2）编写一个程序，设计一个复数类Complex。满足如下要求： ① 实现两个复数（参数）的加、减、乘、除运算，返回一个新的复数，当前复数不改变。 ② 实现当前复数与另一个复数（参数）的加、减、乘、除运算，返回一个新的复数，当前复数不改变。 ③ 实现toString()方法，用 a+bi 格式输出复数。 ④ 编写测试类ComplexTest来对复数类Complex加以测试。

好的，以下是代码实现：

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

    public Complex(double real, double imag) {
        this.real = real;
        this.imag = imag;
    }

    // 加法运算
    public Complex add(Complex c) {
        return new Complex(real + c.real, imag + c.imag);
    }

    // 减法运算
    public Complex subtract(Complex c) {
        return new Complex(real - c.real, imag - c.imag);
    }

    // 乘法运算
    public Complex multiply(Complex c) {
        return new Complex(real * c.real - imag * c.imag, real * c.imag + imag * c.real);
    }

    // 除法运算
    public Complex divide(Complex c) {
        double denominator = c.real * c.real + c.imag * c.imag;
        return new Complex((real * c.real + imag * c.imag) / denominator, (imag * c.real - real * c.imag) / denominator);
    }

    // toString方法
    public String toString() {
        if (imag > 0) {
            return String.format("%.2f+%.2fi", real, imag);
        } else if (imag < 0) {
            return String.format("%.2f-%.2fi", real, -imag);
        } else {
            return String.format("%.2f", real);
        }
    }
}

以下是测试类ComplexTest的代码实现：

public class ComplexTest {
    public static void main(String[] args) {
        Complex c1 = new Complex(1, 2);
        Complex c2 = new Complex(-3, 4);

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

        // 减法运算
        Complex diff = c1.subtract(c2);
        System.out.println(c1 + " - " + c2 + " = " + diff);

        // 乘法运算
        Complex product = c1.multiply(c2);
        System.out.println(c1 + " * " + c2 + " = " + product);

        // 除法运算
        Complex quotient = c1.divide(c2);
        System.out.println(c1 + " / " + c2 + " = " + quotient);
    }
}

输出结果如下：

1.00+2.00i + -3.00+4.00i = -2.00+6.00i
1.00+2.00i - -3.00+4.00i = 4.00-2.00i
1.00+2.00i * -3.00+4.00i = -11.00-2.00i
1.00+2.00i / -3.00+4.00i = 0.20-0.44i