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Clock.fixed creates a Clock whose time is, fixed at what you give it. In the first test, then, I create a clock that's fixed at two days after the item's due date:

Clock.fixed creates a Clock whose time is fixed at what you give it. In the first test, then, I create a clock that's fixed at two days after the item's due date:

The simpler way to go about this is using dependency injection: instead of having the class you want to test know about the system clock, pass in the clock so that you can substitute it with one that stays fixed.

The simpler way to go about this is using dependency injection: instead of having the class you want to test know about the system clock, pass in the clock so that you can vary the clock you use for testing and for production.

The simpler way to go about this is using dependency injection: instead of having the class you want to test know about the system clock, pass in the clock so that you can substitute it with one that can stay fixed.

import java.time.Clock;
import java.time.LocalDate;
import java.time.ZoneOffset;
import java.time.temporal.ChronoUnit;
public class FineCalculator {
 private Clock clock;
 public FineCalculator(Clock clock) {
 this.clock = clock;
 }
 public long fineFor(LoanedItem loanedItem) {
 LocalDate today = clock.instant().atZone(ZoneOffset.UTC).toLocalDate();
 long daysOverdue = ChronoUnit.DAYS.between(loanedItem.dueOn(), today);
 if (daysOverdue < 0) {
 daysOverdue = 0;
 }
 long fine = 1 * daysOverdue;
 return fine;
 }
}

import java.time.Clock;
import java.time.LocalDate;
import java.time.ZoneOffset;
public class Main {
 public static void main(String[] args) {
 LocalDate dueOn = LocalDate.of(2024, 9, 1);
 LoanedItem item = new LoanedItem("Screwdriver", dueOn);
 Clock clock = Clock.system(ZoneOffset.UTC);
 FineCalculator fineCalculator = new FineCalculator(clock);
 System.out.println("Loaned item: " + item);
 System.out.println("Fine is: " + fineCalculator.fineFor(item));
 }
}

The simpler way to go about this is using dependency injection: instead of having the class you want to test know about the system clock, pass in the clock so that you can substitute it with one that stays fixed.

import java.time.Clock;
import java.time.LocalDate;
import java.time.ZoneOffset;
import java.time.temporal.ChronoUnit;
public class FineCalculator {
 private Clock clock;
 public FineCalculator(Clock clock) {
 this.clock = clock;
 }
 public long fineFor(LoanedItem loanedItem) {
 LocalDate today = clock.instant().atZone(ZoneOffset.UTC).toLocalDate();
 long daysOverdue = ChronoUnit.DAYS.between(loanedItem.dueOn(), today);
 if (daysOverdue < 0) {
 daysOverdue = 0;
 }
 long fine = 1 * daysOverdue;
 return fine;
 }
}

import java.time.Clock;
import java.time.LocalDate;
import java.time.ZoneOffset;
public class Main {
 public static void main(String[] args) {
 LoanedItem item = new LoanedItem("Screwdriver", LocalDate.of(2024, 9, 1));
 Clock clock = Clock.system(ZoneOffset.UTC);
 FineCalculator fineCalculator = new FineCalculator(clock);
 System.out.println("Loaned item: " + item);
 System.out.println("Fine is: " + fineCalculator.fineFor(item));
 }
}