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How do I accept the user input and put it into the while condition

This is my code, the while loop does not have an input and the rep variable does not accept an input:
import java.util.Scanner;
public class MixedData {
public static void main(String[] args) {
String rep = "";
do {
Scanner keyboard = new Scanner(System.in);
System.out.print("Enter your full name");
String name = keyboard.nextLine();
System.out.print("Enter your GPA: ");
double gpa = keyboard.nextDouble();
System.out.println("Name: " + name + ", GPA: " + gpa);
System.out.println("Do you want to enter the data for another student?(y/n)");
rep = keyboard.nextLine();
} // This does not accept input
while (rep.equals("y"));
}
}

Either just add one more keyboard.nextLine() before rep = keyboard.nextLine(); (in order to clear the newline character), or read your double gpa value with:
double gpa = Double.parseDouble(keyboard.nextLine());
Important point to understand here (especially if you're a novice Java developer), about why your code doesn't work, is, that you invoke nextDouble() as a last method on your Scanner instance, and it doesn't move the cursor to the next line.
A bit more details:
All the methods patterned nextX() (like nextDouble(), nextInt(), etc.), except nextLine(), read next token you enter, but if the token isn't a new line character, then the cursor isn't moved to the next line. When you enter double value and hit Enter, you actually give to the input stream two tokens: a double value, and a new line character, the double value is initialized into the variable, and the new line character stays into input stream. The next time you invoke nextLine(), that very new line character is read, and that's what gives you an empty string.

Here's the same code using a while loop instead of do-while. It works the way you want it to.
import java.util.Scanner;
public class MixedData {
public static void main(String[] args) {
String rep = "y";
while (!rep.equals("n")) {
Scanner keyboard = new Scanner(System.in);
System.out.print("Enter your full name: ");
String name = keyboard.nextLine();
System.out.print("Enter your GPA: ");
double gpa = keyboard.nextDouble();
System.out.println("Name: " + name + ",GPA: " + gpa);
System.out.println("Do you want to enter the data for another student?(y/n)");
rep = keyboard.next();
}
}
}

You need to skip blank lines.
public static void main(String[] args) {
String rep;
Scanner keyboard = new Scanner(System.in);
do {
System.out.print("Enter your full name");
String name = keyboard.nextLine();
System.out.print("Enter your GPA: ");
double gpa = keyboard.nextDouble();
System.out.println("Name: " + name + ", GPA: " + gpa);
System.out.println("Do you want to enter the data for another student?(y/n)");
rep = keyboard.next();
keyboard.skip("\r\n"); // to skip blank lines
}
while (rep.equalsIgnoreCase("y"));
keyboard.close();
}

Use nextLine instead of nextDouble:
public static void main(String[] args) {
Scanner keyboard = new Scanner(System.in);
String rep = "";
do {
System.out.println("Enter your full name:");
String name = keyboard.nextLine();
System.out.println("Enter your GPA:");
// double gpa = keyboard.nextDouble();
double gpa = Double.parseDouble(keyboard.nextLine());
System.out.println("Name: " + name + ", GPA: " + gpa);
System.out.println("Do you want to enter the data for another student?(y/n)");
rep = keyboard.nextLine();
} while (rep.equals("y"));
keyboard.close();
}

Is it safe and a better practice to accept Strings instead of integers or doubles?

Is the second part a good practice in order to avoid user input mismatch?
public class StringsAsPrimitiveDate{
Scanner input= new Scanner(System.in);
System.out.println("Please enter your number:");
int numberOne=input.nextInt();
}
Or
public class StringsAsPrimitiveDate{
Scanner input= new Scanner(System.in);
System.out.println("Please enter your number:");
String numberOne=input.next();
int result = Integer.parseInt(numberOne);
}
Thank you

