Design SparseVector

Related: Design Sparse Matrix
http://www.1point3acres.com/bbs/thread-137068-1-1.html
设计一个SparseVector class，包含set(long idx, int val), get(long idx), dotProduct(SparseVector otherVec)三个方法，

public class SparseVector {
    private int d;                   // dimension
    private ST<Integer, Double> st;  // the vector, represented by index-value pairs

    public SparseVector(int d) {
        this.d  = d;
        this.st = new ST<Integer, Double>();
    }
    public void put(int i, double value) {
        if (i < 0 || i >= d) throw new IndexOutOfBoundsException("Illegal index");
        if (value == 0.0) st.delete(i);
        else              st.put(i, value);
    }
    public double get(int i) {
        if (i < 0 || i >= d) throw new IndexOutOfBoundsException("Illegal index");
        if (st.contains(i)) return st.get(i);
        else                return 0.0;
    }

    public double dot(SparseVector that) {
        if (this.d != that.d) throw new IllegalArgumentException("Vector lengths disagree");
        double sum = 0.0;

        // iterate over the vector with the fewest nonzeros
        if (this.st.size() <= that.st.size()) {
            for (int i : this.st.keys())
                if (that.st.contains(i)) sum += this.get(i) * that.get(i);
        }
        else  {
            for (int i : that.st.keys())
                if (this.st.contains(i)) sum += this.get(i) * that.get(i);
        }
        return sum;
    }
    public double dot(double[] that) {
        double sum = 0.0;
        for (int i : st.keys())
            sum += that[i] * this.get(i);
        return sum;
    }

    public SparseVector plus(SparseVector that) {
        if (this.d != that.d) throw new IllegalArgumentException("Vector lengths disagree");
        SparseVector c = new SparseVector(d);
        for (int i : this.st.keys()) c.put(i, this.get(i));                // c = this
        for (int i : that.st.keys()) c.put(i, that.get(i) + c.get(i));     // c = c + that
        return c;
    }

}

public class ST<Key extends Comparable<Key>, Value> implements Iterable<Key> {
    private TreeMap<Key, Value> st;
    public ST() {
        st = new TreeMap<Key, Value>();
    }
    public Value get(Key key) {
        if (key == null) throw new NullPointerException("called get() with null key");
        return st.get(key);
    }
    public void put(Key key, Value val) {
        if (key == null) throw new NullPointerException("called put() with null key");
        if (val == null) st.remove(key);
        else             st.put(key, val);
    }
    public void delete(Key key) {
        if (key == null) throw new NullPointerException("called delete() with null key");
        st.remove(key);
    }
    public boolean contains(Key key) {
        if (key == null) throw new NullPointerException("called contains() with null key");
        return st.containsKey(key);
    }
    public int size() {
        return st.size();
    }
    public boolean isEmpty() {
        return size() == 0;
    }
    public Iterable<Key> keys() {
        return st.keySet();
    }eprecated Replaced by {@link #keys()}.
     */
    public Iterator<Key> iterator() {
        return st.keySet().iterator();
    }
    public Key min() {
        if (isEmpty()) throw new NoSuchElementException("called min() with empty symbol table");
        return st.firstKey();
    }throws NoSuchElementException if this symbol table is empty
     */
    public Key max() {
        if (isEmpty()) throw new NoSuchElementException("called max() with empty symbol table");
        return st.lastKey();
    }
    public Key ceiling(Key key) {
        if (key == null) throw new NullPointerException("called ceiling() with null key");
        Key k = st.ceilingKey(key);
        if (k == null) throw new NoSuchElementException("all keys are less than " + key);
        return k;
    }
    public Key floor(Key key) {
        if (key == null) throw new NullPointerException("called floor() with null key");
        Key k = st.floorKey(key);
        if (k == null) throw new NoSuchElementException("all keys are greater than " + key);
        return k;
    }
}