evaluating expressions containing functions in java

I have written a project that calculates different functions like sine,MCM etc without using existing packages like math in java now I want to get an expression from the user in form of a string and then print out the result
like :

import java.util.Scanner;

public class Phase2main {

public static void main(String[] args) {

    Scanner s = new Scanner(System.in);
    String expression   ;
    double result = 0 ;

    System.out.println(" Enter your desired expression from the available functions ");

    expression = s.nextLine();


    System.out.println("Result is : " + result);                
 }
}

then it should run like this: Enter an Expression: ADD(DIV(SIN(FACT(3)),CEIL(TAN(MUL(1.5,FIB(4))))),GCD(2,10)) The Result is: 1.94

how can I make the program to identify my functions like CEIL and their input ? I've checked many of the similar questions but the ones that I found are rather libraries that are too complex for me to understand or do basic arithmetic without identifying functions and their inputs

so how can I write a simple evaluator for this specific problem?

Solution

May be use JavaScript interpreter?

First create engine instance and init:

// Manager creates engines by mime/language names.
// It has own global scope for engiges created by it.
ScriptEngineManager manager = new ScriptEngineManager();
// Create JavaScript interpreter instance.
// (Nashorn is bundled JavaScript interpreter)
ScriptEngine scope = manager.getEngineByName("JavaScript");
// Define functions you need
String initialScript = "cos = Math.cos;" // ; or \n
    + "sin = Math.sin;"
    + "tg  = Math.tan;"
    + "PI  = Math.PI;"
// Any other function
    + "ctg = function (x) { return cos(x)/sin(x); };";
// ...

try {
    // Add these functions to scope
    scope.eval(initialScript);
} catch(ScriptException ex) {
    // Evaluating exceptions, syntax errors are thrown here
}

And then you can evaluate expressions in the "scope" many times:

try {
    double d = (double)scope.eval("sin(PI/2) + cos(PI/2)");
    System.out.println("Calculated: " + d);
} catch(ScriptException e) {
    // ...
}

Be warned:

There is language interpreting - user can pass any script and...
... it can reduce perfomance of application.

You can also use, for example, Jython or JRuby as interpreter.