java - toString() method and recursion -

i creating class implements generic set using binary search tree. implementing tostring() method displays parenthetic expression, having trouble getting work. example, here basic binary tree:

i want tostring method output binary tree such:

(10 (5 (4 () ())   (6 () ()))   (20 (15 () ())   (25 () ()))) 

where () signifies empty tree.

here class tostring method. appreciated!

// allow short name access following classes import csc143.data_structures.*;  public class myset<e> implements simpleset<e> {    // root of "tree" structures set   private btnode root;   // current number of elements in set   private int numelems;    public myset() {     root = null;      numelems = 0;   }    /**    * add element set.     *     * @param e element added set.    * @return  <tt>true</tt> if operation updated contents of set.    */   public boolean add(e e) {     try {       root = addtosubtree(root, (comparable) e);       return true;     } catch(duplicateadded exc) {       // duplicate trying added       return false;     }    }    // helper method adds element "e" tree rooted @ r. returns   // (possibly new) tree containing "e", or throws duplicateadded exception   // if "e" exists in tree.   private btnode addtosubtree(btnode r, comparable elem)     throws duplicateadded {     if(r == null) {       numelems++;       return new btnode(elem);     }      int compare = elem.compareto(r.item);     // element in tree     if(compare == 0) {       throw new duplicateadded();     } else if(compare < 0) {       r.left = addtosubtree(r.left, elem);     } else {  // compare > 0       r.right = addtosubtree(r.right, elem);     }      // element has been added     return r;   }    /**    * remove elements set.    */   public void clear() {     root = null;      numelems = 0;   }    /**    * checks existance of specified value within set.    *     * @param e value sought.    * @return  <tt>true</tt> if value exists in set.    */   public boolean contains(e e) {     return subtreecontains(root, (comparable) e);   }    // helper method returns whether element "elem" in   // (sub-)tree root "r".   private boolean subtreecontains(btnode r, comparable elem) {     if(r == null) {       return false;     } else {       int compare = elem.compareto(r.item);       // found element       if(compare == 0){         return true;       } else if(compare < 0) {         return subtreecontains(r.left, elem);       } else {  // compare > 0         return subtreecontains(r.right, elem);       }      }    }    /**    * check existance of elements in set.    *     * @return  <tt>true</tt> if there no elements in set.    */   public boolean isempty() {     return root == null;   }    /**    * return number of elements in set.    *     * @return number of elements in set.    */   public int size() {     return numelems;   }    /**    * returns string representation of contents of set.    *     * @return  string representation of set.    */   public string tostring() {     return subtreetostring(root);   }    private string subtreetostring(btnode r) {     string str = "";     if(r == null) {       str += "() ";       return str;     }       str += "(" + r.item;     str += subtreetostring(r.left) +        subtreetostring(r.right) + ")";      return str;   }    // inner class creates node compose binary tree structure   class btnode<e> {      /**      * item stored in node.      */     public e item;      /**      * node left of "this" node.      */     public btnode left;      /**      * node right of "this" node.      */     public btnode right;      /**      * constructs btnode object (three parameters).      *       * @param item item stored in node.      * @param left node left of "this" node.      * @param right node right of "this" node.      */     @suppresswarnings("unchecked")     public btnode(object item, btnode left, btnode right) {       // bind references       this.item = (e) item;       this.left = left;       this.right = right;     }      /**      * constructs btnode (one parameter).      *       * @param item stored in node.      */     public btnode(object item) {       // call 3 parameter constructor       this(item, null, null);     }    }  } 

i ran few issues when tried running code, assuming issues resolved - following worked me:

private string subtreetostring(btnode r) {     string str = "";     if(r == null) {         return str;     }     str += r.item;     str += " (" + subtreetostring(r.left) + ") (" + subtreetostring(r.right) + ")";     return str; } 

output:

10 (5 (4 () ()) (6 () ())) (20 () (25 () ())) 

(there's mistake in example output provided above - closing bracket should added after children of 6)