updated the skeleton for Code Review Activity

src/main/java/StringManipulation.java

@@ -1,33 +1,134 @@
+
+import java.util.Arrays;
+
 public class StringManipulation implements StringManipulationInterface {
 
+    private String stringToManipulate;
+
     @Override
     public String getString() {
-        return null;
+        return stringToManipulate;
     }
 
     @Override
     public void setString(String string) {
+        stringToManipulate = string;
     }
 
     @Override
     public int count() {
-        return 0;
+
+        // if the current stringToManipulate is null, throws a NullPointerException
+        checkForNullString();
+
+        // If the stringToManipulate is an empty string, returns 0
+        if(stringToManipulate.length()==0){
+            return 0;
+        }
+
+        // split the string by whitespace(s), period, comma, and other symbols
+        // and stores each word into a String array
+        String[] words = stringToManipulate.split("[\\s.(),:?!]+");
+
+        // returns the length of array as the word count
+        return words.length;
     }
 
     @Override
     public String removeNthCharacter(int n, boolean maintainSpacing) {
-        return null;
+        // if the current stringToManipulate is null, throws a NullPointerException
+        checkForNullString();
+
+        // If the given n is smaller or equal to 0, throws IllegalArgumentException
+        if(n <= 0){
+            throw new IllegalArgumentException("n must be greater than zero.");
+        }
+        // If the given n is larger than the length of the string, throws IndexOutOfBoundsException
+        if(n > stringToManipulate.length()){
+            throw new IndexOutOfBoundsException("n must be less than or equal to the length of the string.");
+        }
+
+        StringBuilder str = new StringBuilder();
+        int length = stringToManipulate.length();
+
+        // Traverse the entire stringToManipulate
+        for(int i = 1; i <= length ;i++){
+            // As long as the index of stringToManipulate is NOT divisible by n, append the
+            // corresponding character to the str
+            if(i % n != 0) {
+                str.append(stringToManipulate.charAt(i - 1));
+            }
+            // if the index is divisible by n AND maintainSpacing is true, append a space
+            else if(maintainSpacing){
+                str.append(' ');
+            }
+        }
+
+        return str.toString();
     }
 
     @Override
-    public String[] getSubStrings(int startWord, int endWord) {
-        return null;
+    public String[] getSubStrings(int startWord, int endWord){
+
+        // if the current stringToManipulate is null, throws a NullPointerException
+        checkForNullString();
+
+        // If the startWord or the endWord is zero or negative, OR the endWord is smaller than the startWord,
+        // throws an exception
+        if(startWord <= 0 || endWord <= 0 || startWord > endWord)
+        {
+            throw new IllegalArgumentException("Invalid argument(s) were entered.");
+        }
+
+        // Gets the word count by using split()
+        String[] words = stringToManipulate.split("[\\s.,_:?!]+");
+
+        // If the endWord is larger than the length of string, throws an exception
+        if(endWord > words.length)
+        {
+            throw new IndexOutOfBoundsException("The endWord entered is out of bounds");
+        }
+
+        // copies the words in the specified range into a new array called substring
+        String[] substring = Arrays.copyOfRange(words, startWord-1, endWord);
+
+        return substring;
     }
 
     @Override
-    public String restoreString(int[] indices) {
-        return null;
+    public String restoreString(int[] indices){
+
+        // if the current stringToManipulate is null, throws a NullPointerException
+        checkForNullString();
+
+        // If the number of indices are not the same as the length of the string,
+        // throws an exception
+        if(indices.length != stringToManipulate.length()){
+            throw new IllegalArgumentException("The number of indices must match the length of the string.");
+        }
+
+        StringBuilder newString = new StringBuilder();
+
+        // Traverse the indices array to the end
+        for(int i = 0; i < indices.length; i++){
+            // if the index is negative, or larger than equal to the length of the string,
+            // throws an exception
+            if(indices[i] < 0 || indices[i]>= stringToManipulate.length()){
+                throw new IndexOutOfBoundsException("Each index must be between 0 and the length of the string.");
+            }
+            // otherwise append the character at the index to the new string
+            newString.append(stringToManipulate.charAt(indices[i]));
+        }
+        return newString.toString();
     }
 
+    /* Helper Functions */
 
+    // Helper function to check if the string is null. If so, throw an NullPointerException
+    private void checkForNullString(){
+        if(stringToManipulate == null) {
+            throw new NullPointerException("The string is null.");
+        }
+    }
 }
+