Validation input to be string only and numbers only JAVA [duplicate]

You can use Regular expression to check if the input match your constraint as follow :

String name;
System.out.println("Enter name:");
name = keyboard.nextLine();
if (!name.matches("[a-zA-Z_]+")) {
    System.out.println("Invalid name");
}
String number;
System.out.println("Enter number:");
number = keyboard.nextLine();
if (!number.matches("[0-9]+")) {
    System.out.println("Invalid number");
}

Here is a good tutorial to learn regex .

You can loop through each character of the String and check if it's not alphabetic using Character.isAlphabetic(char):

Scanner keyboard = new Scanner(System.in);
System.out.println("Enter name:");
String name = keyboard.nextLine();

for (char c : name.toCharArray()) {
    if (!Character.isAlphabetic(c)){
        System.out.println("INVALID");
        break;
    }
}

To only accept numbers, you can do something similar using the Character.isDigit(char) function, but note that you will have to read the input as a String not a double, or get the input as a double and the converting it to String using Double.toString(d).

double number = 0;
try {
    number = Double.parseDouble(name)
} catch (NumberFormatException ex) {
    System.out.println("Name is not a double.");
}

If number is not a double, you can catch a NumberFormatException.

It seems you are using scanner. If you are, you can use the Scanner class' hasNextDouble() to check if the input is a double before reading the double as shown below:

double number;
System.out.println("Enter number:");
if (keyboard.hasNextDouble()){
    name = keyboard.nextDouble();
}

Have a look at the Scanner class docs for more information.

There is also the "fancier" regex solution. Java Pattern Documentation

You can use this (untested code) given you used nextLine() for reading BOTH inputs:

 boolean isWordOnly = Pattern.matches("\w*", name);    //name is in your code
 boolean isFloatOnly = Pattern.matches("\d*.?\d*", number);    //number is in your code too

Now the too boolean values tell you if there is the desired format in your input. You can add it in a do - while loop or anything you want.

It is a good idea to start studying reg(ular) ex(presions) because they are useful in string formatting (Imagine that you have to test if the input is a valid email...). Also used to check for SQL injections and many key things in apps and programs generally.