A common problem with nextInt is that it leaves behind a new line character which may be accidentally automatically consumed by the next scan. Consider the following code:
import java.util.Scanner;
class Main{
public static void main(String args[]) {
int rollNo;
String name;
Scanner scan=new Scanner(System.in);
while (true) {
System.out.print("Enter the roll no of the student: ");
rollNo=scan.nextInt();
System.out.print("Enter the name of the student: ");
name=scan.nextLine();
System.out.printf("Roll no. %d, Name: %s has been registered%n",rollNo, name);
}
}
}
A sample run:
Enter the roll no of the student: 1
Enter the name of the student: Roll no. 1, Name: has been registered
Enter the roll no of the student: Arvind
Exception in thread "main" java.util.InputMismatchException
at java.base/java.util.Scanner.throwFor(Scanner.java:939)
at java.base/java.util.Scanner.next(Scanner.java:1594)
at java.base/java.util.Scanner.nextInt(Scanner.java:2258)
at java.base/java.util.Scanner.nextInt(Scanner.java:2212)
at Main.main(Main.java:10)
Note that this problem can occur even with next but not with nextLine as shown with the following code:
import java.util.Scanner;
class Main {
public static void main(String args[]) {
int rollNo = 0;
String name;
boolean valid = true;
Scanner scan = new Scanner(System.in);
while (true) {
do {
System.out.print("Enter the roll no of the student: ");
try {
rollNo = Integer.parseInt(scan.nextLine());
} catch (Exception e) {
System.out.println("The value should be an integer.");
valid = false;
}
} while (!valid);
System.out.print("Enter the name of the student: ");
name = scan.nextLine();
System.out.println("Roll no. " + rollNo + ", Name: " + name + " has been registered");
}
}
}
A sample run:
Enter the roll no of the student: 1
Enter the name of the student: Arvind
Roll no. 1, Name: Arvind has been registered
Enter the roll no of the student: a
The value should be an integer.
Enter the roll no of the student: 12.5
The value should be an integer.
Enter the roll no of the student: 2
Enter the roll no of the student: Avinash
The value should be an integer.
Enter the roll no of the student:
Conclusion: it is better to use nextLine with proper logic (e.g. exception handling, loop back on invalid input etc.) instead of next or nextInt to avoid issues mentioned above. I also recommend you go through documentation on Scanner.

FileWriter not receiving all input and mysterious dangling newline issue

I am trying to write a Java program in which the user specifies how many "student records" they would like to input, followed by the student's name, age, and GPA, which then gets stored as text. However, I am having a problem with my text not including all entered data and a mysterious dangling newline that I cannot get rid of.
Here is my program:
import java.io.*;
import java.util.Scanner;
public class CreateFile {
public static void main(String[] args) throws IOException {
Scanner input = new Scanner(System.in);
FileWriter fwriter = new FileWriter("c:\\Students.dat");
PrintWriter StudentFile = new PrintWriter(fwriter);
String name = " ";
String next = " ";
int age = 0;
int hm = 0;
double gpa = 0.0;
System.out.print("How many student records would you like to enter: ");
hm = input.nextInt();
for (int x = 1; x <= hm; x++) {
System.out.print("Enter Name: ");
name = input.nextLine();
input.nextLine();
System.out.print("Enter Age: ");
age = input.nextInt();
System.out.print("Enter GPA: ");
gpa = input.nextDouble();
next = input.nextLine();
StudentFile.println(name);
StudentFile.println(age);
StudentFile.println(gpa);
}
StudentFile.close();
System.exit(0);
}
}
Here is sample input and output to illustrate my issues:
run:
How many student records would you like to enter: 3
Enter Name: Jon
Enter Age: 20
Enter GPA: 3.4
Enter Name: Bill
Enter Age: 24
Enter GPA: 3.6
Enter Name: Ted
Enter Age: 34
Enter GPA: 3.9
This is the produced text file:
20
3.4
Bill
24
3.6
Ted
34
3.9
Why doesn't it store the first name entered? Why isn't there a newline in the first entry, but it is in the others?

The problem is that you're using nextLine() when you need to be using next(). I'm assuming you put the second input.nextLine() in there because you were initially having a problem where it would print out "Enter Name: " and then immediately "Enter Age: ". nextLine() is telling your program to skip whatever is there, and not to wait for it. The reason that this paradigm worked at all for any of your entries is that you put next = input.nextLine() at the bottom of your loop. Here's a fix:
package createfile;
import java.io.*;
import java.util.Scanner;
public class CreateFile {
public static void main(String[] args) throws IOException {
Scanner input = new Scanner(System.in);
FileWriter fwriter = new FileWriter("c:Students.dat");
PrintWriter StudentFile = new PrintWriter(fwriter);
String name = " ";
String next = " ";
int age = 0;
int hm = 0;
double gpa = 0.0;
System.out.print("How many student records would you like to enter: ");
hm = input.nextInt();
for (int x = 1; x <= hm; x++) {
System.out.print("Enter Name: ");
name = input.next();
System.out.print("Enter Age: ");
age = input.nextInt();
System.out.print("Enter GPA: ");
gpa = input.nextDouble();
StudentFile.println(name);
StudentFile.println(age);
StudentFile.println(gpa);
}
StudentFile.close();
System.exit(0);
}
}
You could also just move your input.nextLine() above name=input.nextLine() and it would have the same effect.

