问Java在2d中的移动

EN

this(x,y): (500.0, 250.0)
dest(x,y): (400.0, 300.0)
DeltX: 100.0
DeltY: -50.0
Angle: 0.46364760900080615 //about 26.56°, should be 206.56° (26.56+pi) i think
XVELOC: 0.08944272
YVELOC: 0.04472136

public void Update(long deltaTime){
        if(this.destination == null){
            return;
        }

        this.x += this.x_velocity * deltaTime;
        this.y += this.y_velocity * deltaTime;

        if(Math.abs(this.x-destination.x) < 1 && Math.abs(this.y-destination.y) < 1){
            destination.arrive(this);
        }
    }

    public void setPath(){
        if(this.destination == null){
            return;
        } 

        double delta_x = (double) (this.x-this.destination.x);
        double delta_y = (double) (this.y-this.destination.y);
        int sign_x = (int)Math.signum(delta_x);
        int sign_y = (int)Math.signum(delta_y);
        double radian_angle = Math.atan((delta_x/delta_y));
        if(sign_x > 0 && sign_y > 0)
            radian_angle += Math.PI;
        else if(sign_x > 0 && sign_y < 0)
            radian_angle += Math.PI/2;
        else if(sign_x < 0 && sign_y > 0)
            radian_angle += 3*Math.PI/2;

        System.out.println("DeltX: "+delta_x);
        System.out.println("DeltY: "+delta_y);
        System.out.println("Angle: "+radian_angle);

        this.x_velocity = this.max_velocity*(float)Math.cos(radian_angle);
        this.y_velocity = this.max_velocity*(float)Math.sin(radian_angle);

        System.out.println("XVELOC: "+x_velocity);
        System.out.println("YVELOC: "+y_velocity);
    }