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Why can't Java variable names start with a number?

In Java, variable names start with a letter, currency character ($) etc. but not with number, :, or .
Simple question: why is that?
Why doesn't the compiler allow to have variable declarations such as
int 7dfs;

Simply put, it would break facets of the language grammar.
For example, would 7f be a variable name, or a floating point literal with a value of 7?
You can conjure others too: if . was allowed then that would clash with the member selection operator: would foo.bar be an identifier in its own right, or would it be the bar field of an object instance foo?

Because the Java Language specification says so:
IdentifierChars:
JavaLetter {JavaLetterOrDigit}
So - yes, an identifier must start with a letter; it can't start with a digit.
The main reasons behind that:
it is simply what most people expect
it makes parsing source code (much) easier when you restrict the "layout" of identifiers; for example it reduces the possible ambiguities between literals and variable names.

Java pound (#) character syntax [duplicate]

What characters are valid in a Java class name? What other rules govern Java class names (for instance, Java class names cannot begin with a number)?

You can have almost any character, including most Unicode characters! The exact definition is in the Java Language Specification under section 3.8: Identifiers.
An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter. ...
Letters and digits may be drawn from the entire Unicode character set, ... This allows programmers to use identifiers in their programs that are written in their native languages.
An identifier cannot have the same spelling (Unicode character sequence) as a keyword (§3.9), boolean literal (§3.10.3), or the null literal (§3.10.7), or a compile-time error occurs.
However, see this question for whether or not you should do that.

Every programming language has its own set of rules and conventions for the kinds of names that you're allowed to use, and the Java programming language is no different. The rules and conventions for naming your variables can be summarized as follows:
Variable names are case-sensitive. A variable's name can be any legal identifier — an unlimited-length sequence of Unicode letters and digits, beginning with a letter, the dollar sign "$", or the underscore character "_". The convention, however, is to always begin your variable names with a letter, not "$" or "_". Additionally, the dollar sign character, by convention, is never used at all. You may find some situations where auto-generated names will contain the dollar sign, but your variable names should always avoid using it. A similar convention exists for the underscore character; while it's technically legal to begin your variable's name with "_", this practice is discouraged. White space is not permitted.
Subsequent characters may be letters, digits, dollar signs, or underscore characters. Conventions (and common sense) apply to this rule as well. When choosing a name for your variables, use full words instead of cryptic abbreviations. Doing so will make your code easier to read and understand. In many cases it will also make your code self-documenting; fields named cadence, speed, and gear, for example, are much more intuitive than abbreviated versions, such as s, c, and g. Also keep in mind that the name you choose must not be a keyword or reserved word.
If the name you choose consists of only one word, spell that word in all lowercase letters. If it consists of more than one word, capitalize the first letter of each subsequent word. The names gearRatio and currentGear are prime examples of this convention. If your variable stores a constant value, such as static final int NUM_GEARS = 6, the convention changes slightly, capitalizing every letter and separating subsequent words with the underscore character. By convention, the underscore character is never used elsewhere.
From the official Java Tutorial.

Further to previous answers its worth noting that:
Java allows any Unicode currency symbol in symbol names, so the following will all work:
$var1
£var2
€var3
I believe the usage of currency symbols originates in C/C++, where variables added to your code by the compiler conventionally started with '$'. An obvious example in Java is the names of '.class' files for inner classes, which by convention have the format 'Outer$Inner.class'
Many C# and C++ programmers adopt the convention of placing 'I' in front of interfaces (aka pure virtual classes in C++). This is not required, and hence not done, in Java because the implements keyword makes it very clear when something is an interface.
Compare:
class Employee : public IPayable //C++
with
class Employee : IPayable //C#
and
class Employee implements Payable //Java
Many projects use the convention of placing an underscore in front of field names, so that they can readily be distinguished from local variables and parameters e.g.
private double _salary;
A tiny minority place the underscore after the field name e.g.
private double salary_;

As already stated by Jason Cohen, the Java Language Specification defines what a legal identifier is in section 3.8:
"An identifier is an unlimited-length sequence of Java letters and Java digits, the
first of which must be a Java letter. [...] A 'Java letter' is a character for which the method Character.isJavaIdentifierStart(int) returns true. A 'Java letter-or-digit' is a character for which the method Character.isJavaIdentifierPart(int) returns true."
This hopefully answers your second question. Regarding your first question; I've been taught both by teachers and (as far as I can remember) Java compilers that a Java class name should be an identifier that begins with a capital letter A-Z, but I can't find any reliable source on this. When trying it out with OpenJDK there are no warnings when beginning class names with lower-case letters or even a $-sign. When using a $-sign, you do have to escape it if you compile from a bash shell, however.

