of testing a value: if/else if statement, and switch statement. The thing I'm confused with is the string that is used to hold the season. When I declare it as just String season; it works with the if statements. But with the switch statement, doing that produces a "The local variable season may not have been initialized" error.
public class IfElse {
public static void main(String args[]) {
int month = 5;
String season;
// isn't initialized, works fine
if(month == 12 || month == 1 || month == 2)
season = "Winter";
else if(month == 3 || month == 4 || month == 5)
season = "Spring";
else if(month == 6 || month == 7 || month == 8)
season = "Summer";
else
season = "Fall";
// this is okay
System.out.println("May is a " + season + " month.");
}
}
Not initializing season at the same time as declaration works fine for the above code, but the season variable in the last println() for the switch produces an error if it's declared the same way.
The following code doesn't work:
public class Switch {
public static void main(String args[]) {
int month = 5;
String season;
// HAS to be initialized, currently causes error
switch(month) {
case(12):
case(1):
case(2):
season = "Winter";
break;
case(3):
case(4):
case(5):
season = "Spring";
break;
case(6):
case(7):
case(8):
season = "Summer";
break;
case(9):
case(10):
case(11):
season = "Fall";
break;
default:
System.out.println("Invalid month");
break;
}
System.out.println("May is a " + season + " month");
} // produces an error if season isn't initialized to null or ""
}
What causes this? Is it the braces enclosing the switch statement, or a problem with the switch statement itself? How is initializing a string inside an if statement any different than initializing it inside a switch statement? I just can't seem to understand this.
Sorry if this is extremely obvious or if it seems like a dumb question.

That is because you did not specify what season has to be in the default case. What happens when month is not within 1-12? season will not be initialized.
If you are expecting strictly only 1-12 as month input, then you might want to consider throwing an Exception in default:
default:
throw new IllegalArgumentException("Invalid month");

In your first example, there is no path through the code that fails to assign a value to 'season'. In the second example, the default case does not assign a value, so the last print ("May is...") can be executed with an uninitialized value.

In your if/else code, there is an assurance that the variable season will get a value. That is, the else statement.
Your switch code does not have it. Look what will happen to the variable season if the given value for month is 13 -- it will not get a value, and will remain un-initialised.

You should use this
public class Switch {
public static void main(String args[]) {
int month = 5;
String season;
// HAS to be initialized, currently causes error
switch(month) {
case 12:
case 1:
case 2:
season = "Winter";
break;
case 3:
case 4:
case 5:
season = "Spring";
break;
case 6 :
case 7 :
case 8 :
season = "Summer";
break;
case 9 :
case 10 :
case 11 :
season = "Fall";
break;
default:
season = "Invalid";
break;
}
System.out.println("May is a " + season + " month");
} // produces an error if season isn't initialized to null or ""
}

Java - Method to loop until a Y or N is entered

I have looked around and haven't found any questions that has been directly answered for my problem, if i'm wrong, sorry. I'm writing a program that is supposed to take in your birth information, year, month, day, hour, minute and then ask if they would like to do it again but using a method and loop until a Y/N is entered.
My problem is im not able to get the method to take in the Y/N and end the program, as well as end when a N is entered.
I think there is supposed a while or some sort of loop that needs to be in the method but im having a hard time figuring that out.
All my other methods work except for this one, any help would be appreciated, thanks.
This is the code I have for my method now:
public static boolean getYNConfirm(Scanner pipe, String prompt)
{
String choice="";
System.out.println(prompt);
choice = pipe.nextLine();
if(choice.equalsIgnoreCase("Y"))
{
return true;
}
else
{
return false;
}
}
And this is the code for my main program:
public static void main(String[] args)
{
int year, month, day, hour, minutes;
String msg="";
boolean done = false;
Scanner in = new Scanner(System.in);
while(!done)
{
year = SafeInput.getIntInRange(in, "Enter the year you were born: ", 1965, 2000);
month = SafeInput.getIntInRange(in, "Enter your month of birth: ", 1, 12);
switch (month)
{
case 1:
msg = "January";
break;
case 2:
msg = "February";
break;
case 3:
msg = "March";
break;
case 4:
msg = "April";
break;
case 5:
msg = "May";
break;
case 6:
msg = "June";
break;
case 7:
msg = "July";
break;
case 8:
msg = "August";
break;
case 9:
msg = "Septemeber";
break;
case 10:
msg = "October";
break;
case 11:
msg = "November";
break;
case 12:
msg = "December";
break;
}
hour = SafeInput.getIntInRange(in, "Enter the hour you were born in: ", 1, 24);
minutes = SafeInput.getIntInRange(in, "Enter the minutes you were born: ", 1, 59);
System.out.println("You were born: " + year + " , " + msg + " , " + hour + " hr. " + minutes + " mins. ");
SafeInput.getYNConfirm(in, "Would you like to play again?");
}
}
}
Thanks for any help.

