定义和实现复数类，可以完成复数的加减乘除及输出，Java 定义一个抽象类Shape，该类有抽象方法area，圆形、矩形、梯形等均是一种Shape，也都有自己求area方法，定义这些类并写测试类进行测试。Java

在Java中，我们可以创建一个名为`ComplexNumber`的类来代表复数，包含实部（real part）和虚部（imaginary part），然后提供相应的算术操作以及字符串输出的方法。以下是简单的实现：

public class ComplexNumber {
    private double real;
    private double imaginary;

    public ComplexNumber(double real, double imaginary) {
        this.real = real;
        this.imaginary = imaginary;
    }

    // 加法
    public ComplexNumber add(ComplexNumber other) {
        return new ComplexNumber(this.real + other.real, this.imaginary + other.imaginary);
    }

    // 减法
    public ComplexNumber subtract(ComplexNumber other) {
        return new ComplexNumber(this.real - other.real, this.imaginary - other.imaginary);
    }

    // 乘法
    public ComplexNumber multiply(ComplexNumber other) {
        double realPart = this.real * other.real - this.imaginary * other.imaginary;
        double imaginaryPart = this.real * other.imaginary + this.imaginary * other.real;
        return new ComplexNumber(realPart, imaginaryPart);
    }

    // 除法 (假设除数非零)
    public ComplexNumber divide(ComplexNumber divisor) {
        double denominator = divisor.real * divisor.real + divisor.imaginary * divisor.imaginary;
        double realPart = (this.real * divisor.real + this.imaginary * divisor.imaginary) / denominator;
        double imaginaryPart = (this.imaginary * divisor.real - this.real * divisor.imaginary) / denominator;
        return new ComplexNumber(realPart, imaginaryPart);
    }

    @Override
    public String toString() {
        if (imaginary >= 0) {
            return String.format("%.2f + %.2fi", real, imaginary);
        } else {
            return String.format("%.2f - %.2fi", real, -imaginary);
        }
    }
}

// 对复数类进行测试
public class ComplexNumberTest {
    public static void main(String[] args) {
        ComplexNumber c1 = new ComplexNumber(1, 2);
        ComplexNumber c2 = new ComplexNumber(3, 4);

        System.out.println("c1: " + c1); // 输出 1 + 2i
        System.out.println("c1 + c2: " + c1.add(c2)); // 输出 4 + 6i
        System.out.println("c1 - c2: " + c1.subtract(c2)); // 输出 -2 - 2i

        ComplexNumber result = c1.multiply(c2);
        System.out.println("c1 * c2: " + result); // 输出 5 - 10i

        ComplexNumber reciprocal = c1.divide(new ComplexNumber(5, 0));
        System.out.println("c1 / 5: " + reciprocal); // 输出 0.2 - 0.4i (假设浮点数除法准确）
    }
}

在这个例子中，`ComplexNumber`是一个具体的类实现了`Shape`抽象类，但它不需要显式声明，因为我们直接处理复数的运算。`ComplexNumberTest`类用于测试复数的各种操作。