I'd like to add to bosnic's answer that any valid currency character is legal for an identifier in Java. th€is is a legal identifier, as is €this, and € as well. However, I can't figure out how to edit his or her answer, so I am forced to post this trivial addition.

What other rules govern Java class names (for instance, Java class names cannot begin with a number)?
Java class names usually begin with a capital letter.
Java class names cannot begin with a number.
if there are multiple words in the class name like "MyClassName" each word should begin with a capital letter. eg- "MyClassName".This naming convention is based on CamelCase Type.

Class names should be nouns in UpperCamelCase, with the first letter of every word capitalised. Use whole words — avoid acronyms and abbreviations (unless the abbreviation is much more widely used than the long form, such as URL or HTML).
The naming conventions can be read over here:
http://www.oracle.com/technetwork/java/codeconventions-135099.html

Identifiers are used for class names, method names, and variable names. An identifiermay be any descriptive sequence of uppercase and lowercase letters, numbers, or theunderscore and dollar-sign characters. They must not begin with a number, lest they beconfused with a numeric literal. Again, Java is case-sensitive, so VALUE is a differentidentifier than Value.
Some examples of valid identifiers are:
AvgTemp ,count a4 ,$test ,this_is_ok
Invalid variable names include:
2count, high-temp, Not/ok

For with i0 variables

What does it mean this structure-
for (int i0 = 0; i0 < size; i0++) {
I found this using SoapObject.
I don't understand the difference between this and a normal for cycle.

It is a simple variable name. In java any variable name can contains one or more number. But variable name can not contains number at first place.
See the java variable name naming convention here.

In Java, valid variable names can start with a letter (uppercase or lowercase) or an underscore (but not a number), followed by a sequence of letters (uppercase or lowercase), numbers and/or underscore. The regular expression for the recongition of a variable would be:
[a-zA-Z_][a-zA-Z0-9_]*
(Source: gamedev.net - ignore the C# code apart from the regex)
Therefore, i0 is a perfectly valid variable name (where 0i is not).

Which of these are valid variable names?

This is a question from a Java test I took at University
I. publicProtected
II. $_
III. _identi#ficador
I've. Protected
I'd say I, II, and I've are correct. What is the correct answer for this?
Source of the question in spanish: Teniendo la siguiente lista de identificadores de variables, ¿Cuál (es) es (son) válido (s)?

From the java documentation:
Variable names are case-sensitive. A variable's name can be any legal
identifier — an unlimited-length sequence of Unicode letters and
digits, beginning with a letter, the dollar sign "$", or the
underscore character "". The convention, however, is to always begin
your variable names with a letter, not "$" or "". Additionally, the
dollar sign character, by convention, is never used at all. You may
find some situations where auto-generated names will contain the
dollar sign, but your variable names should always avoid using it. A
similar convention exists for the underscore character; while it's
technically legal to begin your variable's name with "_", this
practice is discouraged. White space is not permitted. Subsequent
characters may be letters, digits, dollar signs, or underscore
characters. Conventions (and common sense) apply to this rule as well.
When choosing a name for your variables, use full words instead of
cryptic abbreviations. Doing so will make your code easier to read and
understand. In many cases it will also make your code
self-documenting; fields named cadence, speed, and gear, for example,
are much more intuitive than abbreviated versions, such as s, c, and
g. Also keep in mind that the name you choose must not be a keyword or
reserved word.
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/variables.html
In short: yes, you're right. You can use underscores, dollarsigns, and characters to start a variable name. After the first letter of the variable name, you can also use numbers. Note that using dollar signs is generally not good practice.
From your comment, you said that your teacher rejected "II". Under your question, II is perfectly fine (try it, it will run). However, if the question on your test asked which are "good" variable names, or which variable names follow common practice, then II would be eliminated as explained in the quotation above. One reason for this is that dollar signs do not make readable variable names; they're included because internally Java makes variables that use the dollar sign.
What is the meaning of $ in a variable name?
As pointed out in the comments, IV is not a good name either, since the lower case version "protected" is a reserved keyword. With syntax highlighting, you probably wouldn't get the two confused, but using keyword-variations as variable names is certainly one way to confuse future readers

