用Java实现声明复数类Complex，成员变量包括实部和虚部，成员方法包括实现由字符串构造复数、复数加法、减法、字符串描述、比较相等、计算复数的模√（实部²+虚部²）等操作。

在Java中，我们可以创建一个名为`Complex`的类来表示复数，它有两个私有成员变量`real`和`imaginary`分别代表实部和虚部。以下是一个简单的实现：

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

    // 构造函数，接受字符串解析复数
    public Complex(String complexString) {
        String[] parts = complexString.split(" ");
        if (parts.length == 2) {
            this.real = Double.parseDouble(parts[0]);
            this.imaginary = Double.parseDouble(parts[1]);
        } else {
            throw new IllegalArgumentException("Invalid complex number format");
        }
    }

    // 复数加法
    public Complex add(Complex other) {
        return new Complex(this.real + other.real, this.imaginary + other.imaginary);
    }

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

    // 返回字符串描述
    @Override
    public String toString() {
        return "Complex{" +
                "real=" + real +
                ", imaginary=" + imaginary +
                '}';
    }

    // 比较相等
    @Override
    public boolean equals(Object obj) {
        if (this == obj) return true;
        if (obj == null || getClass() != obj.getClass()) return false;
        Complex complex = (Complex) obj;
        return Double.compare(real, complex.real) == 0 &&
                Double.compare(imaginary, complex.imaginary) == 0;
    }

    // 计算复数模
    public double magnitude() {
        return Math.sqrt(Math.pow(real, 2) + Math.pow(imaginary, 2));
    }

    // 其他方法...
}