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how do i restric a float value to only one placess after decimal in kotlin android

I have a number with some number decimal places,
How can i round this float number with one number decimal places
for example
1.366565646 convert to 1.3

In your case I think you need to trim the number not round it,
You can use this:
double d = 1.366565646;
DecimalFormat df = new DecimalFormat("#.#");
double p = Double.parseDouble(df.format(d));

DecimalFormat not rounding to two decimal places correctly

In the program I am writing I have a percent change function. With the numbers I'm using, it is supposed to return .03, yet I just get 0. Using a calculator, the value not formatted would be 0.03303. I want the number to be truncated after the second decimal place, no rounding at all.
df = new DecimalFormat("###.##;-###.##");
df.setRoundingMode(RoundingMode.DOWN);
//later in code
return df.format(((price-base_price)/price)*100.00D);
Edit: all variable types are doubles. Base price is 756.60 and price is 756.85

Your assumption about the value calculated in ((price-base_price)/price)*100.00D is incorrect (I think you've probably invoked integer math, try ((price-base_price)/(double)price)*100.00D). When I directly execute,
double value = 0.03303;
DecimalFormat df = new DecimalFormat("###.##;-###.##");
df.setRoundingMode(RoundingMode.DOWN);
System.out.println(df.format(value));
I get the (requested) output
0.03

java rounding to two decimals

I have a number
double num = 1.234567;
and I'm trying to keep only two decimals
num = (int)((num * 100) + 0.5) / 100.0;
but the actual number I got is 1.230000000001. How can I get rid of the 0000000001 part?

Try DecimalFormat:
DecimalFormat twoDp= new DecimalFormat("#.##");
Double.valueOf(twoDp.format(num));

You can't, unless you switch to a decimal radix. Doubles and floats don't have decimal places, they have binary places, so you can't round or truncate them to specific numbers of decimal places except in the cases where the value representations are congruent, i.e. the negative powers of 2.
So you have to either use DecimalFormat if you are presenting the result, or BigDecimal if you want to keep computing with it.
Any solution that ends by turning the value back into floating point is incorrect.

String result = String.format("%.2f", num);

Try this .. this should solve it
Double num = //value
num = //arithmetic
String temp =num.toString().split("\\.")[0];
int precision=temp.length();
BigDecimal b =new BigDecimal(num,new MathContext(precision+2));
System.out.println(b.doubleValue());

Java Nice Format Double

How can I format a double to a string so that it shows all (no limit) decimal places if they are there but not show any trailing 0s?

double x = ...;
DecimalFormat fmt = new DecimalFormat();
fmt.setMinimumFractionDigits(0);
fmt.setMaximumFractionDigits(Integer.MAX_VALUE);
System.out.println(fmt.format(x));
Integer.MAX_VALUE is a silly number, but any sufficiently large number is fine, as you'll see from the setMaximumFractionDigits javadoc:
Sets the maximum number of digits allowed in the fraction portion of a number. For formatting numbers other than BigInteger and BigDecimal objects, the lower of newValue and 340 is used. Negative input values are replaced with 0.

Print the number of digits before a decimal point

I know there are ways to get the number of digits after a decimal point, for instance the substring method, but how would I go about doing this for the number of digits before a decimal place?
I need to use this to convert US change (double) into Euro change(double). The way I would like to do this is by taking the number before a decimal (such as $1.) and times it by its euro equivalent (.7498) and take the number after a decimal (.16) and times that by its .01 euro coin value (.0075), add both values together to get the euro equivalent of $1.16 (.8698).

To get the number before decimal point,do this:
String str = new Double(your_double_number).toString().subString(0,str.indexOf('.'));
double v = Double.valueOf(str);
If you are using '$' sign then take 1 in place of 0.
Hope it will help you.

convert US change (double) into Euro change(double)
Please don't do that. Never, never, ever use double or float to represent money, because those datatypes cannot represent most decimal fractions, so you get rounding errors before you even start to do any calculations.
Instead, use BigDecimal.

First of all - just multiplying the Dollar value by the exchange rate will get you the euro value so there's no need to do that as far as i can see - you will just introduce rounding errors.
But if you did need to - just use substring
String dollarVal = "$1.16"
String justFullDollar = dollarVal.substring(1, dollarVal.indexOf("."));
String justCents = dollarVal.substring(dollarVal.indexOf(".")+1);
The Correct way would be to store all you money as integers or arbitrary precision objects that way you get no floating point errors too.
Convert to cents, multiply and convert back again.
e.g.
String dollarVal = "$1.16"
BigDecimal dollars = new BigDecimal(dollarVal.substring(1)); //1.16
BigDecimal cents = dollars.multiply(new BigDecimal(100)); //116
BigDecimal eurocents = cents.multiply(new BigDecimal(exchangeRate)); //86.9768
BigDecimal euros = eurocents.divide(new BigDecimal(100)); //0.869768
DecimalFormat formatter = new DecimalFormat("###.00");
String euroVal = "€" + formatter.format(euros);

You can use
String s[] = new Double(your number).toString().split(".");
The s[0] is the number before decimal point. and s[1] is the number after decimal point.
Both are in String, so you need to parse them into double using
double num1 = Double.parseDouble(s[0]);
double num2 = Double.parseDouble(s[1]);