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。并运用public static void main(String[] args) {

好的，我来为您实现一个Java的复数类，支持加、减、乘和除法运算：

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

    // 构造方法重载
    public Complex(double real, double imag) {
        this.real = real;
        this.imag = imag;
    }
    public Complex() {
        this.real = 0;
        this.imag = 0;
    }
    public Complex(double real) {
        this.real = real;
        this.imag = 0;
    }

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

    // 减法运算
    public Complex sub(Complex other) {
        double real = this.real - other.real;
        double imag = this.imag - other.imag;
        return new Complex(real, imag);
    }

    // 乘法运算
    public Complex mul(Complex other) {
        double real = this.real * other.real - this.imag * other.imag;
        double imag = this.real * other.imag + this.imag * other.real;
        return new Complex(real, imag);
    }

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

    // 输出复数
    public String toString() {
        return real + " + " + imag + "i";
    }

    // 测试
    public static void main(String[] args) {
        Complex a = new Complex(2, 3);
        Complex b = new Complex(4, -5);
        System.out.println("a = " + a);
        System.out.println("b = " + b);
        System.out.println("a + b = " + a.add(b));
        System.out.println("a - b = " + a.sub(b));
        System.out.println("a * b = " + a.mul(b));
        System.out.println("a / b = " + a.div(b));
    }
}

这个类包含了一个实部和一个虚部，以及加、减、乘、除四个运算方法，分别对应加法、减法、乘法和除法。构造方法重载有三种，一种是同时指定实部和虚部，一种是只指定实部，另一种是不指定任何值，实部和虚部默认为0。最后在`main`方法中进行了测试，输出运算结果。

希望这个例子可以帮助您理解复数类的实现。