DateBuilder.java

package com.ontometrics.util;

import java.util.Calendar;
import java.util.Date;

/**
 * Provides a fluent builder interface for constructing Dates and DateTimes.
 */
public class DateBuilder {

  private Calendar calendar = Calendar.getInstance();

	/**
	 * Provides means of starting from a given date, then changing some subset
	 * of the fields
	 * 
	 * @param date
	 *            the starting point date
	 * @return this, for chaining
	 */
	public DateBuilder start(Date date) {
		calendar.setTime(date);
		return this;
	}

	/**
	 * So to build the first of the year, you would set this to 1.
	 * 
	 * @param dayOfMonth
	 *            a number between 1 and 31 (depending on the calendar month of
	 *            course)
	 * @return this, for chaining
	 */
	public DateBuilder day(int dayOfMonth) {
		calendar.set(Calendar.DAY_OF_MONTH, dayOfMonth);
		return this;
	}

	/**
	 * Provide the month of the date you are building, a number between 1 and
	 * 12.
	 * 
	 * @param month
	 *            the value desired in the resulting date, e.g. 5 for May
	 * @return this, for chaining
	 */
	public DateBuilder month(int month) {
		calendar.set(Calendar.MONTH, month);
		return this;
	}

	/**
	 * Provide the year of the date you are building, e.g. 1980.
	 * 
	 * @param year
	 *            the value desired in the resulting date
	 * @return this, for chaining
	 */
	public DateBuilder year(int year) {
		calendar.set(Calendar.YEAR, year);
		return this;
	}

	/**
	 * Provide the hour, in this case of the day. Note the JDK calls this hour
	 * of the day.
	 * 
	 * @param hour
	 *            the value desired in the resulting date, e.g. 14 for 2 pm
	 * @return this, for chaining
	 */
	public DateBuilder hour(int hour) {
		calendar.set(Calendar.HOUR_OF_DAY, hour);
		return this;
	}

	/**
	 * Provide the minutes within the hour of the datetime being constructed
	 * 
	 * @param minutes
	 *            a number between 0 and 59, e.g. if building 12:49, this would
	 *            be 49.
	 * @return this, for chaining
	 */
	public DateBuilder minutes(int minutes) {
		calendar.set(Calendar.MINUTE, minutes);
		return this;
	}

	/**
	 * Provides access to the final product.
	 * 
	 * @return the constructed date with all the desired values
	 */
	public Date build() {
		return calendar.getTime();
	}

}

DateBuilderTest.java

package com.ontometrics.util;

import static org.hamcrest.Matchers.is;
import static org.junit.Assert.assertThat;

import java.util.Calendar;
import java.util.Date;

import org.junit.Test;

public class DateBuilderTest {

  @Test
	public void testCanBuildSimpleDate() {
		Date newDate = new DateBuilder().day(1).month(1).year(2013).build();

		Calendar calendar = Calendar.getInstance();
		calendar.setTime(newDate);
		
		assertThat(calendar.get(Calendar.DAY_OF_MONTH), is(1));
		
	}
	
	@Test
	public void testCanBuildDateTime(){
		Date newDate = new DateBuilder().day(1).month(1).year(2013).hour(12).minutes(49).build();

		Calendar calendar = Calendar.getInstance();
		calendar.setTime(newDate);
		
		assertThat(calendar.get(Calendar.HOUR_OF_DAY), is(12));
		assertThat(calendar.get(Calendar.MINUTE), is(49));
		
	}
	
	@Test
	public void testCanBuildFromToday(){
		Date today = new Date();
		Calendar calendar = Calendar.getInstance();
		calendar.setTime(today);
		int todayYear = calendar.get(Calendar.YEAR);
		int todayMonth = calendar.get(Calendar.MONTH);
		int todayDay = calendar.get(Calendar.DAY_OF_MONTH);
		
		
		Date newDate = new DateBuilder().start(today).hour(12).minutes(49).build();
		calendar.setTime(newDate);
		
		assertThat(calendar.get(Calendar.YEAR), is(todayYear));
		assertThat(calendar.get(Calendar.MONTH), is(todayMonth));
		assertThat(calendar.get(Calendar.DAY_OF_MONTH), is(todayDay));
		
		
	}

}