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Multiple Inheritance in Java

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Multiple Inheritance in Java

Learn more about multiple inheritance and the use of default interface methods in multiple inheritance.

Feb. 21, 22 · Java Zone · Code Snippet

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Ever since Java 8 introduced default interface methods, I felt there had to be a way to use it for multiple inheritance. I have never needed it, but I was bored for a bit today, so decided to try the following idea:

  • Create a non-public class XData to hold the fields the interface needs to work with, and a public interface X in the same X.java source file
  • The interface has one virtual method getXData() that returns the XData instance
  • The remaining interface methods are default methods that call getXData() to read and write the fields as necessary to perform some useful operations.
  • Create another interface Y and class YDatain the same pattern
  • Create a class XY that implements both interfaces X and Y
  • The class XY has XData and YData fields, which are returned by getXData() and getYData(). These are the only two interface methods XY is required to implement.
  • I didn't bother in my example, but XY would have to decide how to implement the Object methods hashCode, equals, and toString. These methods cannot be implemented by interfaces (but they could be implemented in XData and YData classes if desired)

The end result is the XY class is an instance of both X and Y interfaces, and inherits the encapsulated behaviors of both X and Y default methods - multiple inheritance by any reasonable measure.

It is better than C++ multiple inheritance, in that all constructor logic and declared fields are in the XY class. The only problem you could get is if X and Y interfaces declare the same method that differs only by return type - but this is just a general problem of implementing multiple interfaces.

Here are some code examples using Dogand Cow interfaces.
Dog.java:

package mi;

import static java.time.temporal.ChronoField.DAY_OF_MONTH;
import static java.time.temporal.ChronoField.HOUR_OF_DAY;
import static java.time.temporal.ChronoField.MINUTE_OF_DAY;
import static java.time.temporal.ChronoField.MONTH_OF_YEAR;
import static java.time.temporal.ChronoField.NANO_OF_SECOND;
import static java.time.temporal.ChronoField.SECOND_OF_MINUTE;
import static java.time.temporal.ChronoField.YEAR;

import java.time.Instant;
import java.time.OffsetDateTime;
import java.time.ZoneOffset;

/**
 * The fields of data that Foo needs to operate on
 */
class DogData {
	final String breed;
	final String name;
	final OffsetDateTime birthDate;

	DogData(String breed, String name, OffsetDateTime birthDate) {
		this.breed = breed;
		this.name = name;
		this.birthDate = birthDate;
	}
}

public interface Dog {
	/**
	 * The method the class has to implement to return a DogData instance, from one its own fields.
	 * 
	 * @return dog data
	 */
	DogData getDogData();
	
	/**
	 * Return the next time the dog should go to the vet for a checkup,
	 * which is annually, one week after the dog's birthday.
	 * 
	 * @return date of next checkup.
	 */
	default OffsetDateTime nextCheckup() {
		final OffsetDateTime now = OffsetDateTime.now();
		
		final OffsetDateTime today = now.
			minusHours(now.get(HOUR_OF_DAY)).
			minusMinutes(now.get(MINUTE_OF_DAY)).
			minusSeconds(now.get(SECOND_OF_MINUTE)).
			minusSeconds(now.get(NANO_OF_SECOND));
		
		final OffsetDateTime bday = getDogData().birthDate;
		final int thisYear = now.get(YEAR);
		final int bdayMonth = bday.get(MONTH_OF_YEAR);
		final int bdayDay = bday.get(DAY_OF_MONTH);
		
		OffsetDateTime nextCheckup = OffsetDateTime.
			of(thisYear, bdayMonth, bdayDay, 12, 0, 0, 0, ZoneOffset.UTC).
			plusDays(7);

		if (nextCheckup.isBefore(today)) {
			nextCheckup = nextCheckup.plusYears(1);
		}
		
		return nextCheckup;
	}
	
	default String getBreed() {
		return getDogData().breed;
	}
	
	default String getName() {
		return getDogData().name;
	}
	
	default OffsetDateTime getBirthDate() {
		return getDogData().birthDate;
	}
}

Cow.java:

package mi;

import static java.time.temporal.ChronoField.DAY_OF_MONTH;
import static java.time.temporal.ChronoField.HOUR_OF_DAY;
import static java.time.temporal.ChronoField.MINUTE_OF_HOUR;
import static java.time.temporal.ChronoField.MONTH_OF_YEAR;
import static java.time.temporal.ChronoField.NANO_OF_SECOND;
import static java.time.temporal.ChronoField.SECOND_OF_MINUTE;
import static java.time.temporal.ChronoField.YEAR;

import java.time.Instant;
import java.time.OffsetDateTime;
import java.time.ZoneOffset;

class CowData {
	String breed;
	OffsetDateTime birthDate;

	CowData(String breed, OffsetDateTime birthDate) {
		this.breed = breed;
		this.birthDate = birthDate;
	}
}

public interface Cow {
	/**
	 * The method the class has to implement to return a CowData instance, from one its own fields.
	 * 
	 * @return cow data
	 */
	CowData getCowData();
	
