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How to check if a char is null

I want to check if the char is null or not? but why this code is not working? letterChar == null also is not working. I googled for many problems but didn't see any solutions, mostly the solutions are about String.
String letter = enterletter.getText().toString();
char letterChar = letter.charAt(0);
if(letterChar == ' ' || letterChar == NULL) // this is where the code won't works
{
Toast.makeText(getApplicationContext(), "Please enter a letter!", Toast.LENGTH_LONG).show();
}

A char cannot be null as it is a primitive so you cannot check if it equals null, you will have to find a workaround.
Also did you want to check if letterChar == ' ' a space or an empty string? Since you have a space there.
The first two answers here may be helpful for how you can either check if String letter is null first.
or cast char letterChar into an int and check if it equals 0 since the default value of a char is \u0000- (the nul character on the ascii table, not the null reference which is what you are checking for when you say letterChar == null)- which when cast will be 0.

char is a primitive datatype so can not be used to check null.
For your case you can check like this.
if (letterChar == 0) //will be checked implicitly
{
System.out.println("null");
}
//or use this
if (letterChar == '\0')
{
System.out.println("null");
}

From your code it seems like you work with Unicode and the method you search for is into Character class (java.lang)
Character.isLetter(ch);
It also contains lots of useful static method that can suite you.
If so, the code will be
String letter = enterletter.getText().toString();
char letterChar = letter.charAt(0);
if(!Character.isLetter(letterChar))
{
Toast.makeText(getApplicationContext(), "Please enter a letter!", Toast.LENGTH_LONG).show();
}
But if I would need to answer your question (without taking a look to your aim), then the code you need depends on what do you mean by "is char null".
In Java char cannot be == null.
If you want to check if it is not an empty space, you need to do
if (ch == ' ')
If you want to check if there are no line breakers (space, HT, VT, CR, NL and other), you should add the following to the proposed above:
if (ch == ' ' || ch == '\t' || ch == '\r' || ch == '\n' || ch == '\x0b')

java| how to make one variable for all digits (0-9)

i'm working on my project in java, in my project i need to get input from some stream an to parse text and make it to generic char-by-char to some other types, one of them is "ValueNumber".
for that I'm using switch case
Now,because Number can start with ' - ' I need to check if the current char is a Digit between 0 to 9 or ' - ' or something else.
My question is how can I make some variable that will hold all the 10th digits by one variable ?

String will hold it, or StringBuilder for better performance and
then you can parse the string and see if it matches the regex:
return str.matches("[-]?[0-9]+");
if true, it is digit with or without negation sign, if false, it is not a digit you described. The digit can be as long as String allows.

I think you're imagining something like:
switch(aChar) {
case '+':
handlePlus();
break;
case ' ':
handleSpace();
break;
case anyOf("-01234567890");
handlePartOfNumber(aChar);
break;
}
Unfortunately in Java, switch is not this sophisticated. switch deals with exact matches only.
You will need to use a series of if/else blocks instead:
if(aChar == '+') {
handlePlus();
} else if(aChar == ' ') {
handleSpace();
} else if(isMinusOrDigit(aChar)) {
handlePartOfNumber(aChar);
}
Now, how do we implement isMinusOrDigit(char c)?
You've asked about "some variable that holds all the digits". Maybe you mean an array, or a List or a Set. I'll choose Set because it's the purest "bag of items, you don't care the order".
private static Set<Character> MINUS_AND_DIGITS = minusAndDigits();
private static Set<Character> minusAndDigits() {
Set<Character> set = new HashSet<>();
for(char c = '0'; c<='9'; c++) {
set.add(c);
}
set.add('-');
}
private static boolean isMinusOrDigit(char c) {
return MINUS_AND_DIGITS.contains(c);
}
You could also use
a List<Character> (again .contains(c))
an array -- char[] (see How can I test if an array contains a certain value?)
or even a String -- return "0123456789-".indexOf(c) != -1;
But in this case, you don't need a "multi value variable" to work out whether a character is a minus or a digit - because the number characters are next to each other in ASCII:
private static boolean isMinusOrDigit(char c) {
return c == '-' || ( c >= 0 && c<=9 );
}

Determine if character has case in JAVA

In JAVA (Android), I'm trying to determine in a String, if each character has a equivalent in upper or lower case.
My goal is not to lower or upper the case, but to know if it's possible.
For example this function would return true for : 'e' 'é' 'i' 'l' 'L' 'O' 'P'
and false for emojis or chinese characters.
Is there any function that can do this?
EDIT : To be more clear, the function was supposed to take a character for argument, not a String and return false if the character had no uppercase or lowercase version.

You can try this:
boolean validate(char c){
return Character.isUpperCase(c) || Character.isLowerCase(c);
}
This will return true iff it is a Letter in uppercase or lower case only. Otherwise it'll return false.