Value of done is always false. Change on last line:
done = SafeInput.getYNConfirm(in, "Would you like to play again?");
EDIT:
Your logic does not fit if you return true Change here too:
if(choice.equalsIgnoreCase("Y"))
{
return false;
}
else
{
return true;
}

you need to change the value of done base on the return of the getYNConfirm method. So the last line should be
done = SafeInput.getYNConfirm(in, "Would you like to play again?");

Java - error in prompts [duplicate]

I have a question that I can't figure it out , Thank you:
Write a program that prompts the user to enter an integer for today's day of the week (Sunday is 0 ,Monday is 1 ,... and Saturday is 6). Also prompt the user to enter the number of days after today for a future day and display the future day of the week .Here is the sample run:
Enter today's day: 1
Enter number of the day elapsed since today:3
Today is monday and the future day is thursday
My try is:
Scanner input = new Scanner(System.in);
System.out.print("Enter today's day (0 - 6): ");
int day = input.nextInt();
System.out.print("Enter the number of days elapsed since today: ");
int elapsed = input.nextInt();
if(day == 0)
{
System.out.println("Sunday");
}
if(day == 1)
{
System.out.println("Monday");
}
if(day == 2)
{
System.out.println("Tuesday");
}
if(day == 3)
{
System.out.println("Wednesday");
}
if(day == 4)
{
System.out.print("Thursday");
}
if(day == 5)
{
System.out.print("Friday");
}
if(day == 6)
{
System.out.print("Saturday");
}
System.out.print("Today is " + day + " and the future day is " + elapsed);

As you need day-number to day-string twice, put it in a separate function. I want to show you a couple of possible approaches. Version 1, basic, simple and tidy:
// isolate the daynumber --> daystring in a function, that's tidier
String dayFor (int daynumber) {
String dayAsString = "ERROR"; // the default return value
switch(dayNumber) {
case 0 :
dayAsString = "Sunday";
break;
case 1 :
dayAsString = "Monday";
break;
// and so on, until
case 6 :
dayAsString = "Saturday";
break;
}
return dayAsString;
}
A much shorter version that uses an array instead of the switch statement:
String dayFor (int daynumber) {
String dayStrings[] = new String[]{"Sunday","Monday", .... "Saturday"};
// notice that daynumber's modulo is used here, to avoid index out of
// bound errors caused by erroneous daynumbers:
return dayStrings[daynumber % 7];
}
It might be tempting to try something along the lines of the following function where each case returns immediately, but having multiple return statements is discouraged. Just showing it here because it is technically possible, and you'll encounter it sometimes
String dayFor (int daynumber) {
switch(dayNumber) {
case 0 :
return "Sunday";
case 1 :
return "Monday";
// and so on, until
case 6 :
return "Saturday";
}
// normally not reached but you need it because the compiler will
// complain otherwise anyways.
return "ERROR";
}
After this rather long intro the main function becomes short and simple. After the input you just need:
// present day + elapsed modulo 7 = the future day
int future = (day + elapsed) % 7;
System.out.print("Today is " + dayFor(day) + " and the future day is " + dayFor(future) );
Don't forget to add code to check your inputs!

