A runtime error in Java is an application error that occurs during the execution of a program. A runtime error occurs when a program is syntactically correct but contains an issue that is only detected during program execution. These issues cannot be caught at compile-time by the Java compiler and are only detected by the Java Virtual Machine (JVM) when the application is running.
Runtime errors are a category of exception that contains several more specific error types. Some of the most common types of runtime errors are:
- IO errors
- Division by zero errors
- Out of range errors
- Undefined object errors
Runtime Errors vs Compile-Time Errors
Compile-time errors occur when there are syntactical issues present in application code, for example, missing semicolons or parentheses, misspelled keywords or usage of undeclared variables.
These syntax errors are detected by the Java compiler at compile-time and an error message is displayed on the screen. The compiler prevents the code from being executed until the error is fixed. Therefore, these errors must be addressed by debugging before the program can be successfully run.
On the other hand, runtime errors occur during program execution (the interpretation phase), after compilation has taken place. Any code that throws a runtime error is therefore syntactically correct.
Runtime Errors vs Logical Errors
A runtime error could potentially be a legitimate issue in code, for example, incorrectly formatted input data or lack of resources (e.g. insufficient memory or disk space). When a runtime error occurs in Java, the compiler specifies the lines of code where the error is encountered. This information can be used to trace back where the problem originated.
On the other hand, a logical error is always the symptom of a bug in application code leading to incorrect output e.g. subtracting two variables instead of adding them. In case of a logical error, the program operates incorrectly but does not terminate abnormally. Each statement may need to be checked to identify a logical error, which makes it generally harder to debug than a runtime error.
What Causes Runtime Errors in Java
The most common causes of runtime errors in Java are:
- Dividing a number by zero.
- Accessing an element in an array that is out of range.
- Attempting to store an incompatible type value to a collection.
- Passing an invalid argument to a method.
- Attempting to convert an invalid string to a number.
- Insufficient space in memory for thread data.
When any such errors are encountered, the Java compiler generates an error message and terminates the program abnormally. Runtime errors don’t need to be explicitly caught and handled in code. However, it may be useful to catch them and continue program execution.
To handle a runtime error, the code can be placed within a try-catch block and the error can be caught inside the catch block.
Runtime Error Examples
Division by zero error
Here is an example of a java.lang.ArithmeticException, a type of runtime exception, thrown due to division by zero:
public class ArithmeticExceptionExample {
public static void main(String[] args) {
int a = 10, b = 0;
System.out.println("Result: "+ a/b);
}
}In this example, an integer a is attempted to be divided by another integer b, whose value is zero, leading to a java.lang.ArithmeticException:
Exception in thread "main" java.lang.ArithmeticException: / by zero
at ArithmeticExceptionExample.main(ArithmeticExceptionExample.java:4)
Accessing an out of range value in an array
Here is an example of a java.lang.ArrayIndexOutOfBoundsExceptionthrown due to an attempt to access an element in an array that is out of bounds:
public class ValueOutOfRangeErrorExample {
public static void main(String[] args) {
int arr[] = new int[5];
System.out.println("5th element in array: " + arr[5]);
}
}In this example, an array is initialized with 5 elements. An element at position 5 is later attempted to be accessed in the array, which does not exist, leading to a java.lang.ArrayIndexOutOfBoundsException runtime error:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 5 out of bounds for length 5
at ValueOutOfRangeErrorExample.main(ValueOutOfRangeErrorExample.java:4)
How to Solve Runtime Errors
Runtime errors can be handled in Java using try-catch blocks with the following steps:
- Surround the statements that can throw a runtime error in try-catch blocks.
- Catch the error.
- Depending on the requirements of the application, take necessary action. For example, log the exception with an appropriate message.
To illustrate this, the code in the earlier ArithmeticException example can be updated with the above steps:
public class ArithmeticExceptionExample {
public static void main(String[] args) {
try {
int a = 10, b = 0;
System.out.println("Result: " + a/b);
} catch (ArithmeticException ae) {
System.out.println("Arithmetic Exception: cannot divide by 0");
}
System.out.println("Continuing execution...");
}
}
Surrounding the code in try-catch blocks like the above allows the program to continue execution after the exception is encountered:
Arithmetic Exception: cannot divide by 0
Continuing execution…Runtime errors can be avoided where possible by paying attention to detail and making sure all statements in code are mathematically and logically correct.
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