编写一个复数类ComplexNumber,满足以下要求: (1)覆盖其Object.toString()方法,向外界返回“a+bi”形式的字串,此方法的声明为:public String toString() (2)提供相应方法,完成复数的加减乘除和取模运算。

好的，我明白了您的要求。以下是ComplexNumber类的代码：

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

    // 构造函数
    public ComplexNumber(double real, double imag) {
        this.real = real;
        this.imag = imag;
    }

    // 覆盖toString()方法，返回“a bi”形式的字符串
    public String toString() {
        if (imag >= 0) {
            return String.format("%.2f + %.2fi", real, imag);
        } else {
            return String.format("%.2f - %.2fi", real, -imag);
        }
    }

    // 加法运算
    public ComplexNumber add(ComplexNumber other) {
        double newReal = real + other.real;
        double newImag = imag + other.imag;
        return new ComplexNumber(newReal, newImag);
    }

    // 减法运算
    public ComplexNumber subtract(ComplexNumber other) {
        double newReal = real - other.real;
        double newImag = imag - other.imag;
        return new ComplexNumber(newReal, newImag);
    }

    // 乘法运算
    public ComplexNumber multiply(ComplexNumber other) {
        double newReal = real * other.real - imag * other.imag;
        double newImag = real * other.imag + imag * other.real;
        return new ComplexNumber(newReal, newImag);
    }

    // 除法运算
    public ComplexNumber divide(ComplexNumber other) {
        double denom = other.real * other.real + other.imag * other.imag;
        double newReal = (real * other.real + imag * other.imag) / denom;
        double newImag = (imag * other.real - real * other.imag) / denom;
        return new ComplexNumber(newReal, newImag);
    }

    // 取模运算
    public double modulus() {
        return Math.sqrt(real * real + imag * imag);
    }
}

希望我能够回答您的问题。