My doubt is in line 4 how can a TaxCalculator class be declared as field calculator

So, every variable (or field) must have a certain type. it could be int, String or whatever and you declare it as follows;

String myString;
int number;

In the same way calculator is declared as a variable of type TaxCalculator and you declare it as

TaxCalculator calculator;

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Thanks Paul but if we declare datatype as a String we can only give " sentences" for int only numbers. what we can only declare as TaxCalculator

You can do all the things with calculator that the TaxCalculator class allows you to do. So if it has eg a calculateTax method you can call it using calculator.calculateTax().

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