This java code is an implementation of eval() method to solve complex expressions.

EvalPlus.java

package evaluation;

import java.math.BigInteger;
import java.util.EmptyStackException;
import java.util.Scanner;
import java.util.Stack;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class EvalPlus {
	private static Scanner scanner = new Scanner(System.in);

	public static void main(String[] args) {
		System.out.println("This Evaluation is based on BODMAS rule\n");
		evaluate();
	}

	private static void evaluate() {
		StringBuilder finalStr = new StringBuilder();
		System.out.println("Enter an expression to evaluate:");

		/*Example expressions:
		(4*5(3-1)(67+23(25!)(5*6.5%2.2)))
		3+(4*5/3(2)(4)+45-1)
		(89*5/3(2(4(3(2))))(4)+57-1) */

		String expr = scanner.nextLine(); 
		if(isProperExpression(expr)) {
			expr = replaceBefore(expr);
			char[] temp = expr.toCharArray();
			String operators = "(+-*/%)";
			for(int i = 0; i < temp.length; i++) {
				if((i == 0 && temp[i] != '*') || (i == temp.length-1 && temp[i] != '*' && temp[i] != '!')) {
					finalStr.append(temp[i]);
				} else if((i > 0 && i < temp.length -1) || (i==temp.length-1 && temp[i] == '!')) {
					if(temp[i] == '!') {
						StringBuilder str = new StringBuilder();
						for(int k = i-1; k >= 0; k--) {
							if(Character.isDigit(temp[k])) {
								str.insert(0, temp[k] );
							} else {
								break;
							}
						}
						Long prev = Long.valueOf(str.toString());
						BigInteger val = new BigInteger("1");
						for(Long j = prev; j > 1; j--) {
							val = val.multiply(BigInteger.valueOf(j));
						}
						finalStr.setLength(finalStr.length() - str.length());
						finalStr.append("(" + val + ")");
						if(temp.length > i+1) {
							char next = temp[i+1];
							if(operators.indexOf(next) == -1) { 
								finalStr.append("*");
							}
						}
					} else {
						finalStr.append(temp[i]);
					}
				}
			}
			expr = finalStr.toString();
			if(expr != null && !expr.isEmpty()) {
				ScriptEngineManager mgr = new ScriptEngineManager();
				ScriptEngine engine = mgr.getEngineByName("JavaScript");
				try {
					System.out.println("Result: " + engine.eval(expr));
					evaluate();
				} catch (ScriptException e) {
					System.out.println(e.getMessage());
				}
			} else {
				System.out.println("Please give an expression");
				evaluate();
			}
		} else {
			System.out.println("Not a valid expression");
			evaluate();
		}
	}

	private static String replaceBefore(String expr) {
		expr = expr.replace("(", "*(");
		expr = expr.replace("+*", "+").replace("-*", "-").replace("**", "*").replace("/*", "/").replace("%*", "%");
		return expr;
	}

	private static boolean isProperExpression(String expr) {
		expr = expr.replaceAll("[^()]", "");
		char[] arr = expr.toCharArray();
		Stack<Character> stack = new Stack<Character>();
		int i =0;
		while(i < arr.length) {
			try {
				if(arr[i] == '(') {
					stack.push(arr[i]);
				} else {
					stack.pop();
				}
			} catch (EmptyStackException e) {
				stack.push(arr[i]);
			}
			i++;
		}
		return stack.isEmpty();
	}
}