Java Errors
Errors are a natural part of programming—even for experienced Java developers. Understanding how to identify and fix errors is essential for writing reliable, bug-free applications. In Java, errors generally fall into three main categories: compile-time errors, runtime errors, and logical errors
Compile-Time Errors in Java
Compile-time errors occur when the Java compiler detects syntax or type problems before the
program runs.
1. Missing Semicolon
Java requires a semicolon at the end of most statements.
public class MissingSemicolonExample {
public static void main(String[] args) {
int value = 10
System.out.println("Value = " + value);
}
}
Compiler error
';' expected
Fix
int value = 10;
2. Using an Undeclared Variable
Variables must be declared before use.
public class UndeclaredVariableExample {
public static void main(String[] args) {
total = 50;
System.out.println(total);
}
}
Compiler error
cannot find symbol
symbol: variable total
Fix
int total = 50;
3. Type Mismatch
Assigning an incompatible value to a variable causes a compile-time error.
public class TypeMismatchExample {
public static void main(String[] args) {
double price = "19.99";
System.out.println(price);
}
}
Compiler error
incompatible types: String cannot be converted to double
Fix
double price = 19.99;
Runtime Errors in Java
Runtime errors occur after successful compilation, while the program is running. These typically
throw exceptions.
1. Division by Zero
public class DivisionByZeroExample {
public static void main(String[] args) {
int items = 25;
int groups = 0;
int perGroup = items / groups;
System.out.println(perGroup);
}
}
Runtime error
java.lang.ArithmeticException: / by zero
2. Array Index Out of Bounds
Accessing an invalid array index causes an exception.
public class ArrayBoundsExample {
public static void main(String[] args) {
String[] cities = {"Delhi", "Chennai", "Mumbai"};
System.out.println(cities[5]);
}
}
Runtime error
java.lang.ArrayIndexOutOfBoundsException
Logical Errors in Java
Logical errors are the most difficult to detect because the program runs successfully but
produces incorrect results.
public class LogicalErrorExample {
public static void main(String[] args) {
int length = 8;
int width = 4;
int area = length + width; // mistake
System.out.println("Rectangle area = " + area);
}
}
Output
Rectangle area = 12
Expected
Rectangle area = 32
Issue: Area should be calculated using multiplication, not addition.
Fix
int area = length * width;