	/**
	 * Return the next time the cow should go to the vet for a shot,
	 * which is annually, one week before the cow's birthday.
	 * 
	 * @return date of next checkup.
	 */
	default OffsetDateTime nextCheckup() {
		final OffsetDateTime now = OffsetDateTime.now();
		
		final OffsetDateTime today = now.
			minusHours(now.get(HOUR_OF_DAY)).
			minusMinutes(now.get(MINUTE_OF_HOUR)).
			minusSeconds(now.get(SECOND_OF_MINUTE)).
			minusNanos(now.get(NANO_OF_SECOND));
		
		final OffsetDateTime bday = getCowData().birthDate;
		final int thisYear = now.get(YEAR);
		final int bdayMonth = bday.get(MONTH_OF_YEAR);
		final int bdayDay = bday.get(DAY_OF_MONTH);
		
		OffsetDateTime nextCheckup = OffsetDateTime.
			of(thisYear, bdayMonth, bdayDay, 12, 0, 0, 0, ZoneOffset.UTC).
			minusDays(7);
		
		if (nextCheckup.isBefore(today)) {
			nextCheckup = nextCheckup.plusYears(1);
		}
		
		return nextCheckup;
	}
	
	default String getBreed() {
		return getCowData().breed;
	}
	
	default OffsetDateTime getBirthDate() {
		return getCowData().birthDate;
	}
}

DogCow.java:

package mi;

import static java.time.temporal.ChronoField.YEAR;

import java.time.OffsetDateTime;
import java.time.ZoneOffset;

public class DogCow implements Dog, Cow {
	private final OffsetDateTime dogCowBdate = OffsetDateTime.of(
		OffsetDateTime.now().get(YEAR) - (int)(Math.random() * 5),
		(int)(Math.random() * 12) + 1,
		(int)(Math.random() * 28) + 1,
		(int)(Math.random() * 24),
		(int)(Math.random() * 60),
		0,
		0,
		ZoneOffset.UTC
	);
	
	private final DogData dogData = new DogData(
		"Border Collie",
		"Nate",
		dogCowBdate
	);
	
	private final CowData cowData = new CowData(
		"Angus",
		dogCowBdate
	);
	
	@Override
	public DogData getDogData() {
		return dogData;
	}
	
	@Override
	public CowData getCowData() {
		return cowData;
	}
	
	@Override
	public OffsetDateTime nextCheckup() {
		// Check both Dog and Cow, choose whichever one comes first
		final OffsetDateTime nextDogCheckup = Dog.super.nextCheckup();
		final OffsetDateTime nextCowCheckup = Cow.super.nextCheckup();
		
		return nextDogCheckup.isBefore(nextCowCheckup) ? nextDogCheckup : nextCowCheckup;
	}
	
	@Override
	public String getBreed() {
		return Dog.super.getBreed() + " " + Cow.super.getBreed();
	}
	
	@Override
	public OffsetDateTime getBirthDate() {
		return Dog.super.getBirthDate();
	}
}

Main.java:

package mi;

public class Main {
	public static void main(String[] args) {
		for (int i = 1; i < 10; i++) {
			final DogCow dc = new DogCow();
			
			System.out.println(
				"DogCow " + i +
				" is combined breed " +
				dc.getBreed() +
				" named " +
				dc.getName() +
				" born on " +
				dc.getBirthDate() +
				" with next checkup of " +
				dc.nextCheckup()
			);
		}
	}
}

Example Main.java output:

DogCow 1 is combined breed Border Collie Angus named Nate born on 2022-11-22T21:40Z with next checkup of 2022-11-15T12:00Z
DogCow 2 is combined breed Border Collie Angus named Nate born on 2018-09-22T01:53Z with next checkup of 2022-09-15T12:00Z
DogCow 3 is combined breed Border Collie Angus named Nate born on 2022-01-26T18:08Z with next checkup of 2022-02-02T12:00Z
DogCow 4 is combined breed Border Collie Angus named Nate born on 2022-02-23T00:32Z with next checkup of 2022-03-02T12:00Z
DogCow 5 is combined breed Border Collie Angus named Nate born on 2019-03-27T04:24Z with next checkup of 2022-03-20T12:00Z
DogCow 6 is combined breed Border Collie Angus named Nate born on 2022-02-04T19:11Z with next checkup of 2022-02-11T12:00Z
DogCow 7 is combined breed Border Collie Angus named Nate born on 2020-02-20T17:13Z with next checkup of 2022-02-27T12:00Z
DogCow 8 is combined breed Border Collie Angus named Nate born on 2018-09-02T18:02Z with next checkup of 2022-08-26T12:00Z
DogCow 9 is combined breed Border Collie Angus named Nate born on 2018-01-07T21:02Z with next checkup of 2022-01-14T12:00Z

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