The other examples only work if you don't have names like "James Peter" - in their code examples only James would be saved as name.
I'd prefer this:
System.out.print("How many student records would you like to enter: ");
hm = input.nextInt();
input.nextLine();
for (int x = 1; x <= hm; x++) {
System.out.print("Enter Name: ");
name = input.nextLine();
System.out.print("Enter Age: ");
age = input.nextInt();
input.nextLine();
System.out.print("Enter GPA: ");
gpa = input.nextDouble();
input.nextLine();
StudentFile.println(name);
StudentFile.println(age);
StudentFile.println(gpa);
}

This is the corrected for loop:
for ( int x = 1; x <= hm; x++ )
{
System.out.print( "Enter Name: " );
name = input.next();
input.nextLine();
System.out.print( "Enter Age: " );
age = input.nextInt();
input.nextLine();
System.out.print( "Enter GPA: " );
gpa = input.nextDouble();
next = input.nextLine();
StudentFile.println( name );
StudentFile.println( age );
StudentFile.println( gpa );
}
Some things you may want to consider:
Handle the IOException - it should not be ignored!!
Use the methods hasNextXXX() of the Scanner to check if something is available.
Refactor your usage of the variable next, it's never really used.
It's not necessary to call System.exit( 0 ) from the main method - rather use the return statement with a meaningful value.

what is the solution to StringIndexOutOfBoundsException

when i use s.charAt(0) while s is an string input from the user, I get this as an error even though the program runs the first half of the program.
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.lang.String.charAt(String.java:658)
at Shopping.main(Shopping.java:22)
What's the solution to this program? here is my code.
import java.util.Scanner;
public class Shopping {
public static void main(String[] args){
Scanner keyboard = new Scanner(System.in);
System.out.println("Programmed by Raymond Lee");
System.out.println("Welcome to Shopper's Paradise");
ShoppingCart cart = new ShoppingCart();
System.out.print("Enter the name of the first item: ");
String item = keyboard.nextLine();
System.out.print("Enter the quantity: ");
int quantity = keyboard.nextInt();
System.out.print("Enter the price: ");
double price = keyboard.nextDouble();
cart.addToCart(item, price, quantity);
System.out.print("Enter the name of the next item or Q to quit: ");
String quit = keyboard.nextLine();
char choice = quit.charAt(0);
while((choice != 'Q' && choice != 'q') || quit.length() != 1) {
quit = item;
System.out.print("Enter the quantity: ");
quantity = keyboard.nextInt();
System.out.print("Enter the price: ");
price = keyboard.nextDouble();
cart.addToCart(item, price, quantity);
System.out.print("Enter the name of the next item or Q to quit: ");
quit = keyboard.nextLine();
choice = quit.charAt(0);
}
System.out.println(cart);
}
}

The error is occuring at this line
char choice = quit.charAt(0);
This is due to the fact that when you call
double price = keyboard.nextDouble();
then nextDouble leaves the newline in the input stream. So when following is called
String quit = keyboard.nextLine();
then result of nextLine is empty string, results in the given error when you try to use charAt method.
To resolve this error, simply change following
String quit = keyboard.nextLine();
To
String quit = keyboard.next();
Hope this helps

Trouble using nextInt and nextLine()

It's not letting me put my name in but it does the age works fine.
I know i can change the order of the statements but is there another way I could do it?
import java.util.Scanner;
public class ScannerErr2
{
public static void main(String [] args)
{
Scanner keyboard= new Scanner(System.in);
String name;
int age;
System.out.print("Enter your age : ");
age= keyboard.nextInt();
System.out.print("Enter your name: ");
name= keyboard.nextLine();
System.out.println("Age : "+age);
System.out.println("Name: "+name);
}
}

You problem is that the next int doesn't consider the new line character which goes in the input for your name part. Hence name is returned as blank.
You can change your code in 2 ways:
System.out.print("Enter your age : ");
age = keyboard.nextInt();
keyboard.nextLine();
System.out.print("Enter your name: ");
name = keyboard.nextLine();
or
System.out.print("Enter your age : ");
age = Integer.parseInt(keyboard.nextLine().trim());
System.out.print("Enter your name: ");
name = keyboard.nextLine();
I personally like the second way.