Button pls,min,pro,del;
        EditText arg1,arg2;
        TextView result;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            pls = (Button)findViewById(R.id.btnPls);
            min = (Button)findViewById(R.id.btnMin);
            pro = (Button)findViewById(R.id.btnUmn);
            del = (Button)findViewById(R.id.btnDel);

            pls.setOnClickListener(this);
            min.setOnClickListener(this);
            pro.setOnClickListener(this);
            del.setOnClickListener(this);

            arg1 = (EditText)findViewById(R.id.etNum1);
            arg2 = (EditText)findViewById(R.id.etNum2);

            result = (TextView)findViewById(R.id.tvResult);



}

Button pls,min,pro,del;
            EditText arg1,arg2;
            TextView result;