Thread: Multiple scope problems with interfaces

Originally Posted by Norm

Is that referring to the line that is now commented out?

Lets forget about that. I was misusing it anyway

Originally Posted by Norm

What variable was in the full text of the error message? Where is that variable defined?

If you look at my Main class (of which the red around the Main keyword of the class signature is now gone, because I implemented the methods of the implemented interfaces), I have a private void printPlaylists method defined right before the main method. My book told me it's possible to use an interface object as an argument, but I'm having difficulty passing the arguments to the printPlaylists method call in the main method, despite both the printPlaylists and main methods being in the same class.

Also, I don't understand why that is possible, because from what I understand, you can't create instances (assuming objects and instances are the same thing) of interfaces, but rather can only define their methods in the classes that you choose to implement those interfaces in.


That creates a new problem, which shouldn't happen with Java 8 (unless I'm missing something). If you're not allowed to create instances of interfaces, how are you supposed to call non-static default methods from interfaces?

The errors are commented on lines 14, 18, 20, and 22:

package interfaces.ui;
import interfaces.business.*;
import java.util.Scanner;
 
public class Main implements DrivingMusic, Printable, SelfDrivable{
 
	private void printPlaylists(Printable p, DrivingMusic dm, int count) {
		for(int i = 0; i < count; i++) {
			p.print();
		}
	}	
 
	public static void main(String[] args) {
		printPlaylists(p, dm, count);//none of the arguments can be resolved to a variable
 
		//DrivingMusic.showMusicPlaylists();//call the static version of the method
 
		DrivingMusic x = new DrivingMusic();//Cannot instantiate the type DrivingMusic
		x.showMusicPlaylists();
		System.out.println(super.showMusicPlaylists);//Cannot use super in a static context
 
		select();//Cannot make a static reference to the non-static method select() from the type Main
 
		System.out.println("Lets also make sure we can both import and output our constants "
				+ "defined in the DrivingMusic interface directly from there:");
		System.out.println(DrivingMusic.PL1 + ", " + DrivingMusic.PL2 + ", " +
							DrivingMusic.PL3 + ", " + DrivingMusic.PL4);
	}
 
	@Override
	public double totalGasCost(int MPG, int numGasStops) {
		// TODO Auto-generated method stub
		return 0;
	}
 
	@Override
	public void drive(boolean arrived, int milesToGo) {
		// TODO Auto-generated method stub		
	}
 
	@Override
	public void wakeUp() {
		// TODO Auto-generated method stub		
	}
 
	@Override
	public void honkHorn() {
		// TODO Auto-generated method stub		
	}	
 
	@Override
	public void select() {
		System.out.println("Select a playlist: PL1, PL2, PL3, or PL4.");
		String playlist = "No Music Selected.";
		Scanner input = new Scanner(System.in);
		playlist = input.nextLine();
 
		if(playlist.equals("PL1")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL1);
		}
		else if(playlist.equals("PL2")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL2);
		}
		else if(playlist.equals("PL3")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL3);
		}
		else if(playlist.equals("PL4")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL4);
		}
		else {
			System.out.println("Invalid music playlist selected.");
		}		
	}
 
	@Override
	public void print() {
		// TODO Auto-generated method stub		
	}
}

And here are the interfaces I'm trying to use:

DrivingMusic.java

package interfaces.business;
 
import java.util.Scanner;
 
public interface DrivingMusic extends Printable{
 
	String PL1 = "JazzJackrabbitOST";
	String PL2 = "DonkeyKongCountryOST";
	String PL3 = "MegaDeth";
	String PL4 = "Offspring";
 
	String PL_list = ("Our playlists are: " + PL1 + ", " + PL2 + ", " + PL3 + ", " + PL4);
 
	default void showMusicPlaylists() {
		System.out.println(PL_list);
	}
	/*
	static void showMusicPlaylists() {
		System.out.println("Now printing our music playlists from the static version of the "
				+ "showMusicPlaylists method defined in the DrivingMusic interface:");
		System.out.println(PL_list);
	}
	*/
}

