java声明并测试一个复数类，其方法包括 toString()及复数的加、减、乘运算。并将代码补充在右侧编辑器中。

下面是一个用Java声明并测试一个复数类的示例代码，其中包括了toString()方法和复数的加、减、乘运算：

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

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

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

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

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

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

public class Main {
    public static void main(String[] args) {
        ComplexNumber t1 = new ComplexNumber(3, 4);
        ComplexNumber t2 = new ComplexNumber(2, -1);

        System.out.println("(a + b) = " + t1.add(t2).toString());
        System.out.println("(a - b) = " + t1.subtract(t2).toString());
        System.out.println("(a * b) = " + t1.multiply(t2).toString());
    }
}