java 64 lines · 10 steps

A safe Money value type in Java

An immutable Money class uses BigDecimal and currency-aware rounding to handle amounts without losing pennies.

Explained by highlit

Intermediate immutability bigdecimal value-object rounding penny-allocation

1import java.math.BigDecimal;
2import java.math.RoundingMode;
3import java.util.Currency;
4 
5public final class Money {
6 
  private final BigDecimal amount;
  private final Currency currency;
9 
  private Money(BigDecimal amount, Currency currency) {
      this.currency = currency;
      this.amount = amount.setScale(currency.getDefaultFractionDigits(), RoundingMode.HALF_EVEN);
  }
14 
  public static Money of(String amount, String currencyCode) {
      return new Money(new BigDecimal(amount), Currency.getInstance(currencyCode));
  }
18 
  public Money plus(Money other) {
      requireSameCurrency(other);
      return new Money(amount.add(other.amount), currency);
  }
23 
  public Money multipliedBy(BigDecimal factor) {
      return new Money(amount.multiply(factor), currency);
  }
27 
  public Money applyTaxRate(BigDecimal ratePercent) {
      BigDecimal tax = amount.multiply(ratePercent).divide(new BigDecimal("100"), 10, RoundingMode.HALF_UP);
      return new Money(amount.add(tax), currency);
  }
32 
  public Money[] allocate(int parts) {
      BigDecimal share = amount.divideToIntegralValue(new BigDecimal(parts))
              .setScale(currency.getDefaultFractionDigits(), RoundingMode.DOWN);
      Money[] result = new Money[parts];
      Money remainder = new Money(amount, currency);
      for (int i = 0; i < parts; i++) {
          result[i] = new Money(share, currency);
          remainder = remainder.minus(result[i]);
      }
      BigDecimal penny = BigDecimal.ONE.movePointLeft(currency.getDefaultFractionDigits());
      for (int i = 0; remainder.amount.signum() > 0; i++) {
          result[i] = new Money(result[i].amount.add(penny), currency);
          remainder = new Money(remainder.amount.subtract(penny), currency);
      }
      return result;
  }
49 
  private Money minus(Money other) {
      return new Money(amount.subtract(other.amount), currency);
  }
53 
  private void requireSameCurrency(Money other) {
      if (!currency.equals(other.currency)) {
          throw new IllegalArgumentException("Currency mismatch: " + currency + " vs " + other.currency);
      }
  }
59 
  @Override
  public String toString() {
      return currency.getSymbol() + amount.toPlainString();
  }
64}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Model money with BigDecimal and explicit rounding, never floating-point, to avoid silent precision errors.
2Immutable value objects that return new instances make arithmetic predictable and thread-safe.
3Splitting money requires redistributing leftover pennies so the parts sum exactly back to the whole.

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