Java is case-sensitive because it uses a C-style syntax. Case sensitivity is useful because it lets you infer what a name means based on it's case.
For example, the Java standard for class names is uppercasing the first letter of each word (Integer, PrintStream, etc). The standard for variable and method names is lowercase the first letter of the first word, and uppercasing all other first letters (number, println(), checkError(), etc). And the standard for constants is all uppercase with underscores (SOME_CONSTANT).
While you're not required to follow these rules in your code (it'll compile even if you break these rules), Java's built-in code follows them, and all major Java libraries do too. And your code really should follow these rules too, because it helps other developers infer meaning when they see the capitalization.
