ParseInteger.java

package com.thealgorithms.maths;

public final class ParseInteger {
 private ParseInteger() {
 }

 private static void checkInput(final String s) {
 if (s == null) {
 throw new NumberFormatException("Input parameter must not be null!");
 }
 if (s.isEmpty()) {
 throw new NumberFormatException("Input parameter must not be empty!");
 }
 }

 private static void checkDigitAt(final String s, final int pos) {
 if (!Character.isDigit(s.charAt(pos))) {
 throw new NumberFormatException("Input parameter of incorrect format: " + s);
 }
 }

 private static int digitToInt(final char digit) {
 return digit - '0';
 }

 /**
 * Parse a string to integer
 *
 * @param s the string
 * @return the integer value represented by the argument in decimal.
 * @throws NumberFormatException if the {@code string} does not contain a
 * parsable integer.
 */
 public static int parseInt(final String s) {
 checkInput(s);

 final boolean isNegative = s.charAt(0) == '-';
 final boolean isPositive = s.charAt(0) == '+';
 int number = 0;
 for (int i = isNegative || isPositive ? 1 : 0, length = s.length(); i < length; ++i) {
 checkDigitAt(s, i);
 number = number * 10 + digitToInt(s.charAt(i));
 }
 return isNegative ? -number : number;
 }
}