Require a specific format for a String in the constructor???

[charred adder]

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[mellow juniper]

Regular expressions probably

That's the easiest way

import java.util.regex.*;

public class MyClass {
    private static final Pattern PATTERN = Pattern.compile("[A-Z]\\d");
    private final String myString;

    public MyClass(String myString) {
        if (!PATTERN.matcher(myString).matches()) {
            throw new IllegalArgumentException("Invalid string: " + myString);
        }
        this.myString = myString;
    }

    public String getMyString() {
        return myString;
    }
}

This is what it would look like

Give or take

[mellow juniper]

That's a static definition of a regular expression pattern

So you can reuse it every time

[sweet quiver]

One uppercase letter from A to Z, followed by one ASCII digit from 0 to 9

It imposes exactly two chars, a letter and a digit

[spare orchid]

you could also use myString.matches() directly

https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#matches-java.lang.String-

[charred adder]

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