The requirements are still not entirely specified (do you care whether the upper/lowercase equivalent is a different character from the original?), but my most straightforward interpretation of the question is:
For each character ch in a given string, is it true that either toUpperCase(ch) yields an uppercase character, or that toLowerCase(ch) yields a lowercase character?
I phrase it that way because Character.toUpperCase() returns "the uppercase equivalent of the character, if any; otherwise, the character itself".
The doc for String.toUppercase() doesn't mention what happens if there is no uppercase equivalent for some characters, but I think we can assume it returns those characters unchanged, as does Character.toUpperCase().
So a straightforward implementation of that condition would be to test
Character.isUpperCase(s.toUpperCase().charAt(0)) ||
Character.isLowerCase(s.toLowerCase().charAt(0));
for each character as a String.
I'm using the String rather than Character case conversion functions here, in order to take advantage of locale-sensitive mapping. Not only that, but regardless of locale, there are characters that cannot be converted to uppercase by Character.toUpperCase() because their uppercase equivalent is more than one character! For example, we would get incorrect results for \u00df 'ß' (see docs for details).
public class TestUpper {
public static void main(String[] args) {
final String test = "\u0633\u0644\u0627\u0645 World \u00df\u01c8eéilLOP\u76f4!";
for (Character ch : test.toCharArray()) {
System.out.format("'%c' (U+%04x): hasCase()=%b%n", ch, (int)ch, hasCase(ch));
}
}
static boolean hasCase(Character ch) {
String s = ch.toString();
// Does the character s have an uppercase or a lowercase equivalent?
return Character.isUpperCase(s.toUpperCase().charAt(0)) ||
Character.isLowerCase(s.toLowerCase().charAt(0));
}
}
And the results:
'س' (U+0633): hasCase()=false
'ل' (U+0644): hasCase()=false
'ا' (U+0627): hasCase()=false
'م' (U+0645): hasCase()=false
' ' (U+0020): hasCase()=false
'W' (U+0057): hasCase()=true
'o' (U+006f): hasCase()=true
'r' (U+0072): hasCase()=true
'l' (U+006c): hasCase()=true
'd' (U+0064): hasCase()=true
' ' (U+0020): hasCase()=false
'ß' (U+00df): hasCase()=true
'Lj' (U+01c8): hasCase()=true
'e' (U+0065): hasCase()=true
'é' (U+00e9): hasCase()=true
'i' (U+0069): hasCase()=true
'l' (U+006c): hasCase()=true
'L' (U+004c): hasCase()=true
'O' (U+004f): hasCase()=true
'P' (U+0050): hasCase()=true
'直' (U+76f4): hasCase()=false
'!' (U+0021): hasCase()=false
These test cases include Arabic letters and a Chinese character (which are isLetter(), but have no upper/lowercase equivalents), the requested test letters, space and punctuation, and a titlecase letter.
The results are correct according to the criteria currently stated in the question. However, the OP has said in comments that he wants the function to return false for titlecase characters, such as U+01c8, whereas the above code returns true because they have uppercase and lowercase equivalents (U+01c7 and U+01c9). But the OP's statement seems to be based on the mistaken impression that titlecase letters do not have uppercase and lowercase equivalents. Ongoing discussion has not yet resolved the confusion.
Disclaimer: This answer doesn't attempt to take into account supplementary or surrogate code points.

For a simple method, there's Character.isLowerCase. But you actually need to be careful- it depends on language. Some languages may have a lower case 'é' but no uppercase. Or like the turkish "I" may have a different lower case version than other languages.
To work around that, I'd use something like Character.isLetter(myChar) && String.valueOf(myChar).toLowerCase().equals(String.valueOf(myChar)). Remember to use the version of toLowerCase that takes a Locale as parameter if not comparing in the default Locale.

Check if the character is either a lowercase letter or an uppercase letter:
Character.isLowerCase(ch) != Character.isUpperCase(ch)
Alternatively, you can compare the lower and uppercased forms of the character:
Character.toLowerCase(ch) == Character.toUpperCase(ch)
However, you need to be careful about locale (there is one letter in Turkish where I think the lower and uppercase forms are the same).

