parseInteger

Maths/ParseInteger.java

@@ -0,0 +1,33 @@
+package Maths;
+
+public class ParseInteger {
+    public static void main(String[] args) {
+        assert parseInt("123") == Integer.parseInt("123");
+        assert parseInt("-123") == Integer.parseInt("-123");
+        assert parseInt("0123") == Integer.parseInt("0123");
+        assert parseInt("+123") == Integer.parseInt("+123");
+    }
+
+    /**
+     * 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(String s) {
+        if (s == null) {
+            throw new NumberFormatException("null");
+        }
+        boolean isNegative = s.charAt(0) == '-';
+        boolean isPositive = s.charAt(0) == '+';
+        int number = 0;
+        for (int i = isNegative ? 1 : isPositive ? 1 : 0, length = s.length(); i < length; ++i) {
+            if (!Character.isDigit(s.charAt(i))) {
+                throw new NumberFormatException("s=" + s);
+            }
+            number = number * 10 + s.charAt(i) - '0';
+        }
+        return isNegative ? -number : number;
+    }
+}