code: package edu.berkeley.nlp.util;
  
  import java.io.Serializable;
  import java.util.Arrays;
  
  /**
   * Class representing a sparse array. A <code>double[]</code> and
   * <code>int[]</code> arrays store a sparse reprsentation of a double array.
   * Arrays are grown behind the scene when necessary. Setting/getting a count
   * takes time O(log n), where n is the number of non-zero elements.
   * 
   * @author aria42
   * 
   */
  public class SparseArray<T> implements Serializable {
  
  	private static final long serialVersionUID = 42L;
  
  	T[] values = (T[]) new Object[0];
  	int[] indices = new int[0];
  	int length = 0;
  
  	private void grow() {
  		int curSize = values.length;
  		int newSize = curSize + 10;
  
  		T[] newData = (T[]) new Object[newSize];
  		System.arraycopy(values, 0, newData, 0, curSize);
  		values = newData;
  		int[] newIndices = new int[newSize];
  		System.arraycopy(indices, 0, newIndices, 0, curSize);
  		for (int i = curSize; i < newIndices.length; ++i) {
  			newIndices[i] = Integer.MAX_VALUE;
  		}
  		indices = newIndices;
  	}
  
  	public T get(int index) {
  		int res = Arrays.binarySearch(indices, index);
  		if (res >= 0 && res < length) {
  			return values[res];
  		}
  		return null;
  	}
  
  	public int size() {
  		return length;
  	}
  
  	public void put(int index0, T x) {
  		int res = Arrays.binarySearch(indices, index0);
  		// Greater than everything
  		if (res >= 0 && res < length) {
  			values[res] = x;
  			return;
  		}
  		if (length + 1 >= values.length) {
  			grow();
  		}
  		// In the middle
  		int insertionPoint = -(res + 1);
  		assert insertionPoint >= 0 && insertionPoint <= length : String.format(
  				"length: %d insertion: %d", length, insertionPoint);
  		// Shift The Stuff After
  		System.arraycopy(values, insertionPoint, values, insertionPoint + 1,
  				length - insertionPoint);
  		System.arraycopy(indices, insertionPoint, indices, insertionPoint + 1,
  				length - insertionPoint);
  		indices[insertionPoint] = index0;
  		values[insertionPoint] = x;
  		length++;
  	}
  
  	@Override
  	public String toString() {
  		StringBuilder builder = new StringBuilder();
  		builder.append("{ ");
  		for (int i = 0; i < length; ++i) {
  			builder.append(String.format("%d : %.5f", indices[i], values[i]));
  			builder.append(" ");
  		}
  		builder.append(" }");
  		return builder.toString();
  	}
  
  	public String toString(Indexer<?> indexer) {
  		StringBuilder builder = new StringBuilder();
  		builder.append("{ ");
  		for (int i = 0; i < length; ++i) {
  			builder.append(String.format("%s : %.5f",
  					indexer.getObject(indices[i]), values[i]));
  			builder.append(" ");
  		}
  		builder.append("}");
  		return builder.toString();
  	}
  
  	public static void main(String[] args) {
  		SparseArray<Double> sv = new SparseArray<Double>();
  		sv.put(0, 1.0);
  		sv.put(1, 2.0);
  		sv.put(4, -2.0);
  		System.out.println(sv);
  	}
  
  }
  