Lesson 1 — Types of Errors | Module 10: Exception Handling

1 Welcome

Hello, my student. Today we will learn exception handling in Java — one of the most important topics in programming. But before we dive into exceptions, let me ask you a question:

What types of errors have you encountered while programming in CPCS203? Think about it for a moment...

Throughout this course, you have seen three types of errors. Let's review each one, because understanding errors is the foundation for understanding exceptions.

2 Syntax Errors (Compile-Time Errors)

A syntax error happens when your code violates the rules of the Java language. Just like every spoken language has grammar rules, Java has syntax rules.

Language Analogy — Every Language Has Its Own Rules

In Arabic: “جدة جميلة” — the adjective comes AFTER the noun. That is Arabic syntax.

In English: “Beautiful Jeddah” — the adjective comes BEFORE the noun.

Every language has its own rules. If you break them, the sentence doesn’t make sense.

In Java, if your syntax is wrong, the program simply won’t compile.

Example A Program with Syntax Errors

SyntaxDemo.java

public class SyntaxDemo {
    public static void main(String[] args) {
        int x = 10   // ERROR: missing semicolon
        int y = 20;

        System.out.println("Sum = " + x + y) // ERROR: missing semicolon

        if (x > y      // ERROR: missing closing parenthesis
            System.out.println("x is greater");
        }
    }
}

Compile Output

$ javac SyntaxDemo.java SyntaxDemo.java:3: error: ';' expected int x = 10 ^ SyntaxDemo.java:6: error: ';' expected System.out.println("Sum = " + x + y) ^ SyntaxDemo.java:8: error: ')' expected if (x > y ^ 3 errors

The Good News

Syntax errors are the EASIEST to fix. Your IDE tells you exactly where the problem is. The compiler points to the line number and even the exact character. Just follow the arrows!

3 Logical Errors

A logical error is sneaky. The program compiles, runs, and produces output — but the output is WRONG.

Zakah Example — A Real-World Logical Error

In Islam, Zakah is 2.5% of wealth above a certain threshold.

Zakah of 1,000 SAR = 25 SAR

Zakah of 1,000,000 SAR = 25,000 SAR

But imagine your program calculates it as 10% instead of 2.5%...

Example A Program with a Logical Error

ZakahCalculator.java

public class ZakahCalculator {
    public static void main(String[] args) {
        double wealth = 1000000;
        double zakahRate = 0.10;  // BUG! Should be 0.025 (2.5%)

        double zakahDue = wealth * zakahRate;

        System.out.println("Wealth: " + wealth + " SAR");
        System.out.println("Zakah rate: " + zakahRate);
        System.out.println("Zakah due: " + zakahDue + " SAR");
    }
}

Output

$ javac ZakahCalculator.java $ java ZakahCalculator Wealth: 1000000.0 SAR Zakah rate: 0.1 Zakah due: 100000.0 SAR

The Problem

The program says 100,000 SAR, but the correct answer is 25,000 SAR. No crashes, no errors — but completely wrong. This is a logical error.

How Do We Find Logical Errors?

Logical errors are solved by testing. There are techniques like white-box and black-box testing, but that’s not our focus today. The key point is: the compiler cannot help you with logical errors. Only careful testing and code review can catch them.

4 Runtime Errors

Runtime errors are the main character of this chapter. The program compiles, starts running... then suddenly crashes.

Example A Program That Crashes at Runtime

RuntimeDemo.java

public class RuntimeDemo {
    public static void main(String[] args) {
        int a = 10;
        int b = 0;
        int result = a / b;  // Division by zero!

        System.out.println("Result: " + result);
        System.out.println("Program continues...");  // Never reached
    }
}

Output

$ javac RuntimeDemo.java $ java RuntimeDemo Exception in thread "main" java.lang.ArithmeticException: / by zero at RuntimeDemo.main(RuntimeDemo.java:5)

Real-World Impact — Saudi Stock Market

Consider the Saudi stock market — Al-Ashum (الأسهم). Billions of riyals traded every day. If the trading system crashes for 3 hours, billions lost. Medical devices, power distribution, banking systems — all must keep running. Runtime errors in these systems are not just bugs; they are catastrophes.

5 Two Core Goals of This Chapter

Now that we understand the three types of errors, here is the big question: What can we do about runtime errors?

⛔

Graceful Termination

Sometimes the program must stop. But it should stop SAFELY — saving data, closing files, rolling back transactions. Never leave the system in a corrupted state.

✅

Continued Execution

The program can handle the error and keep running. The stock market shouldn’t crash because one transaction failed. Handle the problem and move on.

Remember These Two Goals

Keep these two goals in mind throughout this entire chapter. Every concept — try-catch, finally, throw, throws, custom exceptions — serves these two purposes.

What's Next?

In the next lesson, we will see WHY exception handling is the right approach — Oracle’s three advantages.

Three Types of Errors