Why Wrapper Classes Exist

A generic collection like ArrayList<Integer> can only hold objects, and a primitive int is not an object. Integer is the object form of int that makes it possible to put whole numbers in a list at all.

ArrayList<Integer> scores = new ArrayList<Integer>();
scores.add(95);              // autoboxing: int -> Integer
int first = scores.get(0);   // unboxing: Integer -> int

The compiler inserts the boxing and unboxing automatically. Nothing about the syntax looks different from working with a plain int, which is exactly the point: it should feel invisible until it is not.

Parsing Strings into Numbers

Text typed by a user, or read from a file, always arrives as a String, even when it looks like a number. Converting it into something you can do arithmetic on is a separate step.

String text = "42";
int value = Integer.parseInt(text);

String decimal = "3.14";
double d = Double.parseDouble(decimal);

Comparing Wrapped Values

compareTo compares the VALUES two wrapper objects hold. == on two objects compares whether they are the same object in memory, which is a different question entirely.

Integer a = Integer.valueOf(200);
Integer b = Integer.valueOf(200);
System.out.println(a.compareTo(b));  // 0, meaning equal

The Range Constants

int arithmetic that exceeds Integer.MAX_VALUE wraps around silently rather than throwing, which is why a check has to happen before the overflow, not after.

long doubled = (long) sum * 2;
System.out.println(doubled > Integer.MAX_VALUE);

Integer a = Integer.valueOf(500);
Integer b = Integer.valueOf(500);
System.out.println(a == b);

int sum = 2000000000;
System.out.println((long) sum * 2 > Integer.MAX_VALUE);