Parsing integers: 90% done

Author: rampatra

src/main/java/com/rampatra/strings/IntegerToString.java

@@ -0,0 +1,51 @@
+package com.rampatra.strings;
+
+/**
+ * @author rampatra
+ * @since 2019-04-01
+ */
+public class IntegerToString {
+
+    private static final int[] sizeTable = {9, 99, 999, 9999, 99999, 999999, 9999999, 99999999,
+            999999999, Integer.MAX_VALUE};
+
+    private static String getStringFromInteger(int num) {
+        boolean isNegative = num < 0;
+        num = isNegative ? -num : num;
+        int size = getStringSize(num);
+        size = isNegative ? size + 1 : size;
+        char[] chars = new char[size];
+
+        int rem;
+        for (int i = size - 1; isNegative ? i > 0 : i >= 0; i--) {
+            rem = num % 10;
+            num = num / 10;
+            chars[i] = (char) (rem + '0');
+        }
+
+        if (isNegative) {
+            chars[0] = '-';
+        }
+
+        return new String(chars);
+    }
+
+    private static int getStringSize(int num) {
+        if (num == Integer.MAX_VALUE) return 10;
+
+        for (int i = 0; ; i++) {
+            if (num < sizeTable[i]) {
+                return i + 1;
+            }
+        }
+    }
+
+    public static void main(String[] args) {
+        System.out.println(getStringFromInteger(0));
+        System.out.println(getStringFromInteger(123));
+        System.out.println(getStringFromInteger(+123));
+        System.out.println(getStringFromInteger(-123));
+        System.out.println(getStringFromInteger(Integer.MAX_VALUE));
+        System.out.println(getStringFromInteger(Integer.MIN_VALUE)); // not working
+    }
+}

src/main/java/com/rampatra/strings/StringToInteger.java

@@ -0,0 +1,64 @@
+package com.rampatra.strings;
+
+/**
+ * @author rampatra
+ * @since 2019-04-01
+ */
+public class StringToInteger {
+
+    /**
+     * This method converts a {@code String} to an {@code int}. It assumes the {@code string} contains ASCII
+     * characters only.
+     *
+     * @param str the input string, for example, 0, 123, +123, -123, etc.
+     * @return the equivalent integer.
+     */
+    private static int getIntegerFromString(String str) {
+        int number = 0;
+        int digit;
+        char ch;
+        int weight = 0;
+        boolean isNegative = false;
+
+        // remove all leading and trailing whitespaces
+        str = str.trim();
+        if (str.length() == 0) {
+            throw new NumberFormatException("Empty string");
+        }
+
+        for (int i = str.length() - 1; i >= 0; i--) {
+            ch = str.charAt(i);
+            if (ch == '-' && i == 0) {
+                isNegative = true;
+                continue;
+            } else if (ch == '+' && i == 0) {
+                continue;
+            }
+
+            digit = ch - '0';
+
+            if (digit < 0 || digit > 9) {
+                throw new NumberFormatException("Invalid characters");
+            }
+
+            number += digit * (Math.pow(10, weight++));
+        }
+        return isNegative ? -number : number;
+    }
+
+    public static void main(String[] args) {
+        // normal cases
+        System.out.println(getIntegerFromString("0"));
+        System.out.println(getIntegerFromString("123"));
+        System.out.println(getIntegerFromString("0123"));
+        System.out.println(getIntegerFromString("+123"));
+        System.out.println(getIntegerFromString("-123"));
+
+        // error cases
+        System.out.println(getIntegerFromString("1-23"));
+        System.out.println(getIntegerFromString(""));
+        System.out.println(getIntegerFromString(" "));
+        System.out.println(getIntegerFromString("  "));
+        System.out.println(getIntegerFromString("123L"));
+    }
+}