Timo-UPN

Main.java

public class Main {

    private char[] digits = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'};
    private char[] operators = {'+', '*'};
    private char[] basement = {'$', '_', '_', '_', '_', '_', '_', '_'};
    private int basementPointer = 0;
    private char basementBottom = '$';
    private char basementPlaceholder = '_';
    private char[] word = {'3', '4', '+', '6', '2', '+', '*'};

    public static void main(String[] args) throws Exception {
        new Main();
    }

    public Main() throws Exception {

        // transition from state-0 to state-1
        char nexCharacter = popFromBasement();
        if (isDigit(read()) && nexCharacter == this.basementBottom) {
            char[] result = {nexCharacter, '$'};
            putToBasement(result);
        } else {
            throw new Exception("this word is not acceptable");
        }


    }

    private boolean isDigit(char character) {
        return charsContain(character, this.digits);
    }

    private boolean isOperator(char character) {
        return charsContain(character, this.operators);
    }

    private boolean charsContain(char character, char[] characterList) {
        for (char digit : characterList) {
            if (character == digit) {
                return true;
            }
        }
        return false;
    }

    private char popFromBasement() {
        char value = this.basement[this.basementPointer];
        this.basement[this.basementPointer] = '_';
        this.basementPointer--;
        return value;
    }

    private void putToBasement(char[] word) {
        for (char character : word) {
            putToBasement(character);
        }
    }

    private void putToBasement(char character) {
        this.basementPointer++;
        this.basement[this.basementPointer] = character;
    }

    /**
     * Read the first character of the word and remove it.
     *
     * @return the next character from the left
     */
    private char read() {
        char character = this.word[0];

        char[] newWord = new char[this.word.length - 1];
        System.arraycopy(this.word, 1, newWord, 0, newWord.length);
        this.word = newWord;

        return character;
    }

}