package uk.ac.ebi.quickgo.rest.cache;

import com.google.common.base.Preconditions;
import java.time.DayOfWeek;
import java.time.LocalTime;
import java.util.Objects;
import javax.validation.constraints.NotNull;

/**
 * Data structure holds day of week and time. Used to declare events that have a daily occurrence at a set time.
 *
 * @author Tony Wardell
 * Date: 11/04/2017
 * Time: 16:15
 * Created with IntelliJ IDEA.
 */
class DayTime {

    @NotNull DayOfWeek dayOfWeek;
    @NotNull LocalTime time;

    /**
     * Create an instance of DayTime from {@link DayOfWeek} and {@link LocalTime} instance.
     * @param dayOfWeek
     * @param time
     */
    public DayTime(DayOfWeek dayOfWeek, LocalTime time) {
        Preconditions.checkArgument(Objects.nonNull(dayOfWeek), "Invalid Daytime DayOfWeek parameter passed to " +
                "constructor. Parameter is null, it should be a valid DayOfWeek instance.");
        Preconditions.checkArgument(Objects.nonNull(time), "Invalid DayTime time parameter passed to constructor. " +
                "Parameter was null, it should be a valid LocalTime instance.");
        this.dayOfWeek = dayOfWeek;
        this.time = time;
    }
}

package uk.ac.ebi.quickgo.rest.cache;

import com.google.common.base.Preconditions;
import java.time.DayOfWeek;
import java.time.LocalTime;
import java.util.Objects;
import javax.validation.constraints.NotNull;

/**
 * Data structure holds day of week and time. Used to declare events that have a daily occurrence at a set time.
 *
 * @author Tony Wardell
 * Date: 11/04/2017
 * Time: 16:15
 * Created with IntelliJ IDEA.
 */
class DayTime {

    @NotNull DayOfWeek dayOfWeek;
    @NotNull LocalTime time;

    /**
     * Create an instance of DayTime from {@link DayOfWeek} and {@link LocalTime} instance.
     * @param dayOfWeek this class represents.
     * @param time this class represents.
     */
    DayTime(DayOfWeek dayOfWeek, LocalTime time) {
        Preconditions.checkArgument(Objects.nonNull(dayOfWeek), "Invalid Daytime DayOfWeek parameter passed to " +
                "constructor. Parameter is null, it should be a valid DayOfWeek instance.");
        Preconditions.checkArgument(Objects.nonNull(time), "Invalid DayTime time parameter passed to constructor. " +
                "Parameter was null, it should be a valid LocalTime instance.");
        this.dayOfWeek = dayOfWeek;
        this.time = time;
    }
}

Add to javadoc, make constructor package private.