2.1. Рефакторинг - Площадь треугольника.

https://job4j.ru/profile/exercise/20/task-view/142

src/main/java/ru/j4j/oop/Point.java

@@ -0,0 +1,18 @@
+package ru.j4j.oop;
+
+import static java.lang.Math.sqrt;
+import static java.lang.Math.pow;
+
+public class Point {
+    private final int x;
+    private final int y;
+
+    public Point(int x, int y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    public double distance(Point that) {
+        return sqrt(pow(this.x - that.x, 2) + pow(this.y - that.y, 2));
+    }
+}

src/main/java/ru/j4j/oop/Triangle.java

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+package ru.j4j.oop;
+
+import static java.lang.Math.*;
+
+public class Triangle {
+    private Point ab;
+    private Point bc;
+    private Point ca;
+
+    public Triangle(Point ab, Point bc, Point ca) {
+        this.ab = ab;
+        this.bc = bc;
+        this.ca = ca;
+    }
+
+    public double semiPerimeter(double a, double b, double c) {
+        return (a + b + c) / 2;
+    }
+
+    public boolean exist(double ab, double ac, double bc) {
+        return ab + ac > bc && ac + bc > ab && ab + bc > ac;
+    }
+
+    public double area() {
+        double ab = this.ab.distance(this.bc);
+        double bc = this.ab.distance(this.ca);
+        double ca = this.bc.distance(this.ca);
+        if (exist(ab, bc, ca)) {
+            double p = semiPerimeter(ab, bc, ca);
+            return sqrt(p * (p - ab) * (p - bc) * (p - ca));
+        }
+        return -1;
+    }
+}

src/test/java/ru/j4j/oop/TriangleTest.java

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+package ru.j4j.oop;
+
+import static org.assertj.core.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+class TriangleTest {
+    @Test
+    public void when00and40and04Then8() {
+        Point a = new Point(0, 0);
+        Point b = new Point(4, 0);
+        Point c = new Point(0, 4);
+        Triangle triangle = new Triangle(a, b, c);
+        double rsl = triangle.area();
+        double expected = 8;
+        assertThat(rsl).isCloseTo(expected, offset(0.001));
+    }
+
+    @Test
+    public void when00and00and04Then1() {
+        Point a = new Point(0, 0);
+        Point b = new Point(0, 0);
+        Point c = new Point(0, 4);
+        Triangle triangle = new Triangle(a, b, c);
+        double rsl = triangle.area();
+        double expected = -1;
+        assertThat(rsl).isCloseTo(expected, offset(0.001));
+    }
+
+    @Test
+    public void negativeTest() {
+        Point a = new Point(0, 0);
+        Point b = new Point(0, 0);
+        Point c = new Point(0, 4);
+        Triangle triangle = new Triangle(a, b, c);
+        double rsl = triangle.area();
+        double expected = -1;
+        assertThat(rsl).isEqualTo(expected);
+    }
+}