Java syntax errors refer to mistakes made by a programmer in implementing the grammar of the Java programming language. It doesn't cover mistakes in logic of the program itself.
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Java, like all other programming languages, has its own syntax. For example, one rule of Java syntax is that all commands must end with a semicolon (;). Java syntax is vastly simpler than the syntax of English or any other "natural" language but it is also much stricter. Leaving a comma out of sentence in English merely makes the writer look sloppy. A trivial mistake in Java syntax make the commands unintelligible to the computer.
Syntax errors are a type of compiler error. This means they will be detected immediately when the programmer tries to convert his source code into a program. This is opposed to runtime errors, which are not detected until the program is actually running.
Examples of Syntax Errors
This code -- if x=3 System.out.println("Hello.") -- has three syntax errors:
First, the command does not end in a semicolon. Second, the comparison that occurs after the "if" must be surrounded by parenthesis. Finally, the comparison itself uses the assignment operator "=" rather than the comparison operator "==." Any one of these will generate a Java syntax error. Here is the command written properly:
if (x==3) System.out.println("Hello.");