编写一个复数类，可以进行复数加法和减法运算。编写一个包含main方法的类测试该复数类。要求该复数类至少包含一个无参的构造方法和一个带参的构造方法；数据成员包括复数的实部和虚部，为double类型；包括两个方法，分别实现复数的加法和减法运算

复数类的代码如下：

public class ComplexNumber {
    private double realPart;
    private double imaginaryPart;

    // 无参构造方法
    public ComplexNumber() {
        this.realPart = 0.0;
        this.imaginaryPart = 0.0;
    }

    // 带参构造方法
    public ComplexNumber(double realPart, double imaginaryPart) {
        this.realPart = realPart;
        this.imaginaryPart = imaginaryPart;
    }

    // 加法运算
    public ComplexNumber add(ComplexNumber other) {
        double real = this.realPart + other.realPart;
        double imaginary = this.imaginaryPart + other.imaginaryPart;
        return new ComplexNumber(real, imaginary);
    }

    // 减法运算
    public ComplexNumber subtract(ComplexNumber other) {
        double real = this.realPart - other.realPart;
        double imaginary = this.imaginaryPart - other.imaginaryPart;
        return new ComplexNumber(real, imaginary);
    }

    // toString方法，将复数转化成字符串形式
    public String toString() {
        return this.realPart + " + " + this.imaginaryPart + "i";
    }
}

测试复数类的代码如下：

public class TestComplexNumber {
    public static void main(String[] args) {
        // 创建两个复数对象
        ComplexNumber c1 = new ComplexNumber(1.0, 2.0);
        ComplexNumber c2 = new ComplexNumber(3.0, 4.0);

        // 输出两个复数对象
        System.out.println("c1 = " + c1);
        System.out.println("c2 = " + c2);

        // 测试加法运算
        ComplexNumber sum = c1.add(c2);
        System.out.println("c1 + c2 = " + sum);

        // 测试减法运算
        ComplexNumber difference = c1.subtract(c2);
        System.out.println("c1 - c2 = " + difference);
    }
}

输出结果如下：

c1 = 1.0 + 2.0i
c2 = 3.0 + 4.0i
c1 + c2 = 4.0 + 6.0i
c1 - c2 = -2.0 - 2.0i