声明并测试一个复数类，其方法包括toString()以及复数的加减乘运算。

好的，这是一个关于编程的问题，很高兴回答。下面是一个Java语言的复数类，已经实现了toString()和复数的加减乘运算方法：

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

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

    public String toString() {
        return real + " + " + imaginary + "i";
    }

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

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

    public ComplexNumber multiply(ComplexNumber other) {
        double newReal = real * other.real - imaginary * other.imaginary;
        double newImaginary = real * other.imaginary + imaginary * other.real;
        return new ComplexNumber(newReal, newImaginary);
    }
}

你可以通过创建一个ComplexNumber对象，并调用其方法来测试它的功能。