Averages of numbers in Java

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Input:  [ 1, 3, 5, 1, -10]
Output:  [ 2, 4, 3, -4.5]

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public class Solution {
  public static double[] averages(final int[] numbers) {
    final double res[] = new double[(numbers == null || numbers.length == 0) ? 0 : numbers.length - 1];
    for (int i = 0; i < res.length; i++) res[i] = (numbers[i]+numbers[i+1]) / 2.0;
    return res;
  }
}

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public class Solution {
  public static double[] averages(int[] numbers) {
    if (numbers == null || numbers.length < 2) return new double[] {};
    double [] ans = new double [numbers.length - 1];
    for (int i = 0; i < ans.length; i++) ans[i] = (numbers[i] + numbers[i + 1]) / 2.0;
    return ans;
  }
}

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import java.util.stream.IntStream;

public class Solution {
  public static double[] averages(int[] numbers) {
    if (numbers == null || numbers.length < 2) return new double[0];
    return IntStream.range(1, numbers.length)
                    .mapToDouble(i -> (numbers[i - 1] + numbers[i]) / 2.0)
                    .toArray();
  }
}

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import org.junit.Test;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertArrayEquals;
import org.junit.runners.JUnit4;
import java.util.Arrays;

public class SolutionTest {
    @Test
    public void basicTests() {
      assertEquals(Arrays.toString(new double[] { 2, 2, 2, 2 }), Arrays.toString(Solution.averages(new int[] { 2, 2, 2, 2, 2 })));
      assertEquals(Arrays.toString(new double[] { 0, 0, 0, 0 }), Arrays.toString(Solution.averages(new int[] { 2, -2, 2, -2, 2 })));
      assertEquals(Arrays.toString(new double[] { 2, 4, 3, -4.5 }), Arrays.toString(Solution.averages(new int[] { 1, 3, 5, 1, -10 })));    
    }
    
    @Test
    public void nullEmptyTests() {
      assertEquals("input 'null' should return an empty array", 0, Solution.averages(null).length);
      assertEquals("Empty array as input should return an empty array", 0, Solution.averages(new int[0]).length);
      assertEquals("Array with only one value as input should return an empty array", 0, Solution.averages(new int[] { 2 }).length);
    }
    
    @Test
    public void randomTests() {     
      
      for(int r = 0; r < 100; r++)
      {
        int len = (int)Math.floor(Math.random() * 20);
        int[] numbers = new int[len];
        for(int k = 0; k < len; k++)
        {
          numbers[k] = (int)Math.floor(Math.random() * 41) - 20;
        }
      
        double[] expected = new double[0];
        
        if(numbers.length > 1)
        {
          expected = new double[numbers.length - 1];
          for(int i=0;i<numbers.length-1;i++)
          {
            expected[i] = ((double)numbers[i] + numbers[i+1]) / 2;
          }          
        }
      
        assertArrayEquals("Should work for random input", expected, Solution.averages(numbers), 0.001);
      }
    }
}