声明复数类，成员变量包括实部和虚部，成员方法包括实现由字符串构造复数、复数加法、减法，字符串描述、比较相等，计算复数的模等操作

声明一个复数类，可以按照以下方式设计：

public class ComplexNumber {
    private double real; // 实部
    private double imaginary; // 虚部

    // 构造函数，通过字符串初始化复数
    public ComplexNumber(String complexString) {
        String[] parts = complexString.split(" + i\\b"); // 分割实部和虚部
        if (parts.length == 2) {
            this.real = Double.parseDouble(parts[0]);
            this.imaginary = Double.parseDouble(parts[1]);
        } else if (complexString.endsWith("i")) {
            this.real = 0;
            this.imaginary = Double.parseDouble(complexString.substring(0, complexString.length() - 1));
        } else {
            throw new IllegalArgumentException("Invalid complex number format");
        }
    }

    // 复数加法
    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);
    }

    // 字符串描述
    @Override
    public String toString() {
        return real + " + " + imaginary + "i";
    }

    // 比较相等
    @Override
    public boolean equals(Object obj) {
        if (obj instanceof ComplexNumber) {
            ComplexNumber other = (ComplexNumber) obj;
            return this.real == other.real && this.imaginary == other.imaginary;
        }
        return false;
    }

    // 计算复数的模
    public double magnitude() {
        return Math.sqrt(real * real + imaginary * imaginary);
    }
}