question about compile error or runtime error

[dragon ji]

When the error happens ?At compile time or runtime ?How can I get it clear?Has it regularity or not?

[Levente Szekrenyes]

Ji,
I didn't really get your question. Do you have a specific scenario in your mind (then give us some code) or you're just wondering what's the difference between compile-time and run-tim errors?

[Abu Yoosuf]

Compile time error happens when your program has one or more syntax errors.
What is syntax error? Every programming language dictates how you should write your program. If you want drive a car without getting booked in front of the watchful police officer than you don't jump the red lights. If you don't want the compiler errors than you follow the Java rules book (jls).

int a = 4, b = 5, c = 6;
a = b + c; //compiler salutes you
b + c = a; //compiler gives you a ticket

Runtime error occurs when you violate the rules during the execution of the program. In order to run the program, it should pass the compiler first. So, it doesn't have any syntax violatations.

public static void main (String[] args)
{
   int i = 2, j = 100;
   int result = 200;
   for( int x = 1; x <= 10; x++ )
   {
       i = i - 1;
       result = j / i; 
       //
       //when x = 1, i = 1, result = 100 / 1;
       //when x = 2, i = 0, result = 100 / 0;
       //Java runtime doesn't allow you to divide an int by zero. 
       //so it throws a runtime exception called ArithmeticException  
       
   }
}

[Alfred Kemety]

Simply compile time errors occur when the thing being checked by the compiler can be determined by the compiler during compile time. This is like the syntax of the program, checked exceptions handeling, a call to a non-static variable from the static context, mis-overloading a method, wrong declaration, etc... all these things can be checked by the compiler during compile time and thus you get a compile time error while compiling your code. Compiler though can't catch things that are only determined during run time.
Runtime errors are due to errors that can only be determined by the JVM when running the program, like dividing by a variable that equals to zero, compiler can't determine the value of the variable during compilation cause the compiler doesn't check the real life behavior of your code.
You should know when can compile time errors happen and when does runtime erros occure, you might have to choose between this or that in a question in the exam.

[Ken Krebs]

One particular example of when people are tripped on this is in regard to conversion and casting.
A conversion must be correct at compile time. A cast is a directive to the compiler to ignore a possible conversion error and to let the JVM decide if its correct at runtime.
"What we observe is not nature itself, but nature exposed to our method of questioning,"
--- Werner Heisenberg ---
Ken
SCJP 1.4