“实例变量”和“接口中声明的变量”之间的区别

Question

I was reading about Interfaces in a Book and I came upon this Line that confused me.

Interfaces are syntactically similar to classes, but they lack instance variables.

As far as I know about Interfaces , we can define variables inside an Interface which are by default final .

My question is, What does that Line mean? and What is the Difference between an Instance Variable and the Variable defined in the Interface ??

Answer 1

My question is, What does that Line mean?

Amongst other things, it means the book's terminology is off. By "instance variable," they mean "instance field."

An instance field is a field that is specific to an individual instance of a class. For example:

class Foo {
    // Instance field:
    private int bar;

    // Static field:
    public static final int staticBar;
}

The field bar is per-instance, not class-wide. The field staticBar is class-wide (a static field, sometimes called a "class field").

Interfaces don't have instance fields. They do have static fields. When you do this:

interface FooInterface {
    int staticBar;
}

staticBar is automatically declared public , static , and final (per JLS §9.3 ). So that staticBar is roughly equivalent to the one on our Foo class before.

Answer 2

This means you cant have instance variable but a constant static final variable within an interface as per JLS . For eg

interface MyIface {
    public static final int MY_CONSTANT = 1;
}

And access it using interface name like:

int variable = MyIface.MY_CONSTANT;