NUMBERS-68: Added parse() method to Complex(). TODO: Add testing.

commons-numbers-complex/src/main/java/org/apache/commons/numbers/complex/Complex.java

@@ -20,6 +20,7 @@
 import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.List;
+import java.lang.NumberFormatException;
 import org.apache.commons.numbers.core.Precision;
 /**
  * Representation of a Complex number, i.e. a number which has both a
@@ -131,6 +132,40 @@ public static Complex ofCis(double x) {
         return new Complex(Math.cos(x), Math.sin(x));
     }
 
+    /**
+     * Parses a text expression in a String object to produce
+     * a Cartesian complex number. Acceptable formats are:
+     * <p><ul> * <li>Single number (parsed as real). Example: "3.14"
+     * <li>Single number with appended "i" (parsed as imaginary): "1.42i"
+     * <li>Pair of numbers, separated by plus sign and with appended i: "3 + 4i"
+     * </ul>
+     * @throws{ParseException, NumberFormatException}
+     */
+    public static Complex parse(String s) {
+        final String[] terms = s.split("+");
+        if (terms.length == 2) {
+            if (terms[1].indexOf("i") == terms[1].length()) {
+                String imagTerm = (String) terms[1].subSequence(0, terms[1].length()-1);
+                final double real = Double.parseDouble(terms[0]);
+                final double imaginary = Double.parseDouble(imagTerm);
+                return Complex.ofCartesian(real, imaginary);
+            } else {
+                throw new NumberFormatException("Expression must end with unique \"i\"");
+            }
+        } else if (terms.length == 1){
+            if (terms[0].indexOf("i") == terms[0].length()) {
+                String imagTerm = (String) terms[0].subSequence(0, terms[0].length()-1);
+                final double imaginary = Double.parseDouble(imagTerm);
+                return Complex.ofCartesian(0, imaginary);
+            } else {
+                final double real = Double.parseDouble(terms[0]);
+                return Complex.ofReal(real);
+            }
+        } else {
+            throw new NumberFormatException("Invalid expression for Complex. See documentation for further information.");
+        }
+    }
+
     /**
      * Returns true if either real or imaginary component of the Complex
      * is NaN