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Java Errors

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In every programming language, errors are a normal part of development. They help identify what went wrong in the program and guide the programmer toward fixing it. In Java, errors occur when something prevents the program from running correctly — like missing syntax, invalid logic, or system-level failures.

In this tutorial, you’ll learn about the different types of errors in Java, how they occur, and how to handle or prevent them effectively.

What Are Errors in Java?

An error in Java is any unexpected condition that stops a program from compiling or executing correctly. Errors can occur during compilation or runtime depending on the nature of the issue.

There are mainly three types of errors in Java:

  1. Syntax Errors (Compile-time Errors)

  2. Runtime Errors

  3. Logical Errors

Let’s understand each type in detail.

Syntax Errors (Compile-Time Errors)

Syntax errors happen when you write code that doesn’t follow Java’s grammar or rules. These are detected by the Java compiler before the program runs.

Common syntax errors include:

  • Missing semicolons

  • Incorrect variable names

  • Missing brackets or parentheses

  • Using undeclared variables

Example

class Example {
    public static void main(String[] args) {
        System.out.println("Hello World")
    }
}

Error: Missing semicolon at the end of the println() statement.
The compiler will display:

error: ';' expected

You must fix all syntax errors before your program can compile successfully.

Runtime Errors

Runtime errors occur while the program is running. These errors cause the program to stop unexpectedly or behave incorrectly. They usually happen due to invalid input, file handling issues, or memory problems.

Some common runtime errors are:

  • Dividing by zero

  • Accessing an invalid array index

  • Using a null object reference

  • Opening a file that doesn’t exist

Example

class Example {
    public static void main(String[] args) {
        int number = 10 / 0;
        System.out.println("Result: " + number);
    }
}

Output:

Exception in thread "main" java.lang.ArithmeticException: / by zero

The compiler won’t detect this issue, but it will crash at runtime.

Logical Errors

Logical errors don’t cause the program to crash, but they produce incorrect output. These are the hardest to find because the syntax is correct, and the program runs — it just gives wrong results.

Example

class Example {
    public static void main(String[] args) {
        int a = 10, b = 5;
        int sum = a - b; // Logic mistake, should be a + b
        System.out.println("Sum: " + sum);
    }
}

Output will be Sum: 5 instead of Sum: 15.

Since no error message appears, logical mistakes require debugging and careful testing to find.

Types of Error Categories in Java

Apart from the basic error types, Java also classifies errors based on their severity:

1. Compile-Time Errors

Detected by the compiler (e.g., syntax errors, missing imports, incorrect method calls).

2. Runtime Errors

Occur while the program executes (e.g., dividing by zero, null pointer exceptions).

3. Logical Errors

Caused by incorrect logic or formulas in your code.

4. System Errors

These occur due to hardware or system-level issues, such as running out of memory or JVM crashing.

Common Examples of Java Errors

Error Type Example Description
Syntax Error Missing semicolon Compiler fails to compile
Runtime Error Divide by zero Program stops while running
Logical Error Wrong calculation Output is incorrect
Null Pointer Object not initialized Program crashes
Array Index Accessing invalid index Out of bounds exception

Preventing Errors in Java

While you can’t completely avoid all errors, you can reduce them by following some best practices:

  1. Use meaningful variable names — helps avoid logical confusion.

  2. Test your code frequently — find and fix issues early.

  3. Add validation for user input — avoid runtime failures.

  4. Use IDE features — modern IDEs highlight syntax and runtime warnings.

  5. Write clean, modular code — easier to debug and test.

Difference Between Errors and Exceptions

It’s important to note that errors and exceptions are not the same in Java.

Basis Error Exception
Origin System or environment issues Caused by program or user
Recoverable Usually not recoverable Can be handled with try-catch
Example OutOfMemoryError, StackOverflowError IOException, ArithmeticException

Errors are more serious and often indicate something that cannot be fixed while the program is running. Exceptions, on the other hand, can often be caught and handled gracefully.

Summary of the Tutorial

In this tutorial, you learned what errors are in Java, their main types (syntax, runtime, and logical), and how they differ from exceptions. You also saw how to identify and prevent common mistakes that can break or mislead your program.

Understanding errors is the first step before learning debugging and exception handling — two powerful tools that help you write cleaner, error-free Java programs.

Practice Questions

  1. Write a Java program that intentionally creates a syntax error and then fix it to make the program run correctly.

  2. Create a program that causes a runtime error by dividing a number by zero.

  3. Write a Java program that produces a logical error while calculating the average of three numbers.

  4. Create a program that generates a NullPointerException and explain why it happens.

  5. Write a Java program that demonstrates an ArrayIndexOutOfBoundsException by accessing an invalid array index.

  6. Create a simple program that checks for input validation before performing division to prevent runtime errors.

  7. Write a Java program where a user enters two integers, and handle the case where the second number is zero to avoid errors.

  8. Create a program that demonstrates a compile-time error by using an undeclared variable.

  9. Write a Java program that performs subtraction instead of addition (a logical mistake), then fix it.

  10. Create a program that prints different types of errors and exceptions using comments to explain each one.

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