Autoboxing and Wrapper Classes

Part of: ArrayList

Objects Only. An ArrayList can only store objects , never primitive types like int, double, or boolean. You cannot write ArrayList<int . Instead Java provides wrapper classes that wrap a primitive in an object: - int - Integer - double - Double - boolean - Boolean So you write ArrayList<Integer or ArrayList<Double . Autoboxing. Autoboxing is the automatic conversion from a primitive to its wrapper. Unboxing is the reverse. The compiler inserts these conversions for you: Because of autoboxing, you can mostly treat ArrayList<Integer as if it held plain ints. But the values are really Integer objects under the hood. Pitfalls to Know. - Comparing with == : Comparing two Integer objects with == may give wrong results because it compares references, not values. Use .equals() or unbox to int first. - Null unboxing : Unboxing a null Integer throws a NullPointerException. - remove(int) trap : As seen earlier, list.remove(5) on an Integer list removes index 5, not the value 5, precisely because the int overload is preferred over autoboxing. What usually goes wrong. What you see What caused it How to fix it --- --- --- unexpected type on ArrayList<int A type parameter has to be an object type

Challenge: Sum and Average