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# Quantiles/DoublesSketch Java Example

    // simplified file operations and no error handling for clarity

    import java.io.FileInputStream;
    import java.io.FileOutputStream;
    import java.util.Arrays;
    import java.util.Random;

    import org.apache.datasketches.memory.Memory;

    import org.apache.datasketches.quantiles.DoublesSketch;
    import org.apache.datasketches.quantiles.DoublesUnion;
    import org.apache.datasketches.quantiles.UpdateDoublesSketch;

    // this section generates two sketches from random data and serializes them into files
    {
      Random rand = new Random();

      UpdateDoublesSketch sketch1 = DoublesSketch.builder().build(); // default k=128
      for (int i = 0; i < 10000; i++) {
        sketch1.update(rand.nextGaussian()); // mean=0, stddev=1
      }
      FileOutputStream out1 = new FileOutputStream("QuantilesDoublesSketch1.bin");
      out1.write(sketch1.toByteArray());
      out1.close();
    
      UpdateDoublesSketch sketch2 = DoublesSketch.builder().build(); // default k=128
      for (int i = 0; i < 10000; i++) {
        sketch2.update(rand.nextGaussian() + 1); // shift the mean for the second sketch
      }
      FileOutputStream out2 = new FileOutputStream("QuantilesDoublesSketch2.bin");
      out2.write(sketch2.toByteArray());
      out2.close();
    }

    // this section deserializes the sketches, produces a union and prints some results
    {
      FileInputStream in1 = new FileInputStream("QuantilesDoublesSketch1.bin");
      byte[] bytes1 = new byte[in1.available()];
      in1.read(bytes1);
      in1.close();
      DoublesSketch sketch1 = DoublesSketch.wrap(Memory.wrap(bytes1));

      FileInputStream in2 = new FileInputStream("QuantilesDoublesSketch2.bin");
      byte[] bytes2 = new byte[in2.available()];
      in2.read(bytes2);
      in2.close();
      DoublesSketch sketch2 = DoublesSketch.wrap(Memory.wrap(bytes2));

      DoublesUnion union = DoublesUnion.builder().build(); // default k=128
      union.update(sketch1);
      union.update(sketch2);
      DoublesSketch result = union.getResult();
      // Debug output from the sketch
      System.out.println(result.toString());

      System.out.println("Min, Median, Max values");
      System.out.println(Arrays.toString(result.getQuantiles(new double[] {0, 0.5, 1})));

      System.out.println("Probability Histogram: estimated probability mass in 4 bins: (-inf, -2), [-2, 0), [0, 2), [2, +inf)");
      System.out.println(Arrays.toString(result.getPMF(new double[] {-2, 0, 2})));

      System.out.println("Frequency Histogram: estimated number of original values in the same bins");
      double[] histogram = result.getPMF(new double[] {-2, 0, 2});
      for (int i = 0; i < histogram.length; i++) {
        histogram[i] *= result.getN(); // scale the fractions by the total count of values
      }
      System.out.println(Arrays.toString(histogram));
    }

    Output:
    ### Quantiles HeapUpdateDoublesSketch SUMMARY: 
    Empty                        : false
    Direct, Capacity bytes       : false, 
    Estimation Mode              : true
    K                            : 128
    N                            : 20,000
    Levels (Needed, Total, Valid): 7, 7, 4
    Level Bit Pattern            : 1001110
    BaseBufferCount              : 32
    Combined Buffer Capacity     : 1,152
    Retained Items               : 544
    Compact Storage Bytes        : 4,384
    Updatable Storage Bytes      : 9,248
    Normalized Rank Error        : 1.725%
    Min Value                    : -4.113
    Max Value                    : 4.363
    ### END SKETCH SUMMARY

    Min, Median, Max values
    [-4.113097775288085, 0.49496152841809893, 4.362712872544037]
    Probability Histogram: estimated probability mass in 4 bins: (-inf, -2), [-2, 0), [0, 2), [2, +inf)
    [0.01445, 0.3071, 0.58545, 0.093]
    Frequency Histogram: estimated number of original values in the same bins
    [289.0, 6142.0, 11709.0, 1860.0]