The final field cannot be assigned, for an interface

Question

I have a class Product and an interface LargeProduct. Product implements LargeProduct.

LargeProduct has a variable height which has getter and setter methods which have to be implemented in the Product class.

The height variable in LargeProduct is defined like so:

public int height = null;

The getter method works fine:

public int getHeight() {
    return height;
}

But the setter method does not:

public void setHeight(int height) {
    this.height = height;
}

Errors:

The final field LargeProduct.height cannot be assigned
The static field LargeProduct.height should be accessed in a static way

I'm not sure which error it's actually giving.. I'm using Eclipse and when I hover it gives the first error, and at the bottom of the window it gives the second error.

Answer 1

By default fields (member variable) in interface are public static final

and you don't have setter for final

Answer 2

Interfaces can only include constants, not general-purpose variables. Interfaces should only contain constants if they're genuinely relevant to the rest of the interface, and they should use SHOUTY_CASE as per any other constant. It sounds like LargeProduct shouldn't have a constant called height - but instead, your implementation should declare the height field.

Note that interfaces are meant to be APIs, showing the abilities of types. Fields shouldn't be part of the API - they're an implementation detail. After all, who's to say that you'll write getHeight and setHeight based on a simple variable? Those methods could query a database, or delegate to an instance of some other type. The consumer of the interface shouldn't know or care about that.

For further information about fields in interfaces, I suggest you read section 9.3 of the Java Language Specification:

Every field declaration in the body of an interface is implicitly public, static, and final. It is permitted to redundantly specify any or all of these modifiers for such fields.