java 定义一个二维向量<a,b>类，其中a、b为其属性，主要操作如下： 向量相加：<a,b>+<c,d>=<a+c,b+d> 向量相减：<a,b>-<c,d>=<a-c,b-d> 向量内积：<a,b>*<c,d>=a*c+b*d 编程定义该类，并使用该类

以下是 Java 代码实现：

public class Vector2D {
    private double x;
    private double y;

    public Vector2D(double x, double y) {
        this.x = x;
        this.y = y;
    }

    public Vector2D add(Vector2D other) {
        return new Vector2D(this.x + other.x, this.y + other.y);
    }

    public Vector2D subtract(Vector2D other) {
        return new Vector2D(this.x - other.x, this.y - other.y);
    }

    public double dotProduct(Vector2D other) {
        return this.x * other.x + this.y * other.y;
    }

    public void print() {
        System.out.println("(" + this.x + ", " + this.y + ")");
    }
}

可在 `main` 函数中进行测试：

public static void main(String[] args) {
    Vector2D v1 = new Vector2D(1, 2);
    Vector2D v2 = new Vector2D(3, 4);

    Vector2D v3 = v1.add(v2);
    v3.print(); // 输出 (4.0, 6.0)

    Vector2D v4 = v1.subtract(v2);
    v4.print(); // 输出 (-2.0, -2.0)

    double dotProduct = v1.dotProduct(v2);
    System.out.println(dotProduct); // 输出 11.0
}