Thread: How to make Math.round() round to the nearest .1?

I want round to round to the nearest 10th instead of the nearest integer... Any ideas?

public class Triangle 
{
    private final int leg;
    private final double hypotenuse;
 
    public Triangle( int l )
    {
        leg = l;
        hypotenuse = leg*Math.sqrt( 2.0 );
    }    
 
    public int getLeg()
    {
        return leg;
    }
 
    public double getHypotenuse()
    {
        return hypotenuse;
    }
 
    @Override
    public String toString()
    {
        return String.format( "Triangle(%d)", leg );
    }
}

Triangle(5)
Leg: 5
Hypotenuse: 7.0 (this should be 7.1)

Do do you want to round the value hypotenuse - ie produce a completely different double value - or merely format the value so that it appears with one decimal place?

I ask because people often confuse rounding (double->double) with formatting (double->String). And often imagine that double values have decimal places: in fact decimal places are a property of the String (base 10) representation of double values.

Assuming you want to format you would use

String.format("%.1f", hypotenuse);

The "f" means you are formatting a floating point value (a double) and the ".1" means you want one decimal place to appear.

Full details in the Formatter API docs. There is rather a lot to absorb there: too much to attempt to remember. But it's worth bookmarking and going back to whenever formatting problems arise.

String.format("%.1f", hypotenuse);

Ahhh, so easy. I can't believe I didn't think of that. Thank you, as always.

You're welcome.