Java Docs says interfaces cannot have fields.Why?

interface, java

Solution

How could this be possible because Java Docs also says all fields in interface are `public`, `static` and `final`.

From Java Language Specification. Chapter 9. Interfaces. 9.3. Field (Constant) Declarations (emphasis mine):

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.

If two or more (distinct) field modifiers appear in a field declaration, it is customary, though not required, that they appear in the order consistent with that shown above in the production for `ConstantModifier`.

This means that every field in the interface will be considered as a `public` constant field. If you add `public`, `static` and `final` or some of these modifiers or none of them, the compiler will add them for you. That's the meaning of implicitly.

So, having this:

interface Foo {
    String bar = "Hello World";
}

Is similar to have

interface Foo {
    public String bar = "Hello World";
}

That is similar to

interface Foo {
    static bar = "Hello World";
}

The three interfaces will compile to the same bytecode.

Also, make sure that your fields declaration in an interface follow the subnote. This means, you cannot narrow the visibility of a field in an interface. This is illegal:

interface Foo {
    //compiler error!
    private String bar = "Hello World";
}

Because `bar` won't be `public` and it defies the standar definition for fields in an interface.

Problem

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