Interface help

[Archer]

Hi there... I'm teaching myself Java, and I'm trying to understand interfaces.

Here's a bit of code I'm playing with, and I'm wondering what I'm doing wrong

public class StudentDemo
{
	public void main(String[] args)
	{
		Student stu = new Student();
		stu.setName("wallace");
		stu.setYearOfCommencement(1101);
	}
	
	public class Student implements StudentSpecification
	{
		String stuName = null;
		int stuYear = 0;
		public void setName(String name);
		{
			stuName = name;
		}

		public void setYearOfCommencement(int year);	
		{
			stuYear = year;
		}

		public String getName()
		{
			return stuName;
		}

		public int getYearOfCommencement();	
		{
			return stuYear;
		}
	}

interface StudentSpecification
	{	
		// Change the name of the student
		void setName(String name);

		// Change the year of commencement
		void setYearOfCommencement(int year); 

		// Get the name
		String getName();

		int getYearOfCommencement();
	}
}

Can anyone point me in the right direction?

I'm finding alot of information online, but the examples the use never pass arguments like mine does.

[sunde887]

What problems are you running into? Try creating classes and interfaces as separate things, not inside the tester class.

[Junky]

How about prividing relevant information about what is wrong. Do you get errors? then post the EXACT error messages and indicate on which line they occur. Doe the code run but not produce correct output? Then show the output you get and what you expect instead. Don't make us guess.

[Junky]

Just spotted some problems. In the Student class compare the getName method to all your other methods. Notice anything amiss?

[Archer]

Terribly sorry... When I compile, I see this:

StudentDemo.java:14: missing method body, or declare abstract
		public void setName(String name);
		            ^
StudentDemo.java:16: cannot find symbol
symbol  : variable name
location: class StudentDemo.Student
			stuName = name;
			          ^
StudentDemo.java:19: missing method body, or declare abstract
		public void setYearOfCommencement(int year);	
		            ^
StudentDemo.java:21: cannot find symbol
symbol  : variable year
location: class StudentDemo.Student
			stuYear = year;
			          ^
StudentDemo.java:29: missing method body, or declare abstract
		public int getYearOfCommencement();	
		           ^
StudentDemo.java:31: return outside method
			return stuYear;
			^
6 errors

And I dont see what difference having the interface class does in this case?

[Junky]

See my reply above

[Archer]

Oh god... That was a classic monday moment!

[Archer]

Ok... I need to have my main method static, but in doing so I run into:

non-static variable this cannot be referenced from a static context

At line 5

[Junky]

Post your latest code.

[Archer]

public class StudentDemo
{
	public static void main(String[] args)
	{
		Student stu = new Student();
		stu.setName("wallace");
		stu.setYearOfCommencement(1101);
	}
	
	public class Student implements StudentSpecification
	{
		String stuName = null;
		int stuYear = 0;
		public void setName(String name)
		{
			stuName = name;
		}

		public void setYearOfCommencement(int year)	
		{
			stuYear = year;
		}

		public String getName()
		{
			return stuName;
		}

		public int getYearOfCommencement()	
		{
			return stuYear;
		}
	}

interface StudentSpecification
	{	
		// Change the name of the student
		void setName(String name);

		// Change the year of commencement
		void setYearOfCommencement(int year); 

		// Get the name
		String getName();

		int getYearOfCommencement();
	}
}

Returns this:

javac *.java
StudentDemo.java:5: non-static variable this cannot be referenced from a static context
		Student stu = new Student();
		              ^
1 error

[Junky]

It is a rather odd error but it is caused by having Student and StudentSpecification in the same file as StudentDemo. You can either make Student static (not recommended) or place Student and StudentSpecification in their own files.

[Archer]

Thanks so much for your help!

[Tolls]

It is a rather odd error but it is caused by having Student and StudentSpecification in the same file as StudentDemo. You can either make Student static (not recommended) or place Student and StudentSpecification in their own files.

It's actually caused by having Student inside StudentDemo, not by them simply being in the same file. Consequently Student can only be defined with respect to an instance of StudentDemo, unless Student was to be defined as static.

But the best idea is, as you say, to make them their own files.

[Junky]

It's actually caused by having Student inside StudentDemo, not by them simply being in the same file.

DOH!

Of course. I did wonder why OP wasn't getting the "only one top level class can be public" error.