Printable.java

package interfaces.business;
 
public interface Printable {
 
	void print();
	void select();
}

SelfDrivable.java

package interfaces.business;
import java.util.Scanner;
 
public interface SelfDrivable {
 
	public static final int SUPERSONIC = 768;//This constant should be in a final class instead of here.
 
	default String setDestination(String destination) {
		System.out.println("Where do you want to go?");
		Scanner dest = new Scanner(System.in);
		destination = dest.nextLine();
		return destination;
	}
	public abstract double totalGasCost(int MPG, int numGasStops); 
	void drive(boolean arrived, int milesToGo);
	void wakeUp();
	void honkHorn();	
}

Originally Posted by Norm

Yes it is an interface not a class.

You're dead right. So why are default methods (non-static interface methods with a body) useful then? How do you implement them? Even if you can pass them around interfaces, what good does that do if you can't call them in your program's main method non-statically? In my updated code below, I have even tried overriding my methods outside of the main method but in still in the same Main class, and yet my program is still complaining about me trying to make a static reference to a non-static method when I try to call them in my code:

Main.java

package interfaces.ui;
import interfaces.business.*;
import java.util.Scanner;
 
public class Main implements DrivingMusic, Printable, SelfDrivable{
 
	private void printMultiple(Printable p, int count) {
		for(int i = 0; i < count; i++) {
			p.print();
		}
	}
 
	public static void main(String[] args) {
 
		Driver z = new Driver(5580,"Sam","Peterson");		
 
		printMultiple(z, 2);
 
		//DrivingMusic.showMusicPlaylists();//call the static version of the method
 
		showMusicPlaylists();
 
		/*[Code to call the non-static version of showMusicPlaylists here]*/
 
		select();
 
		System.out.println("Lets also make sure we can both import and output our constants "
				+ "defined in the DrivingMusic interface directly from there:");
		System.out.println(DrivingMusic.PL1 + ", " + DrivingMusic.PL2 + ", " +
							DrivingMusic.PL3 + ", " + DrivingMusic.PL4);
	}
 
	@Override
	public double totalGasCost(int MPG, int numGasStops) {
		// TODO Auto-generated method stub
		return 0;
	}
 
	@Override
	public void drive(boolean arrived, int milesToGo) {
		// TODO Auto-generated method stub		
	}
 
	@Override
	public void wakeUp() {
		// TODO Auto-generated method stub		
	}
 
	@Override
	public void honkHorn() {
		// TODO Auto-generated method stub		
	}	
 
	@Override
	public void select() {
		System.out.println("Select a playlist: PL1, PL2, PL3, or PL4.");
		String playlist = "No Music Selected.";
		Scanner input = new Scanner(System.in);
		playlist = input.nextLine();
 
		if(playlist.equals("PL1")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL1);
		}
		else if(playlist.equals("PL2")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL2);
		}
		else if(playlist.equals("PL3")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL3);
		}
		else if(playlist.equals("PL4")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL4);
		}
		else {
			System.out.println("Invalid music playlist selected.");
		}		
	}
 
	@Override
	public void showMusicPlaylists() {
		System.out.println(PL_list);
	}
 
	@Override
	public void print() {
		// TODO Auto-generated method stub		
	}
}

DrivingMusic.java

package interfaces.business;
 
import java.util.Scanner;
 
public interface DrivingMusic extends Printable{
 
	String PL1 = "JazzJackrabbitOST";
	String PL2 = "DonkeyKongCountryOST";
	String PL3 = "MegaDeth";
	String PL4 = "Offspring";
 
	String PL_list = ("Our playlists are: " + PL1 + ", " + PL2 + ", " + PL3 + ", " + PL4);
 
	default void showMusicPlaylists() {
		System.out.println(PL_list);
	}
	/*
	static void showMusicPlaylists() {
		System.out.println("Now printing our music playlists from the static version of the "
				+ "showMusicPlaylists method defined in the DrivingMusic interface:");
		System.out.println(PL_list);
	}
	*/
}

Printable.java

package interfaces.business;
 
public interface Printable {
 
	void print();
	void select();
}

SelfDrivable.java

package interfaces.business;
import java.util.Scanner;
 
public interface SelfDrivable {
 
	public static final int SUPERSONIC = 768;//This constant should be in a final class instead of here.
 
