Code Smell ve Refactoring — Davranışı Değiştirmeden Değiştirmek

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// Pins what the fee code does today, before anyone tidies it.
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// Expected values were copied from the current output, not from a spec.
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class EftFeeCharacterizationTest {
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    private final EftFeeCalculator calculator = new EftFeeCalculator();
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    @ParameterizedTest
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    @CsvSource({
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        // segment, years, amount,   fee
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        "STANDARD,  1,     10000.00, 25.00",
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        "STANDARD,  1,     500.00,   2.50",    // the minimum fee applies
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        "PREMIUM,   3,     10000.00, 15.00",   // exactly 3 years is not loyal yet
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        "PREMIUM,   5,     12345.00, 12.35",   // 12.345 is rounded up today
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    })
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    void feeStaysExactlyAsToday(Segment segment, int years, BigDecimal amount, BigDecimal fee) {
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        Account account = new Account("TR330006100519786457841326", segment, years);
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        assertThat(calculator.feeFor(account, amount)).isEqualTo(fee);
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    }
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}

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public class Account {
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    private static final BigDecimal STANDARD_RATE = new BigDecimal("0.0025");
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    private static final BigDecimal PREMIUM_RATE = new BigDecimal("0.0015");
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    private static final BigDecimal LOYAL_PREMIUM_RATE = new BigDecimal("0.0010");
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    private final String iban;
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    private final Segment segment;
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    private final int yearsAsCustomer;
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    public Account(String iban, Segment segment, int yearsAsCustomer) {
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        this.iban = iban;
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        this.segment = segment;
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        this.yearsAsCustomer = yearsAsCustomer;
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    }
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    // Moved here from EftFeeCalculator: the rule only reads this account's own data.
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    public BigDecimal eftFeeRate() {
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        if (segment == Segment.PREMIUM && yearsAsCustomer > 3) {
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            return LOYAL_PREMIUM_RATE;
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        }
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        if (segment == Segment.PREMIUM) {
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            return PREMIUM_RATE;
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        }
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        return STANDARD_RATE;
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    }
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}

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// Domestic EFT fees, charged in TRY.
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public class EftFeeCalculator {
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    private static final BigDecimal MIN_FEE = new BigDecimal("2.50");
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    public BigDecimal feeFor(Account account, BigDecimal amount) {
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        return amount.multiply(account.eftFeeRate())
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                .setScale(2, RoundingMode.HALF_UP)   // today's rounding, kept on purpose
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                .max(MIN_FEE);
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    }
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}

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--- a/src/main/java/com/bank/fee/EftFeeCalculator.java
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+++ b/src/main/java/com/bank/fee/EftFeeCalculator.java
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@@ -7,5 +7,5 @@ public class EftFeeCalculator {
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     public BigDecimal feeFor(Account account, BigDecimal amount) {
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-        return amount.multiply(rateFor(account))
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-                .setScale(2, RoundingMode.HALF_UP)
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+        return amount.multiply(account.eftFeeRate())
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+                .setScale(2, RoundingMode.HALF_EVEN)
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                 .max(MIN_FEE);
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     }