Java Errors
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Java Errors
Even experienced Java developers make mistakes. The key is learning how to spot and fix them!
These pages cover common errors and helpful debugging tips to help you understand what's going wrong and how to fix it.
Types of Errors in Java
| Error Type |
Description |
| Compile-Time Error |
Detected by the compiler. Prevents code from running. |
| Runtime Error |
Occurs while the program is running. Often causes crashes. |
| Logical Error |
Code runs but gives incorrect results. Hardest to find. |
Common Compile-Time Errors
Compile-time errors occur when the program cannot compile due to syntax or type issues.
Here are some examples:
1) Missing Semicolon
Example
int x = 5
System.out.println(x);
Result:
error: ';' expected
Try it Yourself »
Tip: Java requires a semicolon at the end of every statement (int x = 5;).
2) Undeclared Variables
Example
System.out.println(myVar);
Result:
cannot find symbol
symbol: variable myVar
Try it Yourself »
Tip: You must declare a variable before using it (int myVar = 50;).
3) Mismatched Types
Example
int x = "Hello";
Result:
incompatible types: String cannot be converted to int
Try it Yourself »
Tip: Make sure the value matches the variable type (String x = "Hello";).
Common Runtime Errors
Runtime errors occur when the program compiles but crashes or behaves unexpectedly.
Here are some examples:
1) Division by Zero
Example
int x = 10;
int y = 0;
int result = x / y;
System.out.println(result);
Result:
Exception in thread "main" java.lang.ArithmeticException: / by zero
Try it Yourself »
2) Array Index Out of Bounds
Example
int[] numbers = {1, 2, 3};
System.out.println(numbers[8]);
Result:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 8 out of bounds for length 3
Try it Yourself »
Logical Errors
Logical errors happen when the code runs, but the result is not what you thought:
Example:
int x = 10;
int y = 2;
int sum = x - y;
System.out.println("x + y = " + sum);
Result:
x + y = 8
Try it Yourself »
Expected Result: 12
Logical Error: The code mistakenly subtracts instead of adds.
Tip: Test your program with different inputs to catch logic flaws (try using x + y instead).
This is part of debugging, which you will learn more about in the next chapter.
Good Habits to Avoid Errors
- Use meaningful variable names
- Read the error message carefully. What line does it mention?
- Check for missing semicolons or braces
- Look for typos in variable or method names
In the next chapter, you will learn how to debug your code - how to find and fix bugs/errors in your program.
