Find sum of top-left to bottom-right diagonals in Java

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array = [[1, 2],
         [3, 4]]

diagonal sum: 1 + 4 = 5

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array = [[5, 9, 1, 0],
         [8, 7, 2, 3],
         [1, 4, 1, 9],
         [2, 3, 8, 2]]

diagonal sum: 5 + 7 + 1 + 2 = 15

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public class Diagonal {
  public static int diagonalSum(final int[][] matrix) {
    int count = 0;
    for(int i=0; i<matrix.length; i++) {
      count += matrix[i][i];
    }
    return count;
  }
}

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import java.util.stream.IntStream;

public class Diagonal {
  public static int diagonalSum(final int[][] matrix) {
    return IntStream.range(0, matrix.length).map(i -> matrix[i][i]).sum();
  }
}

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public class Diagonal {
  public static int diagonalSum(final int[][] matrix) {
    return java.util.stream.IntStream.range(0, matrix.length)
        .map(i -> matrix[i][i])
        .sum();
  }
}

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import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;

public class SolutionTest {

    @Test
    public void testMatrix1() {
       final int[][] matrix = new int[][]{{12}};
       assertEquals(12, Diagonal.diagonalSum(matrix));
    }
    
    @Test
    public void testMatrix2() {
      final int[][] matrix = new int[][]{{1, 2}, {3, 4}};
      assertEquals(5, Diagonal.diagonalSum(matrix));
    }
    
    @Test
    public void testMatrix3() {
      final int[][] matrix = new int[][]{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
      assertEquals(15, Diagonal.diagonalSum(matrix));
    }
    
    @Test
    public void testMatrix4() {
      final int[][] matrix = new int[][]{{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}, {13, 14, 15, 16}};
      assertEquals(34, Diagonal.diagonalSum(matrix));
    }
}