StateMachile.java

public class Solution {
   
    public boolean isNumber(String s) {
        
        if (s == null) return false;
        s = s.trim();
        if (s.length() == 0) return false;
        s = s+ ' ';
        
        State state = State.INIT;
        try {
            for (int i = 0; i < s.length(); i++) {
                char c = s.charAt(i);
                if (i == 0) state = State.INIT.next(c);
                else state = state.next(c);
            }
        } catch (RuntimeException e) {
            return false;
        }
        return true;
    }
    
 public enum State {

        INIT {
            @Override
            public State next(char c) {
                if (c == '-' || c == '+') return SIGN;
                if (c == '.') return START_DOT;
                int num = c - '0';
                if (0 <= num && num <= 9 ) return BASE_NUM;
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        },
        START_DOT {
            @Override
            public State next(char c) {
                int num = c - '0';
                if (0 <= num && num <= 9 ) return BASE_NUM_AFTER_DOT;
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        },
        SIGN {
            @Override
            public State next(char c) {
                int num = c - '0';
                if (c == '.') return START_DOT;
                if (0 <= num && num <= 9 ) return BASE_NUM;
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        },
        BASE_NUM {
            @Override
            public State next(char c) {
                if (c == ' ') return END;
                if (c == '.') return DOT;
                if (c == 'e') return E;
                int num = c - '0';
                if (0 <= num && num <= 9 ) return BASE_NUM;
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        },
        DOT {
            @Override
            public State next(char c) {
                if (c == ' ') return END;
                if (c == 'e') return E;
                int num = c - '0';
                if (0 <= num && num <= 9 ) return BASE_NUM_AFTER_DOT;
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        },
        BASE_NUM_AFTER_DOT {
            @Override
            public State next(char c) {
                if (c == ' ') return END;
                if (c == 'e') return E;
                int num = c - '0';
                if (0 <= num && num <= 9 ) return BASE_NUM_AFTER_DOT;
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        },
        E {
            @Override
            public State next(char c) {
                if (c == '-' || c == '+') return EXP_SIGN;
                int num = c - '0';
                if (0 <= num && num <= 9 ) return EXP_NUM;
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        },
        EXP_SIGN {
            @Override
            public State next(char c) {
                int num = c - '0';
                if (0 <= num && num <= 9 ) return EXP_NUM;
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        },
        EXP_NUM {
            @Override
            public State next(char c) {
                if (c == ' ') return END;
                int num = c - '0';
                if (0 <= num && num <= 9 ) return EXP_NUM;
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        },
        END {
            @Override
            public State next(char c) {
                throw new RuntimeException("Validation error. Wrong char '" + c + "'.");
            }
        };

        public abstract State next(char c);
    }
}