You can do it better by using an array to store the the day names.
String[] dayNames = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
Now you can use the user input as the index
int nameIndex = //... get input
//validate input
//dayNames[nameIndex] is the day of the week
Now get the input for number of days to add
int numDays = //...get input
Then just add that many days to compute the index for future day of week
int futureNameIndex = nameIndex; //start with entered day of week index
for(int i=0; i<numDays; i++) {
futureNameIndex++; //increment by 1 for numDays times
if(futureNameIndex == dayNames.length) { //if the index reaches lenght of the array
futureNameIndex = 0; //reset it to 0
}
}
I think you will find that one easier to understand. Finally
//dayNames[futureNameIndex] is the future day of week.

The question gives you the days ranging from 0-6, instead of 1-7(conventional). Now, for example, if the day today is 1(Monday) and the daysElapsed since today is 3, then the day should be Thursday. Since this question has the initial day inclusive, the resulting day will be after 1(Monday),2,3(Wednesday) have passed, that is Thursday.
Let's take an example and apply it to the code below.
day = 1;
daysElased = 3;
else if(day > 0 && day < 7) , which is the case
{
sum = 1(day) + 3(daysElapsed); // sum = 4
}
If sum is in the range of 0-6, each if case can be created corresponding to each day. In the case above, the sum is less than 6, so it will be having its own if clause. Had the sum been greater, for example, days = 1 and daysElapsed = 6, then sum = 1(days) + 6(daysElapsed) = 7.
In this case it will match the clause if(sum > 6), then sum = sum % 7 = 7 % 7 = 0 = Sunday. This means that the days from 1(Monday) to 6(Saturday) have been elapsed, so the day will be Sunday(0).
if(day == 0) // If the present day entered is Zero(0 is for Sunday)
{
sum = daysElapsed; // daysElapsed will be entered by the user
}
else if(day > 0 && day < 7) // If the present day is > 0 but < 7 (1 - 6 days)
{
sum = day + daysElapsed; //
}
if(sum>6) // if 0<= sum <=6 , 6 if cases can be created. If sum > 6 :
{
sum = sum % 7;
}
if(sum == 0)
{
System.out.println("Day is Sunday.");
}
.
.
.
.
else if(sum == 6)
{
System.out.println("Day is Saturday.");
}

As I know, this question is from the book "Introduction To Java Programming". Where this question is asked, you don't have any knowledge of methods, loops, arrays etc. so I will just use Selections.
Here, when I tried to solve with a better way, I could not find any since we cannot use arrays which could be very helpful or methods which is even better. That's why this question is a little redundant in book.
And you really should not use if statements because switch is much better in this case.
System.out.println("Enter today's number (0 for Sunday, 1 for Monday...) :");
int todayNo = in.nextInt();
System.out.println("Enter the number of days elapsed since today:");
int elapsedDay = in.nextInt();
int futureDay = (todayNo + elapsedDay) % 7;
switch (todayNo) {
case 0:
System.out.print("Today is Sunday and");
break;
case 1:
System.out.print("Today is Monday and");
break;
case 2:
System.out.print("Today is Tuesday and");
break;
case 3:
System.out.print("Today is Wednesday and");
break;
case 4:
System.out.print("Today is Thursday and");
break;
case 5:
System.out.print("Today is Friday and");
break;
case 6:
System.out.print("Today is Saturday and");
break;
}
switch (futureDay) {
case 0:
System.out.print(" the future day is Sunday.");
break;
case 1:
System.out.print(" the future day is Monday.");
break;
case 2:
System.out.print(" the future day is Tuesday.");
break;
case 3:
System.out.print(" the future day is Wednesday.");
break;
case 4:
System.out.print(" the future day is Thursday.");
break;
case 5:
System.out.print(" the future day is Friday.");
break;
case 6:
System.out.print(" the future day is Saturday.");
break;
}
Here, the only thing that you maybe don't know is System.out.print();. The only difference with the System.out.println(); is with this method, this one doesn't print on a new line, it prints on the same line which is what we need here. Tinker with it to understand better.