	default String setDestination(String destination) {
		System.out.println("Where do you want to go?");
		Scanner dest = new Scanner(System.in);
		destination = dest.nextLine();
		return destination;
	}
	public abstract double totalGasCost(int MPG, int numGasStops); 
	void drive(boolean arrived, int milesToGo);
	void wakeUp();
	void honkHorn();	
}

Driver.java

package interfaces.business;
public class Driver{
 
	private int DriverID;
	private String firstName;
	private String lastName;
 
	public Driver(int DriverID, String firstName, String lastName) {
		this.DriverID = DriverID;
		this.firstName = firstName;
		this.lastName = lastName;
	}
 
	public void setDriverID(int DriverID) {
		this.DriverID = DriverID;
	}
	public int getDriverID() {
		return DriverID;
	}
 
	public void setFirstName(String firstName) {
		this.firstName = firstName;
	}
	public String getFirstName() {
		return firstName;
	}
 
	public void setLastName(String lastName) {
		this.lastName = lastName;
	}
	public String getLastName() {
		return lastName;
	}			
}

Originally Posted by Norm

Please copy and post here the FULL text of the error messages, not just a short reference to it.

Line 17: "The method printMultiple(Printable, int) in the type Main is not applicable for the arguments (Driver, int)"

Line 21: "Cannot make a static reference to the non-static method showMusicPlaylists() from the type Main"

Line 25: "Cannot make a static reference to the non-static method select() from the type Main"

Thank you Norm. On to the next thing. Even with the tutorial link you gave me, I'm still having trouble with calling default methods. Here's my updated code:

Printable.java

package interfaces.business;
 
public interface Printable{
 
	void print();
	void printMusicPlaylists();
	void select();	
}

SelfDrivable.java

package interfaces.business;
import java.util.Scanner;
 
public interface SelfDrivable {
 
	public static final int SUPERSONIC = 768;//This constant should be in a final class instead of here.
 
	default String setDestination(String destination) {
		System.out.println("Where do you want to go?");
		Scanner dest = new Scanner(System.in);
		destination = dest.nextLine();
		return destination;
	}
	public abstract double totalGasCost(int MPG, int numGasStops); 
	void drive(boolean arrived, int milesToGo);
	void wakeUp();
	void honkHorn();	
}

DrivingMusic.java

package interfaces.business;
 
import java.time.ZonedDateTime;
import java.util.Scanner;
 
public interface DrivingMusic extends Printable{
 
	String PL1 = "JazzJackrabbitOST";
	String PL2 = "DonkeyKongCountryOST";
	String PL3 = "MegaDeth";
	String PL4 = "Offspring";
 
	String PL_list = ("Our playlists are: " + PL1 + ", " + PL2 + ", " + PL3 + ", " + PL4);
 
	default void showMusicPlaylists() {
		System.out.println("Calling the default showMusicPlaylists method defined in the DrivingMusic interface.");
		System.out.println(PL_list);
	}		
}

CarClock.java

package interfaces.business;
import java.time.*;
 
public interface CarClock {
 
	//ZonedDateTime getZonedDateTime(String zoneStr);//abstract method
 
	LocalDateTime getLocalDateTime();	
 
	static ZoneId getZoneId(String zoneStr) {
		try {
			return ZoneId.of(zoneStr);
		}catch(DateTimeException e) {
			System.err.println("Invalid time zone: " + zoneStr + "; using default time zone instead.");
			return ZoneId.systemDefault();
		}
	}
 
	default ZonedDateTime getZonedDateTime(String zoneStr) {
		return ZonedDateTime.of(getLocalDateTime(), getZoneId(zoneStr));
	}
}

