Mahmoud Abas, Full-Stack Software Engineer & AI

Software Construction - Java Exceptions and Debugging Lab

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Java Exceptions and Debugging Lab

Error types, debugging technique, and handling exceptions gracefully

Types of Errors

Java Errors

Compile-Time

Detected before the program runs: it won't even compile (missing semicolon, undeclared variable)

Runtime

Occur while the program is running and crash it (division by zero, array index out of bounds)

Logical

The program runs and produces a result, but the result is wrong (hardest to find)

Compile-Time Errors

java

int x = 5   // Error: ';' expected ,  fix: add a semicolon

System.out.println(myVar);  // Error: cannot find symbol
int myVar = 50;              // fix: declare before use

int x = "Hello";   // Error: incompatible types
String x = "Hello"; // fix: use the correct type

Compile-time errors stop compilation entirely, the compiler names the exact problem

Runtime Errors

java

int x = 10, y = 0;
int result = x / y;   // ArithmeticException: / by zero

int[] numbers = {1, 2, 3};
System.out.println(numbers[10]);  // ArrayIndexOutOfBoundsException: 10

Runtime errors compile fine but crash the program the moment the bad operation executes

Logical Errors

java

int x = 10, y = 2;
int result = x - y;   // meant x + y
System.out.println(result);  // prints 8, not 12

No crash, no error message, the program just gives the wrong answer
Hardest category to catch; requires careful tracing of expected vs actual values

Debugging Basics

Debugging: Finding and fixing errors in your code.

Simplest technique: add System.out.println() calls to trace state before the failure point

Code has bug

Add print statements

Run program

Check output

Found problem?

Yes

Fix bug

Test again

java

System.out.println("x = " + x);
System.out.println("y = " + y);
int result = x / y;  // crash happens here ,  trace shows exactly what led to it

Debugging Checklist

Read the full error message, it usually names the exact exception and line
Check that every variable is initialized before use
Print variable values to trace the problem step by step
Watch for off-by-one errors in loops
Test with different inputs, including edge cases

Exception Handling: try-catch

When an error occurs, Java throws an exception, an error object. Left unhandled, it crashes the program
try-catch lets you handle it gracefully instead

Execute try block

Error occurs?

Skip catch block

Execute catch block

Continue program

java

try {
    int[] myNumbers = {1, 2, 3};
    System.out.println(myNumbers[10]);
} catch (Exception e) {
    System.out.println("Something went wrong.");
}
// Output: Something went wrong.  (program continues)

The finally Block

finally runs always, whether an exception occurred or not

try block

Error?

Yes

catch block

Skip catch

finally block

Continue

java

try {
    int[] myNumbers = {1, 2, 3};
    System.out.println(myNumbers[10]);
} catch (Exception e) {
    System.out.println("Something went wrong.");
} finally {
    System.out.println("The 'try catch' is finished.");
}

Use it for cleanup code that must run regardless of success or failure

The throw Keyword

Create and raise your own exceptions with throw

java

static void checkAge(int age) {
    if (age < 18) {
        throw new ArithmeticException("Access denied - must be 18+.");
    }
    System.out.println("Access granted!");
}

try {
    checkAge(15);
} catch (Exception e) {
    System.out.println("Error: " + e.getMessage());
}
// Output: Error: Access denied - must be 18+.

Try-with-Resources

Manual resource cleanup (files, streams) needs a finally block that closes the resource, verbose and error-prone
Try-with-resources (Java 7+) closes the resource automatically, even if an exception occurs

java

try (FileWriter fw = new FileWriter("output.txt");
     BufferedWriter bw = new BufferedWriter(fw)) {
    bw.write("Hello World!");
} catch (Exception e) {
    e.printStackTrace();
}
// Both resources close automatically, in reverse order

Open Resource 1

Open Resource 2

Use Resources

Close Resource 2

Close Resource 1

Common Exception Types

Exception	Cause
`ArithmeticException`	Division by zero
`ArrayIndexOutOfBoundsException`	Invalid array index
`NullPointerException`	Using a null object
`FileNotFoundException`	File doesn't exist
`NumberFormatException`	Invalid string-to-number conversion

Key Takeaways

Errors fall into three categories: compile-time (won't build), runtime (crashes), and logical (silently wrong)
Debug systematically, read the full error message, print variable state, and test varied inputs
try-catch prevents crashes by handling exceptions gracefully; finally always runs for cleanup
throw lets you raise your own exceptions with meaningful messages
Try-with-resources automatically and safely closes files, streams, and connections, even on failure
The more you debug deliberately, the faster you get at spotting and fixing problems

Java Exceptions and Debugging

Error Types

Compile-Time, Runtime, Logical

Debugging

Print statements, IDE debugger, check variables

Exception Handling

try-catch, finally, throw

Best Practices

try-with-resources, specific exceptions, meaningful messages

Open this lesson

Mahmoud AbasJava Exceptions and Debugging Lab

int x = 5 // Error: ';' expected , fix: add a semicolon System.out.println(myVar); // Error: cannot find symbol int myVar = 50; // fix: declare before use int x = "Hello"; // Error: incompatible types String x = "Hello"; // fix: use the correct type

try { int[] myNumbers = {1, 2, 3}; System.out.println(myNumbers[10]); } catch (Exception e) { System.out.println("Something went wrong."); } // Output: Something went wrong. (program continues)

try { int[] myNumbers = {1, 2, 3}; System.out.println(myNumbers[10]); } catch (Exception e) { System.out.println("Something went wrong."); } finally { System.out.println("The 'try catch' is finished."); }

static void checkAge(int age) { if (age < 18) { throw new ArithmeticException("Access denied - must be 18+."); } System.out.println("Access granted!"); } try { checkAge(15); } catch (Exception e) { System.out.println("Error: " + e.getMessage()); } // Output: Error: Access denied - must be 18+.

try (FileWriter fw = new FileWriter("output.txt"); BufferedWriter bw = new BufferedWriter(fw)) { bw.write("Hello World!"); } catch (Exception e) { e.printStackTrace(); } // Both resources close automatically, in reverse order

Exception

Cause

ArithmeticException

Division by zero

ArrayIndexOutOfBoundsException

Invalid array index

NullPointerException

Using a null object

FileNotFoundException

File doesn't exist

NumberFormatException

Invalid string-to-number conversion