Java Errors in AP CSA: Find, Understand, and Fix Them

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Java Errors in AP CSA: Find It, Understand It, Fix It

Every Java error an AP Computer Science A student runs into, explained in plain language with broken-and-fixed code, the common multiple-choice trap, and a link to the exact spot in the official College Board CED. Search an error below or browse by type.

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Runtime errors (code compiles, then crashes)

These pass compilation and only fail while the program is running. They are the errors students hit most on real assignments and the ones the exam loves to test with "what is the result?" questions.

NullPointerExceptionguide Calling a method on a null reference. Covered in the Runtime Errors guide. ArrayIndexOutOfBoundsguide Index outside 0 to length-1. Covered in Top Array Mistakes. StringIndexOutOfBounds charAt or substring past the end of a String. NumberFormatException Integer.parseInt on text that is not a number. ArrayList IndexOutOfBoundsguide get/remove past the list size. Covered in ArrayList Traps. ConcurrentModificationguide Modifying a list inside a for-each. Covered in the ArrayList Exam Guide. ArithmeticExceptionguide Integer divide or mod by zero. Related: casting and type conversion. StackOverflowErrorguide Runaway recursion. Covered in the Recursion Tracing guide.

Compiler errors (code will not build)

These stop the program before it ever runs. The compiler is telling you something is wrong with the code itself. Fixing them is about reading the message and knowing what it really means, and the message often points one line below the real mistake.

cannot find symbol A name the compiler does not recognize: typo, scope, or missing declaration. incompatible types Assigning a value to a variable of the wrong type, like double to int. missing return statement A non-void method that can reach its end without returning. illegal start of expression Usually a missing brace that throws off the parser above this line. ';' expected A statement missing its terminating semicolon, often on the line above. non-static from static context Using an instance member from static main without an object. variable might not be initialized Using a local variable before it is assigned on every path. unreachable statement Code placed where it can never run, such as after a return.

Studying the concepts, not just fixing a crash?

These error pages get you unstuck fast. For full exam prep, work through the lessons and the unit study guides.

AP CSA Lessons Unit 4 Study Guide

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