code is ready for review

src/main/java/StringManipulation.java

@@ -1,32 +1,92 @@
+import java.util.Arrays;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import java.util.regex.MatchResult;
+/**
+ * junit Assignment
+ * kunga n ngochetsang
+ * date 6/12/2023
+ * */
 public class StringManipulation implements StringManipulationInterface {
-
+    private String stringData ="";
     @Override
     public String getString() {
-        return null;
+        return stringData;
     }
 
     @Override
     public void setString(String string) {
+        stringData = string;
     }
 
     @Override
     public int count() {
-        return 0;
+        if(stringData == null) {
+            throw new NullPointerException("String is null.");
+        }
+        int wordCount = 0;
+        String[] stringArr;
+
+        stringArr = stringData.trim().split("[^a-zA-Z0-9_'@&-]+");
+        wordCount = stringArr.length;
+
+        if(wordCount==1&&stringArr[0].isEmpty()){
+            wordCount=0;
+        }
+        return wordCount;
     }
 
     @Override
     public String removeNthCharacter(int n, boolean maintainSpacing) {
-        return null;
+        if(stringData==null){
+            throw new NullPointerException("String is null.");
+        }
+        if (n < 1) {
+            throw new IllegalArgumentException("n must be greater than 0");
+        }
+        if (n > stringData.length()) {
+            throw new IllegalArgumentException("n must be less than the length of the string");
+        }
+        StringBuilder builtString = new StringBuilder(stringData);
+        for (int i = n - 1; i < stringData.length(); i += n) {
+            builtString.setCharAt(i, '|');
+        }
+        if(maintainSpacing){
+            return builtString.toString().replace("|"," ");
+        }else
+            return builtString.toString().replace("|","");
     }
 
     @Override
-    public String[] getSubStrings(int startWord, int endWord) {
-        return null;
+    public String[] getSubStrings(int startWord, int endWord){
+        if (startWord < 1||endWord<1||startWord>endWord) {
+            throw new IllegalArgumentException("start word or end word is invalid");
+        }
+        if (endWord > count()) {
+            throw new IndexOutOfBoundsException("string has less than endWord words in it");
+        }
+        String[] stringArr;
+        stringArr = stringData.trim().split("[^a-zA-Z0-9_'@&]+");
+        String[] stringArrSubString = Arrays.copyOfRange(stringArr,startWord-1,endWord);
+        return stringArrSubString;
     }
 
     @Override
-    public String restoreString(int[] indices) {
-        return null;
+    public String restoreString(int[] indices){
+        if (stringData ==null){
+            return null;
+        }
+        if (stringData.length() != indices.length) {
+            throw new IllegalArgumentException("not s.length == indices.length == n");
+        }
+       StringBuilder builtString = new StringBuilder();
+        for (int i = 0;i<indices.length;i++){
+            if (indices[i]< 0  ||  indices[i]> stringData.length()) {
+                throw new IndexOutOfBoundsException("indices[i]< 0  or  indices[i]> string length");
+            }
+            builtString.append(stringData.charAt(indices[i]));
+        }
+        return builtString.toString();
     }