写一个抽象父类shape，有抽象方法area(求面积)，perimeter(求周长)，子类有等边三角形，梯形，正方形，求出这三个面积周长

首先，我们创建一个名为`Shape`的抽象父类，它包含两个抽象方法`area()`和`perimeter()`：

public abstract class Shape {
    protected double side; // 等边三角形的边长

    public abstract double area();
    public abstract double perimeter();

    // 构造函数示例，用于设置等边三角形的边长
    public Shape(double side) {
        this.side = side;
    }
}

接下来，我们定义三个具体的子类：`EquilateralTriangle`(等边三角形)，`Trapezoid`(梯形)和`Square`(正方形)：

public class EquilateralTriangle extends Shape {
    @Override
    public double area() {
        return Math.sqrt(3) / 4 * Math.pow(side, 2);
    }

    @Override
    public double perimeter() {
        return 3 * side;
    }
}

public class Trapezoid extends Shape {
    private double base1, base2; // 梯形的两底

    public Trapezoid(double base1, double base2, double height) {
        super(base1 + base2); // 周长基于两底之和
        this.base1 = base1;
        this.base2 = base2;
    }

    @Override
    public double area() {
        return (base1 + base2) * height / 2;
    }

    @Override
    public double perimeter() {
        return base1 + base2 + 2 * side; // 假设梯形的高已知，side为其他边长
    }
}

public class Square extends Shape {
    private double side;

    public Square(double side) {
        super(side);
        this.side = side;
    }

    @Override
    public double area() {
        return Math.pow(side, 2);
    }

    @Override
    public double perimeter() {
        return 4 * side;
    }
}

现在你可以通过创建每个子类的实例并调用它们的`area()`和`perimeter()`方法来获取各个形状的面积和周长：

public class Main {
    public static void main(String[] args) {
        EquilateralTriangle triangle = new EquilateralTriangle(5);
        System.out.println("等边三角形的面积: " + triangle.area());
        System.out.println("等边三角形的周长: " + triangle.perimeter());

        Trapezoid trapezoid = new Trapezoid(3, 7, 4);
        System.out.println("梯形的面积: " + trapezoid.area());
        System.out.println("梯形的周长: " + trapezoid.perimeter());

        Square square = new Square(4);
        System.out.println("正方形的面积: " + square.area());
        System.out.println("正方形的周长: " + square.perimeter());
    }
}