Driver.java

package interfaces.business;
 
import java.util.Scanner;
import java.time.*;
 
public class Driver implements Printable, DrivingMusic, CarClock{
 
	private int DriverID;
	private String firstName;
	private String lastName;
 
	public Driver(int DriverID, String firstName, String lastName) {
		this.DriverID = DriverID;
		this.firstName = firstName;
		this.lastName = lastName;
	}
 
	public void setDriverID(int DriverID) {
		this.DriverID = DriverID;
	}
	public int getDriverID() {
		return DriverID;
	}
 
	public void setFirstName(String firstName) {
		this.firstName = firstName;
	}
	public String getFirstName() {
		return firstName;
	}
 
	public void setLastName(String lastName) {
		this.lastName = lastName;
	}
	public String getLastName() {
		return lastName;
	}
 
	@Override
	public void print() {
		System.out.println("calling the print method definition from the Driver class.");
		System.out.println();
	}
 
	@Override
	public void printMusicPlaylists() {
		System.out.println("calling the printMusicPlaylists method definition from the Driver class.");
		System.out.println(DrivingMusic.PL_list);
		System.out.println();
	}
 
	@Override
	public void select() {
		System.out.println("calling the select method definition from the Driver class.");
		System.out.println("Select a playlist: PL1, PL2, PL3, or PL4.");
		String playlist = "No Music Selected.";
		Scanner input = new Scanner(System.in);
		playlist = input.nextLine();
 
		if(playlist.equals("PL1")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL1);
		}
		else if(playlist.equals("PL2")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL2);
		}
		else if(playlist.equals("PL3")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL3);
		}
		else if(playlist.equals("PL4")) {
			System.out.println("You have chosen to listen to the " + DrivingMusic.PL4);
		}
		else {
			System.out.println("Invalid music playlist selected.");
		}
		System.out.println();
	}
 
	public void showMusicPlaylists();
 
	public ZonedDateTime getZonedDateTime(String zoneStr);
}

Main.java

package interfaces.ui;
import interfaces.business.*;
import java.util.Scanner;
import java.time.*;
//Remember, we don't need to implement the Printable interface in this class,
//because the DrivingMusic interface is already extending the Printable interface.
public class Main implements DrivingMusic, SelfDrivable, CarClock{
 
	private static void printMultiple(Printable p, int count) {
		for(int i = 0; i < count; i++) {
			System.out.println("From the for loop in the printMultiple method we are now");				
			p.print();			
		}
	}
 
	public static void main(String[] args) {
 
		Driver z = new Driver(5580,"Sam","Peterson");		
 
		printMultiple(z, 3);
 
		//DrivingMusic.showMusicPlaylists();//call the static version of the method
 
		//You need a reference to an instance of the class to call non-static methods:
		z.print();
		z.printMusicPlaylists();
		z.select();
		z.showMusicPlaylists();
		z.getZonedDateTime(zoneStr);
 
		System.out.println("Again, our playlists are:");
		System.out.println(DrivingMusic.PL1 + ", " + DrivingMusic.PL2 + ", " +
							DrivingMusic.PL3 + ", " + DrivingMusic.PL4);
	}
 
	@Override
	public double totalGasCost(int MPG, int numGasStops) {
		// TODO Auto-generated method stub
		return 0;
	}
 
	@Override
	public void drive(boolean arrived, int milesToGo) {
		// TODO Auto-generated method stub		
	}
 
	@Override
	public void wakeUp() {
		// TODO Auto-generated method stub		
	}
 
	@Override
	public void honkHorn() {
		// TODO Auto-generated method stub		
	}
 
	@Override
	public void showMusicPlaylists() {
		System.out.println("Calling the showMusicPlaylists method definition from the Main class.");
		System.out.println(PL_list);
	}
 
	@Override
	public void print() {
		// TODO Auto-generated method stub
		System.out.println("Calling the print method definition from the Main class.");
	}
 
	@Override
	public void select() {
		// TODO Auto-generated method stub		
	}	
 
	@Override
	public void printMusicPlaylists() {
		// TODO Auto-generated method stub
	}		
}

Lines 78 and 80 in Driver.java both have the same error, "This method requires a body instead of a semicolon".

Line 29 in Main.java has an error that says, "zoneStr cannot be resolved to a variable".

You can't instantiate methods directly from an interface, they have to be defined in a class and called from there. I get that. But why does it keep telling me they need a body? They're default methods. DEFAULT! That means they already have a body in their interfaces. So what is going on here?