Here is your code, we have modified it.

import java.io.*;
interface arith {
    int a=5; 
    int b=8; 
    void add(); 
    void mul();
    void sub();
    void div(); 
    } 
    class exampleinterface implements arith{
        public void add() {
            int c=a+b;
            System.out.println("method add: "+c);
            }
                public void mul() {
                    int d=a*b;
                    System.out.println("method mul: "+d); 
                    } 
                        public void sub() {
                            int e=a-b; 
                            System.out.println("method sub: "+e);
                            }
                                public void div() {
                                    int f=a/b;
                                    System.out.println("method div: "+f);
                                    }

                                        public static void main(String []arg) {
                                            exampleinterface s=new exampleinterface();

                                            s.add();
                                            s.mul();
                                            s.sub();
                                            s.div();
                                            s.div();
                                            }
                                            }