Thread: String verification.

For an assignment I'm to make a program that takes in a user input as a password containing at least ten characters and at least one letter and one number.

Take a look at regular expressions (regex) - Lesson: Regular Expressions (The Java™ Tutorials > Essential Classes). That link might be overwhelming at first, but regular expressions are an important concept for String manipulation and will readily allow you to accomplish String validations. You could do this using basic String manipulation, but a regex is much easier

First, let's clear up a few things with the small sample you provided. StringBuilder is a class. You can declare a variable to be of type StringBuilder. That is to say the variable can hold/refer to StringBuilder objects. The variable must have a valid identifier (a.k.a. name).

StringBuilder my_string_builder; // declares a variable named my_string_builder which can hold StringBuilder objects

You can create StringBuilder objects using the StringBuilder's constructor. A constructor is a special method inside of StringBuilder with the same name as the class, StringBuilder. To call a constructor you must use the new keyword. Some classes can have more than one constructor defined. As you indicated, there's a constructor which takes a String object and creates a StringBuilder object initialized with the String's data.

new StringBuilder("a test string");

Let's put the two parts together and create a StringBuilder object and assign it to a variable so we can use that variable to manipulate that StringBuilder object.

StringBuilder my_string_builder = new StringBuilder("a test string");

You don't need to provide a String literal, you just need to give it a valid String object.

String test_string = "this is a string";
 
StringBuilder my_string_builder = new StringBuilder(test_string);

Now that bit out of the way, why do you think you need a StringBuilder object? What does the StringBuilder object give you? I would suggest reading the StringBuilder JavaDoc to find out what a StringBuilder is.

The very first sentence describes exactly a StringBuilder is.

A mutable sequence of characters.

mutable means you can change it. Strings in Java, on the other hand, are immutable. That means once you have a String object it will be the same from the time it's created to the time it's destroyed. Note that I was careful to say that a String object is immutable. String variables can be changed to refer to other Strings if they're not declared with the final keyword.

final String str1 = "a fine day";
String str2 = "Java programming";
 
str1 = str2; // compile error, the variable str1 can't be made to refer to a different String object.
str2 = str1; // perfectly fine, str2 can refer to the same String object str1 refers to.

So back to the original question of why do you need a StringBuilder object, and why isn't a String object sufficient?

To answer this question think about what steps you have to take in order to figure out if the password inputted is valid. How would you (using a piece of paper and a pencil) figure out if the password contains at least one letter and one number.