Java variables and primitive types

Working model

Use integer types for counts. For this receipt, money is stored as an integer number of paise; the sample excludes currencies whose smallest unit differs. Floating-point values are unsuitable for exact decimal accounting.

Code

long unitPricePaise = 129900L;
int quantity = 3;
long invoiceTotalPaise = Math.multiplyExact(unitPricePaise, quantity);
boolean eligibleForDispatch = quantity > 0;
System.out.println(invoiceTotalPaise);
System.out.println(eligibleForDispatch);

The output is 389700, followed by true. Math.multiplyExact reports overflow rather than silently wrapping. A long has a wider range than an int, but neither grows without a limit.

Cost and design choices

Primitive assignment copies a value. Reference assignment copies a reference; two variables can then identify the same mutable object. That distinction matters when a method changes a list it receives.

Range and initialization boundaries

int arithmetic can wrap without throwing. Widening the destination variable after an overflowing int calculation does not recover the lost value. Convert an operand before the calculation or use checked arithmetic when overflow should reject the operation.

Fields and array elements receive default values; local variables need a definite assignment before use. A default zero may also be a valid business value, so explicitly track absence instead of assuming an unassigned field will be obvious.

Connected lessons

Continue with Java operators and numeric promotion, Java Tutorial.

Common Mistakes

Local variables require initialization before use. Integer overflow is not automatically an exception for ordinary arithmetic. For financial systems, validate units, currency, and rounding rules explicitly.

Choose the representation for the allowed values

A quantity that can never be fractional should not become double merely because a parser accepted a decimal token. Integral ranges, overflow behavior and nullability belong to the field contract. A primitive has no null sentinel; an Integer may be absent and can fail during unboxing.

Local variables need a definitely assigned value before reading. Fields have initialization defaults, but a default zero is not evidence that a business value was supplied. Model missing input explicitly at the parsing boundary rather than allowing field defaults to stand in for validation.

Assignments to reference variables copy references. They do not duplicate a list, an array or its nested members. Declare which component owns mutation and test changes through a second alias before describing a value as a snapshot. final prevents reassignment of that variable; it does not freeze a referenced array.