4.7 Wrapper Classes

What you need to know

• Integer and Double are wrapper classes: objects that hold one int or double. Needed because ArrayList<int> is illegal; you write ArrayList<Integer>.
• Autoboxing: Java automatically converts int → Integer when needed (list.add(5)). Unboxing: Integer → int (int x = list.get(0)).
• Integer.MAX_VALUE and Integer.MIN_VALUE — the int range limits, on the reference sheet. Useful as initial values for min/max algorithms.
• Integer.parseInt(String) and Double.parseDouble(String) convert text to numbers — essential for file input.
• Comparing two Integer objects with == compares references and is unreliable; use .equals() or unbox to ints first.
• Unboxing a null Integer throws a NullPointerException.
• Arithmetic on Integers unboxes automatically: Integer a = 5; int b = a + 3; works.

Worked example

ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(7);                    // autobox int → Integer
int first = nums.get(0);        // unbox Integer → int
Integer big = Integer.MAX_VALUE;
int n = Integer.parseInt("42");   // 42
double d = Double.parseDouble("3.5");

int min = Integer.MAX_VALUE;     // start high for a min search
for (int x : nums)
{
    if (x < min) min = x;
}

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• Why wrappers exist: generics (ArrayList<T>) only work with reference types. int is a primitive. Integer is a class whose objects hold one int. So ArrayList<Integer> is the way to store ints.
• Autoboxing (int → Integer) happens automatically on add, on assignment to an Integer variable, on passing an int where an Integer is expected. Unboxing (Integer → int) happens on assignment to an int, in arithmetic, in comparisons with < >. You rarely write the conversion.
• The == trap: Integer a = 1000, b = 1000; a == b is false (different objects). For small values (−128 to 127) Java caches, so it's true — which makes the bug intermittent. Use .equals or unbox to int. The exam avoids relying on caching.
• Null unboxing: Integer x = null; int y = x; throws NullPointerException. An ArrayList slot that was never set is null; unboxing it crashes.
• Integer.MAX_VALUE / MIN_VALUE: the int range. Standard initial values for min/max searches — guaranteed to be replaced by any real value.
• Integer.parseInt("42") → 42. Double.parseDouble("3.5") → 3.5. Non-numeric input throws NumberFormatException. These are how file and keyboard text become numbers.
• Arithmetic on Integers auto-unboxes: Integer a = 5; a + 3 is 8 (an int). list.get(0) + list.get(1) adds two unboxed ints.

Mistakes that cost points

• ArrayList<int>. Compile error. Guaranteed question.
• == on Integer objects. Unreliable. Unbox or use equals.
• Unboxing null. NPE.
• Starting a min search at 0. Use Integer.MAX_VALUE or the first element.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.