how to solve cos 90 problem in java?

This question already has an answer here:

Is floating point math broken? 23 answers

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What you're getting is most likely very, very small numbers, which are being displayed in exponential notation. The reason you're getting them is because pi/2 is not exactly representable in IEEE 754 notation, so there's no way to get the exact cosine of 90/270 degrees.

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Just run your source and it returns:

cos 90 : 1.8369701987210297E-16
sin 90 : 4.0

That's absolutely correct. The first value is nearly 0. The second is 4 as expected.

3 * cos(90°) = 3 * 0 = 0

Here you have to read the Math.toRadians() documentation which says:

Converts an angle measured in degrees to an approximately equivalent angle measured in radians. The conversion from degrees to radians is generally inexact.

Update: You can use for example the MathUtils.round() method from the Apache Commons repository and round the output to say 8 decimals, like this:

System.out.println("cos 90 : " + MathUtils.round(x, 8));

That will give you:

cos 90 : 0.0
sin 90 : 4.0

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尝试这个：

public class calc
{
    private double x;
    private double y;
    public calc(double x,double y)
    {
        this.x=x;
        this.y=y;
    }
    public void print(double theta)
    {
        if( ((Math.toDegrees(theta) / 90) % 2) == 1)
        {
            x = x*0;
            y = y*Math.sin(theta);
        }
        else if( ((Math.toDegrees(theta) / 90) % 2) == 0)
        {
            x = x*Math.cos(theta);
            y = y*0; 
        }
        else
        {
           x = x*Math.cos(theta);
           y = y*Math.sin(theta); 
        }
        System.out.println("cos 90 : "+x);
        System.out.println("sin 90 : "+y);
    }
    public static void main(String[]args)
    {
        calc p = new calc(3,4);
        p.print(Math.toRadians(90));
    }
}

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