Added some test files.

app/src/test/java/com/example/hpos/CubicEquationSolver.kt

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+package com.example.hpos
+
+import org.junit.Test
+
+class CubicEquationSolver {
+
+    @Test
+    fun test() {
+        val a: Double = 12.toDouble()
+        val b: Double = (15).toDouble()
+        val c: Double = (9).toDouble()
+        val d: Double = (-2456).toDouble()
+
+        solve(a, b, c, d)
+    }
+
+    fun solve(a: Double, b: Double, c: Double, d: Double) {
+        val f: Double = getF(a, b, c)
+        val g: Double = getG(a, b, c, d)
+        val h: Double = getH(f, g)
+        println("f = $f & g = $g & h = $h")
+
+        if (f == 0.0 && g == 0.0 && h == 0.0)
+            solveOneRoot(a, d)
+        else if (h <= 0)
+            solveRealRoots(a, b, g, h)
+        else
+            solveNotRealRoots(a, b, g, h)
+    }
+
+    private fun getF(a: Double, b: Double, c: Double): Double {
+        return (3 * c / a - b * b / (a * a)) / 3
+    }
+
+    private fun getG(a: Double, b: Double, c: Double, d: Double): Double {
+        var g: Double = 2 * b * b * b / (a * a * a)
+        g += -9 * b * c / (a * a)
+        g += 27 * d / a
+        g /= 27.0
+        return g
+    }
+
+    private fun getH(f: Double, g: Double): Double {
+        return g * g / 4 + f * f * f / 27
+    }
+
+    private fun solveOneRoot(a: Double, d: Double) {
+        val x: Double = -Math.pow(d / a, 1.0 / 3)
+        println("The solution is: $x")
+
+        println("rounded off solutions are:")
+        println(String.format("%.3g%n", x))
+    }
+
+    private fun solveRealRoots(a: Double, b: Double, g: Double, h: Double) {
+        val i: Double = Math.sqrt(g * g / 4 - h)
+        val j: Double = Math.pow(i, 1.0 / 3)
+        val k: Double = Math.acos(-g / (2 * i))
+
+        val l: Double = -j
+        val m: Double = Math.cos(k / 3)
+        val n: Double = Math.sqrt(3.0) * Math.sin(k / 3)
+        val p: Double = -b / (3 * a)
+
+        val x1: Double = 2 * j * Math.cos(k / 3) - b / (3 * a)
+        val x2: Double = l * (m + n) + p
+        val x3: Double = l * (m - n) + p
+
+        println("The solutions are:")
+        println(x1)
+        println(x2)
+        println(x3)
+
+        println("rounded off solutions are:")
+        println(String.format("%.3g", x1))
+        println(String.format("%.3g", x2))
+        println(String.format("%.3g", x3))
+    }
+
+    private fun solveNotRealRoots(a: Double, b: Double, g: Double, h: Double) {
+
+        val r: Double = -(g / 2) + Math.sqrt(h)
+        val s: Double = if (r >= 0) Math.pow(r, 1.0 / 3) else -Math.pow(-r, 1.0 / 3)
+        val t: Double = -(g / 2) - Math.sqrt(h)
+        val u: Double = if (t >= 0) Math.pow(t, 1.0 / 3) else -Math.pow(-t, 1.0 / 3)
+
+        val x1: Double = s + u - b / (3 * a)
+        val x2: Double = -((s + u) / 2) - b / (3 * a)
+        val immaginary: Double = (s - u) * Math.sqrt(3.0) / 2
+
+        println("The solutions are:")
+        println(x1)
+        println(x2.toString() + " + " + immaginary + "i")
+        println(x2.toString() + " - " + immaginary + "i")
+    }
+}

app/src/test/java/com/example/hpos/OtherTests.kt

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+package com.example.hpos
+
+import org.junit.Test
+import java.util.*
+import java.util.concurrent.atomic.AtomicLong
+
+class OtherTests {
+
+    @Test
+    fun uniqueId() {
+        val date = Date()
+        date.day
+    }
+
+}