feat:w4

w4/Shape.java

@@ -0,0 +1,39 @@
+package learn;
+
+public abstract class Shape {
+    public abstract double getArea();
+}
+class Circle extends Shape{
+    private double radius;
+    public Circle(double radius){
+        this.radius=radius;
+    }
+    @Override
+    public double getArea(){
+        return Math.PI*radius*radius;
+    }
+}
+class Rectangle extends Shape{
+    private double length;
+    private double width;
+    public Rectangle(double length,double width){
+        this.length=length;
+        this.width=width;
+    }
+    @Override
+    public double getArea(){
+        return length*width;
+    }
+}
+class Triangle extends Shape{
+    private double base;
+    private double height;
+    public Triangle(double base,double height){
+        this.base=base;
+        this.height=height;
+    }
+    @Override
+    public double getArea(){
+        return base*height/2;
+    }
+}

w4/ShapeUtil.java

@@ -0,0 +1,11 @@
+package learn;
+
+public class ShapeUtil {
+    public static void printArea(Shape shape){
+        System.out.println(shape.getArea());
+    }
+    public static void main(String[]args){
+        Shape circle=new Circle(5);
+        printArea(circle);
+    }
+}