Generate Random Point in a Circle LeetCode solution in Java

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class Solution {
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  double radius;
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  double x_center;
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  double y_center;
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  Random r = new Random();
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  public Solution(double radius, double x_center, double y_center) {
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    this.radius = radius;
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    this.x_center = x_center;
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    this.y_center = y_center;
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  }
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  public double[] randPoint() {
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    double angle = r.nextDouble(Math.PI * 2);
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    // For probability is inversely proportional to radius, we use sqrt of random number.
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    double rad = Math.sqrt(r.nextDouble()) * radius;
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    double[] ret = new double[2];
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    ret[0] = rad * Math.cos(angle) + x_center;
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    ret[1] = rad * Math.sin(angle) + y_center;
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    return ret;
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  }
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}