Private protected public are reserved or keywords in java.. Use _ or to use that those words.. example
int public_x;
int protected_x;
String private_s;

Java global variables - What is the convention to declare? [closed]

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I think the question speaks for itself but if it doesnt here is the short explanation for what I meant:
Coding a java programm, what convention should I take to declare a global variable? I Mean, which one below should I use?
public class Variables {
private variable;
private Variable;
private VARIABLE;
private largerVariable;
private Larger_Variable;
private larger_variable;
private LARGER_VARIABLE;
}

There are several rules:
Variable names are case-sensitive. A variable's name can be any legal identifier — an unlimited-length sequence of Unicode letters and digits, beginning with a letter, the dollar sign "$", or the underscore character "". The convention, however, is to always begin your variable names with a letter, not "$" or "". Additionally, the dollar sign character, by convention, is never used at all. You may find some situations where auto-generated names will contain the dollar sign, but your variable names should always avoid using it. A similar convention exists for the underscore character; while it's technically legal to begin your variable's name with "_", this practice is discouraged. White space is not permitted.
Subsequent characters may be letters, digits, dollar signs, or underscore characters. Conventions (and common sense) apply to this rule as well. When choosing a name for your variables, use full words instead of cryptic abbreviations. Doing so will make your code easier to read and understand. In many cases it will also make your code self-documenting; fields named cadence, speed, and gear, for example, are much more intuitive than abbreviated versions, such as s, c, and g. Also keep in mind that the name you choose must not be a keyword or reserved word.
If the name you choose consists of only one word, spell that word in all lowercase letters. If it consists of more than one word, capitalize the first letter of each subsequent word. The names gearRatio and currentGear are prime examples of this convention. If your variable stores a constant value, such as static final int NUM_GEARS = 6, the convention changes slightly, capitalizing every letter and separating subsequent words with the underscore character. By convention, the underscore character is never used elsewhere.
From here
http://docs.oracle.com/javase/tutorial/java/nutsandbolts/variables.html
If I read these rules right, valid names(by naming convention) will be:
private Object largerVariable;
private Object LARGER_VARIABLE;

Another thought is to declare constants such that they can't be changed:
private static final boolean POWER_ON = true;
Additionally, if I need to define boolean constants, I try to define it so they are affirmative (true) rather than negative (false) so its easier to grasp without thinking about it too much. Example:
I use this:
private static final boolean POWER_ON = true;
rather than this:
private static final boolean POWER_OFF = false;
Another consideration:
For boolean variables, I might consider preceeding the word with 'is' or 'has' to indicate its boolean if its not obvious from the word. Example: IS_RADIOACTIVE.
Lastly, I only need to define POWER_ON rather the POWER_OFF since !POWER_ON means the same thing in a java statement.

Java has no such thing as 'global' variables.
However, if you want to use constants which can be accessed by any other class, most people will declare a public static final variable and initialize with the corresponding value right away.
public class HttpCodes {
public static final String NOT_FOUND = "404";
}
Can be accessed by any other class: HttpCodes.NOT_FOUND
If you're trying to make it an instance variable, go with camelCase private final String notFound = "404" for example.

I am assumming global means class level...
as a practice class level variables should be written in capital letters
In most cases they are marked as static so that can be access by all the class methods.
Also, if you believe they would be initialized only once then consider making them final.
correct declaration could be:
private static VARIABLE;
private static LARGER_VARIABLE;