Question

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Runtime vs Compile time

How should I know whether a specific line of code in Java may throw a compile time or run-time error? Assuming that the specific line of code anyway throws and error.

Answer 1

In Eclipse, compile time errors will be underlined in red. A compile time error is an error that is detected by the compiler. Common causes for compile time errors include:

• Syntax errors such as missing semi-colon or use of a reserved keyword (such as ‘class’).
• When you try and access a variable that is not in scope.
• When you declare multiple objects with the same name.

If the compiler detects any errors during compilation it will fail to build a new assembly (or class file in Java).

Even if your code has no compile time errors, errors can still occur on run-time. Errors such as ‘logic errors’ and ‘runtime errors’. A good example of a runtime error is as followed:

• Pretend you’re going to store an item in an array at index 5 but the array’s size is only 4. The compiler won’t detect an error here because it understands that the array size is subject to change but on run-time you’ll be thrown an exception.

To detect which line exactly a run-time error occurs on you can use a combination of break points in Eclipse and proper exception handling.