定义一个abstract的Shape类，求形状的面积，由Shape类派生4个子类三角形类、矩形类、梯形类、圆形类，分别求各自面积，然后进行测试。

首先，让我们定义一个抽象的 `Shape` 类，这个类包含一个抽象方法 `getArea()`，表示所有形状的面积计算方法，因为我们需要为每个具体的形状提供实现。然后，我们将创建四个子类，每个子类都覆盖这个抽象方法并提供自己的特定面积计算逻辑。

// Shape接口/抽象类
public abstract class Shape {
    public abstract double getArea();
}

// 子类1: 三角形
public 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 0.5 * base * height;
    }
}

// 子类2: 矩形
public class Rectangle extends Shape {
    private double width;
    private double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double getArea() {
        return width * height;
    }
}

// 子类3: 梯形
public class Trapezoid extends Shape {
    private double a;
    private double b;
    private double height;

    public Trapezoid(double a, double b, double height) {
        this.a = a;
        this.b = b;
        this.height = height;
    }

    @Override
    public double getArea() {
        return (a + b) * height / 2;
    }
}

// 子类4: 圆形
public class Circle extends Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

接下来，我们可以创建一个测试类来实例化这些形状并计算它们的面积：

public class Main {
    public static void main(String[] args) {
        Shape triangle = new Triangle(3, 4);
        Shape rectangle = new Rectangle(5, 6);
        Shape trapezoid = new Trapezoid(4, 5, 7);
        Shape circle = new Circle(3.5);

        System.out.println("Triangle area: " + triangle.getArea());
        System.out.println("Rectangle area: " + rectangle.getArea());
        System.out.println("Trapezoid area: " + trapezoid.getArea());
        System.out.println("Circle area: " + circle.getArea());
    }
}