Variables in interfaces and extension

Implicit variable declaration:

• Variables declared in an interface are automatically public, static and final.
• Useful for creating shared constants in large programs.

Code example:

// Interface que contém constantes
interface IConst {
    int MIN = 0;
    int MAX = 10;
    String ERRORMSG = "Boundary Error";
}

class IConstD implements IConst {
    public static void main(String[] args) {
        int nums[] = new int[MAX];
        for (int i = MIN; i < 11; i++) {
            if (i >= MAX)
                System.out.println(ERRORMSG);
            else {
                nums[i] = i;
                System.out.print(nums[i] + " ");
            }
        }
    }
}

Note: Although useful for constants, this technique can be controversial.

Interfaces can be extended

Inheritance in interfaces:

• Interfaces can inherit other interfaces with the extends keyword.
• A class that implements a derived interface must implement all the methods of the entire interface chain.

Code example:

// Interface A
interface A {
    void meth1();
    void meth2();
}

// Interface B estende A
interface B extends A {
    void meth3();
}

// Classe que implementa A e B
class MyClass implements B {
    public void meth1() {
        System.out.println("Implement meth1().");
    }

    public void meth2() {
        System.out.println("Implement meth2().");
    }

    public void meth3() {
        System.out.println("Implement meth3().");
    }
}

class IFExtend {
    public static void main(String[] args) {
        MyClass ob = new MyClass();
        ob.meth1();
        ob.meth2();
        ob.meth3();
    }
}

Important: If you remove the implementation of meth1(), a compilation error will occur as all interface methods must be implemented.

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