new to Java. I have an assignment due tonight but am stuck with what needs to be assigned within the ( ). This is a verification of an email prefix and I am calling the prefix "example" from the method I used to get the prefix. Any help would be greatly appreciated!

public static boolean isValidPrefixChar(char b) {
        return isAlphaNumeric(b) || b == '.' || b == '-' || b == '_';

This is where I am calling it:

public static boolean isValidPrefix(String str4) {
        getPrefix("[email protected]");
        isValidPrefixChar(???);

Please and thanks!

CodePudding user response：

tl;dr

To directly answer your question, identify the char value of the first character of your string. Call String#charAt. Unfortunately, that method uses annoying zero-based index counting. So the first character is counterintuitively zero rather than one.

isValidPrefixChar( prefix.charAt( 0 ) );

But… Better to avoid char type. Pass a code point number:

isValidPrefixChar( prefix.codePointAt( 0 ) ); // Pass code point number assigned to the first (index zero) character of your input string.

char is obsolete

The char type has been legacy since Java 2. As a 16-bit value it is physically incapable of representing most characters. Instead use code point integer numbers.

Code points

Every known character has been assigned a specific permanent number as an identifier. This number is known as a code point.

• . = FULL STOP = 46 decimal
• - = HYPHEN-MINUS = 45 decimal
• _ = LOW LINE = 95 decimal

Your examination of a single character would take an int argument, as the code point.

public static boolean isValidPrefixCharacter ( int codePoint )
{
    if ( ! Character.isValidCodePoint( codePoint ) ) { throw new IllegalArgumentException( "Invalid code point number passed." ); }
    return
            Character.isLetterOrDigit( codePoint )
                    || codePoint == ".".codePointAt( 0 )  // Annoying zero-based index counting.
                    || codePoint == "-".codePointAt( 0 )
                    || codePoint == "_".codePointAt( 0 )
            ;
}

That method would be called from another method like this.

public static boolean isValidPrefix ( String possibleEmailAddress )
{
    Objects.requireNonNull( possibleEmailAddress );
    if ( possibleEmailAddress.isBlank() ) { throw new IllegalArgumentException( "Possible email address must have some not text, not empty string." ); }
    int codePointOfFirstCharacter = possibleEmailAddress.codePointAt( 0 );   // Annoying zero-based index counting.
    boolean isPrefixValid = isValidPrefixCharacter( codePointOfFirstCharacter );
    return isPrefixValid;
}

Example harness code.

boolean isValid = App8.isValidPrefix( "[email protected]" );
System.out.println( "isValid = "   isValid );

isValid = App8.isValidPrefix( "