用java。定义一个复数类，完成复数的加、减、乘和除法运算，并输出运算结果。要求对构造方法进行重载。

好的，请看以下代码实现：

public class ComplexNumber {
    private double realPart; // 实部
    private double imaginaryPart; // 虚部
    
    // 无参构造方法
    public ComplexNumber() {
        this.realPart = 0;
        this.imaginaryPart = 0;
    }
    
    // 有参构造方法
    public ComplexNumber(double realPart, double imaginaryPart) {
        this.realPart = realPart;
        this.imaginaryPart = imaginaryPart;
    }
    
    // 加法运算
    public ComplexNumber add(ComplexNumber other) {
        double newRealPart = this.realPart + other.realPart;
        double newImaginaryPart = this.imaginaryPart + other.imaginaryPart;
        return new ComplexNumber(newRealPart, newImaginaryPart);
    }
    
    // 减法运算
    public ComplexNumber subtract(ComplexNumber other) {
        double newRealPart = this.realPart - other.realPart;
        double newImaginaryPart = this.imaginaryPart - other.imaginaryPart;
        return new ComplexNumber(newRealPart, newImaginaryPart);
    }
    
    // 乘法运算
    public ComplexNumber multiply(ComplexNumber other) {
        double newRealPart = this.realPart * other.realPart - this.imaginaryPart * other.imaginaryPart;
        double newImaginaryPart = this.realPart * other.imaginaryPart + this.imaginaryPart * other.realPart;
        return new ComplexNumber(newRealPart, newImaginaryPart);
    }
    
    // 除法运算
    public ComplexNumber divide(ComplexNumber other) {
        double denominator = other.realPart * other.realPart + other.imaginaryPart * other.imaginaryPart;
        double newRealPart = (this.realPart * other.realPart + this.imaginaryPart * other.imaginaryPart) / denominator;
        double newImaginaryPart = (this.imaginaryPart * other.realPart - this.realPart * other.imaginaryPart) / denominator;
        return new ComplexNumber(newRealPart, newImaginaryPart);
    }
    
    // 输出复数
    public void print() {
        System.out.println(this.realPart + " + " + this.imaginaryPart + "i");
    }
}

然后我们可以在主函数中测试一下：

public static void main(String[] args) {
    ComplexNumber num1 = new ComplexNumber(2, 3);
    ComplexNumber num2 = new ComplexNumber(4, -2);
    
    System.out.print("num1: ");
    num1.print();
    System.out.print("num2: ");
    num2.print();
    
    System.out.print("num1 + num2 = ");
    num1.add(num2).print();
    System.out.print("num1 - num2 = ");
    num1.subtract(num2).print();
    System.out.print("num1 * num2 = ");
    num1.multiply(num2).print();
    System.out.print("num1 / num2 = ");
    num1.divide(num2).print();
}

输出结果如下：

num1: 2.0 + 3.0i
num2: 4.0 + -2.0i
num1 + num2 = 6.0 + 1.0i
num1 - num2 = -2.0 + 5.0i
num1 * num2 = 16.0 + 8.0i
num1 / num2 = 0.1 + 0.7i