Compile-Time Errors
1. "Cannot find symbol"
Cause: Variable, method, or class not declared or misspelled.
// Error
System.out.println(myVariable); // myVariable not declared
// Fix
int myVariable = 10;
System.out.println(myVariable);2. "Class, interface, or enum expected"
Cause: Code written outside of any class definition.
// Error
System.out.println("Hello"); // Outside class
// Fix
public class Main {
public static void main(String[] args) {
System.out.println("Hello");
}
}3. "Incompatible types"
Cause: Assigning value of wrong type without proper casting.
// Error
int num = "123"; // String to int
// Fix
int num = Integer.parseInt("123");
// or
String str = "123";4. "Missing return statement"
Cause: Method with non-void return type doesn't return a value in all paths.
// Error
public int getValue(boolean flag) {
if (flag) {
return 10;
}
// Missing return for else case
}
// Fix
public int getValue(boolean flag) {
if (flag) {
return 10;
}
return 0; // Default return
}5. "Unreported exception; must be caught or declared"
Cause: Checked exception not handled.
// Error
FileReader fr = new FileReader("file.txt"); // IOException not handled
// Fix 1: try-catch
try {
FileReader fr = new FileReader("file.txt");
} catch (IOException e) {
e.printStackTrace();
}
// Fix 2: throws declaration
public void readFile() throws IOException {
FileReader fr = new FileReader("file.txt");
}6. "Variable might not have been initialized"
Cause: Local variable used before initialization.
// Error
int x;
System.out.println(x); // x not initialized
// Fix
int x = 0;
System.out.println(x);Runtime Errors (Exceptions)
1. NullPointerException
Cause: Calling method or accessing field on null reference.
// Error
String str = null;
int len = str.length(); // NullPointerException
// Fix: Null check
if (str != null) {
int len = str.length();
}
// Fix: Optional (Java 8+)
Optional.ofNullable(str)
.ifPresent(s -> System.out.println(s.length()));2. ArrayIndexOutOfBoundsException
Cause: Accessing array with invalid index.
// Error
int[] arr = {1, 2, 3};
System.out.println(arr[3]); // Index 3 doesn't exist (0-2)
// Fix: Check bounds
int index = 3;
if (index >= 0 && index < arr.length) {
System.out.println(arr[index]);
}3. ClassCastException
Cause: Invalid type casting.
// Error
Object obj = "Hello";
Integer num = (Integer) obj; // String cannot be cast to Integer
// Fix: Check type first
if (obj instanceof Integer) {
Integer num = (Integer) obj;
}4. NumberFormatException
Cause: Parsing non-numeric string to number.
// Error
int num = Integer.parseInt("abc"); // "abc" is not a number
// Fix: Validate or catch
try {
int num = Integer.parseInt(input);
} catch (NumberFormatException e) {
System.out.println("Invalid number format");
}5. ArithmeticException
Cause: Division by zero (integer).
// Error
int result = 10 / 0; // ArithmeticException
// Fix: Check divisor
int divisor = 0;
if (divisor != 0) {
int result = 10 / divisor;
} else {
System.out.println("Cannot divide by zero");
}6. ConcurrentModificationException
Cause: Modifying collection while iterating with for-each.
// Error
List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C"));
for (String s : list) {
list.remove(s); // ConcurrentModificationException
}
// Fix: Use Iterator
Iterator<String> it = list.iterator();
while (it.hasNext()) {
String s = it.next();
it.remove(); // Safe removal
}
// Fix: removeIf (Java 8+)
list.removeIf(s -> s.equals("B"));7. StackOverflowError
Cause: Infinite recursion or too deep recursion.
// Error
public void infiniteRecursion() {
infiniteRecursion(); // No base case
}
// Fix: Add base case
public int factorial(int n) {
if (n <= 1) return 1; // Base case
return n * factorial(n - 1);
}8. OutOfMemoryError
Cause: JVM runs out of heap space.
// Error - creating too many objects
List<byte[]> list = new ArrayList<>();
while (true) {
list.add(new byte[1000000]); // OutOfMemoryError
}
// Fix: Limit data, increase heap (-Xmx), use streamingLogical Errors
1. Off-by-one Error
Cause: Loop iterates one too many or too few times.
// Error
for (int i = 0; i <= arr.length; i++) { // Should be < not <=
System.out.println(arr[i]);
}
// Fix
for (int i = 0; i < arr.length; i++) {
System.out.println(arr[i]);
}2. == vs .equals() for Strings
Cause: Using == to compare string content.
// Error
String s1 = new String("hello");
String s2 = new String("hello");
if (s1 == s2) { ... } // false - compares references
// Fix
if (s1.equals(s2)) { ... } // true - compares content3. Floating Point Comparison
Cause: Direct equality comparison of floating point numbers.
// Error
double a = 0.1 + 0.2;
if (a == 0.3) { ... } // May be false due to precision
// Fix: Use epsilon comparison
double epsilon = 0.0001;
if (Math.abs(a - 0.3) < epsilon) { ... }4. Integer Overflow
Cause: Arithmetic exceeds int range.
// Error
int big = Integer.MAX_VALUE;
int result = big + 1; // Overflows to negative
// Fix: Use long
long big = Integer.MAX_VALUE;
long result = big + 1; // Works correctly5. Forgetting break in switch
Cause: Fall-through to next case.
// Error
switch (day) {
case 1: System.out.println("Monday");
case 2: System.out.println("Tuesday"); // Also prints for day=1
}
// Fix
switch (day) {
case 1: System.out.println("Monday"); break;
case 2: System.out.println("Tuesday"); break;
}