Two strings uppercase and lowercase not matching does not necessarily mean the string is valid. true1 will not equal TRUE1 but fails the test case. You need to check each individual character. This is a rough cut, you'll probably have to do something fancy for emojis and Chinese characters.
public static boolean isAllCase(String value) {
String upper = value.toUpperCase();
String lower = value.toLowerCase();
if(upper.length() != lower.length())
return false;
for(int i = 0; i < upper.length(); i++) {
if(upper.charAt(i) == lower.charAt(i))
return false;
}
return true;
}

public boolean hasEquivalentCase(char ch) {
return (Character.isLowerCase(ch)) || Character.isUpperCase(ch)
}

public boolean validate(char value){
if( (value >= 'a' && value <= 'z') || (value >= 'A' &&
value <= 'Z')
return true;
return false;
}
this for each caracter to your String.
public boolean All( String cad ){
for( int i = 0; i < cad.lenght() ; i++ ){
if( !validate(cad.charAt(i)) ){
#the letter has not upper or lower
return false;
}
}
return true;
}

Methods of comparing strings in java and why this code returns with an error

Right now, I have the code written that is displayed below:
if(letterTwo.equals("a" || "e" || "i" || "o" || "u") && !"a" || "e" || "i" || "o" || "u".equals(letterOne));{}
The errors I am receiving are:
"The operator ! is undefined for the argument types string", and "The operator || is undefined for the argument types string, java.lang,string, java.lang.string"
I was wondering how I could fix this code, and what the operators are used for comparing strings like I need to.

Assumption: letterTwo is a String
letterTwo.equals() expects a single String argument. Not the big boolean expression you are passing it.
You can either do
if (letterTwo.equals("a") || letterTwo.equals("b") || letterTwo.equals("c"))
{
//etc
}
Or if your Java version is new enough you may be able to use a switch
switch(letterTwo)
{
case "a":
case "b":
case "c": // stuff
break;
}
If letterTwo is actually a char (which it may well be given the name...), you can do similar things but it is a bit easier:
if (letterTwo == 'a' || letterTwo == 'b' || letterTwo == 'c')
{
//etc
}

Simple:
if (letter.equals("a") || letter.equals("b")
And so on. The point is that you have to use those boolean operators on expressions that have type boolean!
But of course, code as the above turns unreadable quickly. In the real world, such kind of checks would rather be done using a simple regular expression.

The correct way for you would be :
if( (letterTwo.equals("a") || letterTwo.equals("e") || letterTwo.equals("i") || letterTwo.equals("o") || letterTwo.equals("u")) && !(letterOne.equals("a") || letterOne.equals("e") || letterOne.equals("i") ||letterOne.equals("o") || letterOne.equals("u")) ;{}
Please note, this is not case-sensitive. It would treat "A" and "a" differently.If you want them to be treated in same way use "equalsIgnoreCase" instead of "equals" above.
As pointed out by #GhostCat above, it distrubs readability and regex should be much better to handle such cases

Writing one regular expression for string in java

I am trying to write one regular expression for string. Let us say there is a string RBY_YBR where _ represents empty so we can recursively replace the alphabets and _ and the result is RRBBYY_ . There can be two or more alphabet pairs can be formed or something like this also RRR .
Conditions
1). Left or right alphabet should be the same.
2). If there is no _ then the alphabet should be like RRBBYY not RBRBYY or RBYRBY etc.
3). There can be more than one underscore _ .
From regular expression I am trying to find whether the given string can satisfy the regular expression or not by replacing the character with _ to form a pattern of consecutive alphabets
The regular expression which I wrote is
String regEx = "[A-ZA-Z_]";
But this regular expression is failing for RBRB. since there is no empty space to replace the characters and RBRB is also not in a pattern.
How could I write the effective regular expression to solve this.

Ok, as I understand it, a matching string shall either consist only of same characters being grouped together, or must contain at least one underscore.
So, RRRBBR would be invalid, while RRRRBB, RRRBBR_, and RRRBB_R_ would all be valid.
After comment of question creator, additional condition: Every character must occur 0 or 2 or more times.
As far as I know, this is not possible with Regular Expressions, as Regular Expressions are finite-state machines without "storage". You would have to "store" each character found in the string to check that it won't appear later again.
I would suggest a very simple method for verifying such strings:
public static boolean matchesMyPattern(String s) {
boolean withUnderscore = s.contains("_");
int[] found = new int[26];
for (int i = 0; i < s.length(); i++) {
char ch = s.charAt(i);
if (ch != '_' && (ch < 'A' || ch > 'Z')) {
return false;
}
if (ch != '_' && i > 0 && s.charAt(i - 1) != ch && found[ch - 'A'] > 0
&& !withUnderscore) {
return false;
}
if (ch != '_') {
found[ch - 'A']++;
}
}
for (int i = 0; i < found.length; i++) {
if (found[i] == 1) {
return false;
}
}
return true;
}

Please take my answer with a grain of salt, since it's a bit of a "Fastest gun in the West" post.
It follows the same assumptions as Florian Albrecht's answer. (thanks)
I believe that this will solve your problem:
(([A-Za-z])(\2|_)+)+
https://regex101.com/r/7TfSVc/1
It works by using the second capturing group and ensuring that more of it follow, or there are underscores.
Known bug: it does not work if an underscore starts a string.
EDIT
This one is better, though I forgot what I was doing by the end of it.
(([A-Za-z_])(\2|_)+|_+[A-Za-z]_*)+
https://regex101.com/r/7TfSVc/4