package javaapplication2;
import java.util.Scanner;
public class JavaApplication2 {
public static void main(String[] args) {
int day, eday, fday;
String str, str1;
Scanner S = new Scanner(System.in);
System.out.println("Enter today's day: ");
day = S.nextInt();
System.out.println("Enter the number of days elapsed since today: ");
eday = S.nextInt();
if (day == 0) {
str = "Sunday";
System.out.print("Today is " +str + " and ");
}
else if (day == 1) {
str = "Monday";
System.out.print("Today is " +str + " and ");
}
else if (day == 2) {
str = "Tuesday";
System.out.print("Today is " +str + " and ");
}
else if (day == 3) {
str = "Wednesday";
System.out.print("Today is " +str + " and ");
}
else if (day == 4) {
str = "Thursday";
System.out.print("Today is " +str + " and ");
}
else if (day == 5) {
str = "Friday";
System.out.print("Today is " +str + " and ");
}
else if (day == 6) {
str = "Saturday";
System.out.print("Today is " +str + " and ");
}
fday = day + eday;
if (fday % 7 == 0) {
str1 = "Sunday";
System.out.print("Future day is " +str1);
}
else if (fday % 7 == 1) {
str1 = "Monday";
System.out.print("Future day is " +str1);
}
else if (fday % 7 == 2) {
str1 = "Tuesday";
System.out.print("Future day is " +str1);
}
else if (fday % 7 == 3) {
str1 = "Wednesday";
System.out.print("Future day is " +str1);
}
else if (fday % 7 == 4) {
str1 = "Thursday";
System.out.print("Future day is " +str1);
}
else if (fday % 7 == 5) {
str1 = "Friday";
System.out.print("Future day is " +str1);
}
else if (fday % 7 == 6) {
str1 = "Saturday";
System.out.print("Future day is " +str1);
}
}

The question is from a book titled "Introduction to Java programming" by Y. Daniel Liang. Apart from using the string type, which I believe is covered in the next chapter; the solution I wrote for this exercise uses only what you have been taught so far.
import java.util.Scanner;
public class Exercise_03_06 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter today's day: ");
int todaysDay = input.nextInt();
System.out.print("Enter the number of days elapsed since today: ");
int elapsedDays = input.nextInt();
int futureDay = (todaysDay + elapsedDays) % 7;
String day_of_week = "";
switch (todaysDay) {
case 0: day_of_week = "Sunday"; break;
case 1: day_of_week = "Monday"; break;
case 2: day_of_week = "Tuesday"; break;
case 3: day_of_week = "Wednesday"; break;
case 4: day_of_week = "Thursday"; break;
case 5: day_of_week = "Friday"; break;
case 6: day_of_week = "Saturday";
}
switch (futureDay) {
case 0:
System.out.println("Today is " + day_of_week + " and the future day is Sunday."); break;
case 1:
System.out.println("Today is " + day_of_week + " and the future day is Monday."); break;
case 2:
System.out.println("Today is " + day_of_week + " and the future day is Tuesday."); break;
case 3:
System.out.println("Today is " + day_of_week + " and the future day is Wednesday."); break;
case 4:
System.out.println("Today is " + day_of_week + " and the future day is Thursday."); break;
case 5:
System.out.println("Today is " + day_of_week + " and the future day is Friday."); break;
case 6:
System.out.println("Today is " + day_of_week + " and the future day is Saturday.");
}
}
}
Output:
Enter today's day: 0
Enter the number of days elapsed since today: 31
Today is Sunday and the future day is Wednesday.
Notes:
The first switch statement assigns a day of type string to the variable day_of_week which is later used for printing "today's day".
To obtain the future day, you must find the remainder of the sum of today's day and the number of days elapsed divided by 7.
The last switch statement "matches" a case number that is identical to the number stored within the futureDay variable (which is obtained by performing the mathematical operation noted above).

