Java Keywords & Identifiers
Keywords
Keywords like class, int, if, and return are reserved by the language itself and have fixed meaning to the compiler, so trying to name a variable int or class will fail to compile.
Example: Keywords
public class Main {
public static void main(String[] args) {
// int int = 5; // ILLEGAL - 'int' is a reserved keyword, not a valid identifier
int total = 5;
System.out.println(total);
}
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Identifiers
An identifier is any name you choose for a variable, method, class, or package -- picking clear, descriptive identifiers (like totalPrice instead of x) makes your code self-explanatory without extra comments.
Example: Identifiers
public class Main {
public static void main(String[] args) {
int totalPrice = 250; // clear identifier instead of 'x'
System.out.println(totalPrice);
}
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Naming Rules
Java requires identifiers to start with a letter, underscore, or dollar sign, and forbids spaces or symbols like # or - anywhere in the name -- numbers are allowed, but never as the very first character.
Example: Naming Rules
public class Main {
public static void main(String[] args) {
int _count = 1;
int $price = 2;
int total2 = 3;
// int 2total = 4; // ILLEGAL - cannot start with a digit
System.out.println(_count + $price + total2);
}
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Case Sensitivity in Names
Because page and Page are treated as completely different identifiers, mixing up capitalization when referencing a variable is a very common source of 'cannot find symbol' compiler errors.
Example: Case Sensitivity in Names
// Define the class Main
public class Main {
// Program entry point: the JVM starts running here
public static void main(String[] args) {
// Declare page and set it to "lowercase"
String page = "lowercase";
// Declare Page and set it to "uppercase"
String Page = "uppercase";
// Print a line to the console
System.out.println(page + " is not the same identifier as " + Page);
}
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Best Practices for Names
Favor full words over abbreviations for anything beyond a loop counter -- customerCount over cc -- since future readers (including you, months later) will spend far less time decoding what the variable holds.
Example: Best Practices for Names
public class Main {
public static void main(String[] args) {
int customerCount = 42; // clearer than 'cc'
System.out.println(customerCount);
}
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- Using a keyword such as
classorintas a variable name, which is a compile error. - Starting an identifier with a digit, as in
1name, which is not allowed. - Assuming
Nameandnameare the same identifier, when Java is case-sensitive.
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