Validating date from user input

I'm writing a program where I am supposed to have the user input a date from the year 0 - 4000. I'm supposed to see if the date is valid, and if it was a leap year or not. I'm having problems in my code.
I'm getting an else without error on line 57.
I also am not sure how to how to say if the date is valid or not.
IE: this date is valid, is a leap year - or is not valid is not a leap year ...etc...
I'm still a beginner so I dont want the code written for me but I would like to know how to fix it! Thank you.
import java.util.*;
public class LegalDate //file name
{
public static void main (String [] args)
{
Scanner kb = new Scanner (System.in); //new scanner
//name the variables
int month, day, year;
int daysinMonth;
boolean month1, year1, day1;
boolean validDate;
boolean leapYear;
//ask the user for input
//I asked the MM/DD/YYYY in seperate lines to help me visually with the program
System.out.println("Please enter the month, day, and year in interger form: " );
kb.nextInt();
//now I'm checking to see if the month and years are valid
if (month <1 || month >12)
{ month1 = true;}
if (year <0 || year >4000)
{year1= true;}
//I'm using a switch here instead of an if-else statement, which can also be used
switch (month) {
case 1:
case 3:
case 5: //months with 31 days
case 7:
case 8:
case 10:
case 12:
numDays = 31;
break;
case 4:
case 6: //months with 30 days
case 9:
case 11:
numDays = 30;
break;
case 2:
if (((year % 4 == 0) && !(year % 100 == 0)) || (year % 400 == 0)) //formula for leapyear
numDays = 29;
{
system.out.println("is a leap year");
}
else
numDays = 28;
{
system.out.println("is not a leap year");
}
break;
default:
System.out.println("Invalid month.");
break;
if (month1 == true)
if (day1 == true)
if (year1 == true)
System.out.println ("date is valid ");
else
if (month1 == false)
System.out.println ("date is invalid");
else
if (day1 == false)
System.out.println ("date is invalid");
else
if (year1 == false)
System.out.println ("date is invalid");
}}
}

On line 57, you open a new code block but nothing is able to access it. I believe you meant to type:
else{
numDays = 28;
system.out.println("is not a leap year");
}
As a small tip, you can change this:
if (month1 == true)
if (day1 == true)
if (year1 == true)
System.out.println ("date is valid ");
to this:
if (month1 && day1 && year1)
System.out.println ("date is valid ");
Since the boolean comparison operators return true or false, you can tell that the condition just needs to be boolean. Since month1, day1, and year1 are all boolean values, you dont need to compare them to anything.
What the condition means, in the event you don't know, is if month1 and day1 and year1 are all true, then print date is valid

Why don't you try Java 8 date time API
It validates date and do much more for you
Like
try {
LocalDate date =LocalDate.of(2016, 12, 31);
if(date.isLeapYear())
System.out.println("Leap year");
else
System.out.println("Not leap year");
}
catch(DateTimeException e) {
System.out.println(e.getMessage());
}

You don't appear to be placing the code between your 'if' and 'else' statements in curly brackets, which means that the statement will only apply to the next line. For example:
if (a)
b = true
c = true
else
d = true
is read as
if (a) {
b = true
}
c = true
else {
d = true
}
Hopefully, you can see how the compiler wouldn't understand this, as an 'else' statement must occur directly after its associated 'if' block.
I would suggest adding some methods to simplify your code. For example:
public static boolean isLeapYear(int year) {
return (((year % 4 == 0) && !(year % 100 == 0)) || (year % 400 == 0));
}
Also, if you use boolean variables to store information, you can print it all neatly at the end. For example, you could instantiate a variable 'isValid' to true at the top of your code, set it to false if you calculate that the date is invalid, and use an if statement at the end to print your result.
I know you said you didn't want it written for you, but that's the easiest way to demonstrate the importance of methods. Hopefully you can see how this is more readable than your version?
import java.util.Scanner;
public class LegalDate {
static final int maxYear = 4000;
public static void main (String [] args) {
int month, day, year;
boolean leapYear, validDate = false;
Scanner kb = new Scanner (System.in);
System.out.println("Please enter the month, day, and year in interger form.");
System.out.print("Month: ");
month = kb.nextInt();
System.out.print("Day: ");
day = kb.nextInt();
System.out.print("Year: ");
year = kb.nextInt();
leapYear = isLeapYear(year);
validDate = isValidDate(month, day, year);
System.out.printf("%nThe date is %svalid and is %sa leap year.%n", validDate ? "" : "not ", leapYear ? "" : "not ");
kb.close();
}
public static int numDaysInMonth(int month, boolean isLeapYear) {
switch (month) {
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
return 31;
case 4:
case 6:
case 9:
case 11:
return 30;
case 2:
if (isLeapYear) {
return 29;
} else {
return 28;
}
default:
return 0;
}
}
public static boolean isLeapYear(int year) {
return (((year % 4 == 0) && !(year % 100 == 0)) || (year % 400 == 0));
}
public static boolean isValidDate(int month, int day, int year) {
return (month >= 1 && month <= 12) && (day >= 1 && day <= numDaysInMonth(month, isLeapYear(year))) && (year >= 0 && year <= maxYear);
}
}
If you have any questions I'll try my best to answer them!

The syntax of if-else statement on line 50 of your program is not correct.
It's a dangling else. Enclosing the body of if and else statement within braces should resolve this.
if (((year % 4 == 0) && !(year % 100 == 0)) || (year % 400 == 0))
{
numDays = 29;
system.out.println("is a leap year");
}
else
{
numDays = 28;
system.out.println("is not a leap year");
}
you can make use of IDE or give attention to compiler error message to resolve such errors.

What my system.out.println coming out vertical

everything works perfect and I know more or less java sout formatting, but I ve been following this book and its been explaining how the logic works. One thing I can not seem to figure out is why my output is coming out vertical and in the book its perfectly horizontal? I have it formatted exactly the same way
this is my java code
/*
* To change this license header, choose License Headers in Project Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package b.elsestatements;
import java.util.*;
/**
*
* #author willc86
*/
public class Clock {
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
Calendar now = Calendar.getInstance();
int hour = now.get(Calendar.HOUR_OF_DAY);
int minuite = now.get(Calendar.MINUTE);
int month = now.get(Calendar.MONTH) + 1;
int day = now.get(Calendar.DAY_OF_MONTH);
int year = now.get(Calendar.YEAR);
if (hour > 17){
System.out.println("Good evening");
} else if (hour < 12) {
System.out.println("Goodmorning");
} else {
System.out.println("Good afternoon");
}
System.out.println("It is ");
if (minuite != 0) {
System.out.println("" + minuite + " ");
// if (minuite != 1) {
// System.out.println("minuites");
// } else {
// System.out.println("minuite");
// }
System.out.println((minuite != 1) ? "minuites" : "minuite ");
System.out.println("Past");
System.out.println((hour > 12) ? (hour - 12) : hour);
System.out.println("O clock on ");
//month case
switch (month) {
case 1:
System.out.println("Jan");
break;
case 2:
System.out.println("Feb");
break;
case 3:
System.out.println("Mar");
break;
case 4:
System.out.println("Apr");
break;
case 5:
System.out.println("May");
break;
case 6:
System.out.println("Jun");
break;
case 7:
System.out.println("Jul");
break;
case 8:
System.out.println("Aug");
break;
case 9:
System.out.println("Sept");
break;
case 10:
System.out.println("Oct");
break;
case 11:
System.out.println("Nov");
break;
case 12:
System.out.println("Dec");
}
System.out.println("On " + year + " day " + day);
}
}
}

Maybe on your book it doesn't say: System.out.println maybe it says: System.out.print
System.out.print("" + minuite + " ");
System.out.print((minuite != 1) ? "minuites" : "minuite ");
System.out.print("Past");
System.out.print((hour > 12) ? (hour - 12) : hour);
System.out.print("O clock on ");
Maybe that should solve your problem.
I mean change System.out.println to System.out.print
And your output should look like this:
Good afternoon
It is 52 minuitesPast2O clock on Apr
On 2014 day 28
System.out.println will create a new line (ln on the end stands for line) so if you take it out, it should print horizontal without creating new lines.
Another way to create new lines on a System.out.print is by using escape characters such as "\n" that will do the same as System.out.println
This case:
System.out.println("Hello"); //Will create a new line
System.out.println("World"); //Will create a new line
System.out.print("Hello"); //Won't create a new line
System.out.print("World"); //Won't create a new line
System.out.print("Hello \n"); //Will create a new line
System.out.print("World"); //Won't create a new line
The output for the 3 cases above will be:
Hello
World
Hello World
Hello
World