We Keep Coding

Big Error in Java or incongruency with simple operators

I'm beginning in Java, and could anyone explain me why Java gives me these answers?
I have a very simple class trying to learn how to round a number.
I want 2 decimals
so...
public static void main(String[] args) {
double pi1 = Math.PI;
System.out.println("pi1 = " + pi1);
double p2 ;
p2= Math.round(pi1*100)/100;
//p2= Math.round(pi1*100)
//p2=p2/100;
System.out.println("p2 = " + p2);
}
If I run this result is:
p2 = 3.0
Then I change
//p2= Math.round(pi1*100)/100;
p2 = Math.round(pi1*100);
p2 = p2/100;
Now, result is:
p2 = 3.14
as I wanted
Why with these differences? Why the first option doesn't give me 3.14
I think that I've made a correct code with 1st option.
Please, anyone could tell me why?
These things makes me don't trust Java.
Thank you.

I will assume that you know how integer division works in Java. In short, when both sides of / are integral types, like in 314 / 100, the expression evaluates to an integer too, like 3.
Math.round returns a long, which is an integral type. In your first code, you have the expression Math.round(pi1*100)/100. Math.round(...) returns an integer type, 100 is an integer literal, so integer division occurs.
However, in the second code, you first assigned the result of Math.round to p2. The long returned is implicitly converted to a double first, and stored in p2. You then wrote an expression in p2: p2/100. Here, one of the operands is double, so integer division does not occur.
Therefore, the one liner version that is the same as the second code is:
p2 = ((double)Math.round(pi1*100))/100;
You don't see the double conversion in the second code because it is done implicitly.
A note on rounding
This way rounding doubles should be used if you want to do calculations with the rounded number afterwards. If you just want to display a rounded number as output, you should use String.format, System.out.printf, or DecimalFormat. Read more about these methods here.

No mistake here, it works properly. Method Math.round(double) returns type long.
In your first variant, you receive result long 314 and divide it by 100 and get the result 3, and then assign it back to double.
In your second variant, you receive long result 314 and assign it back to double. Dividing double keeps precision as opposed to dividing long, so you get the correct result of 3.14
There for make p2= Math.round(pi1*100)/100; as
p2= Math.round(pi1*100) / 100.0;
Output -:
pi1 = 3.141592653589793
p2 = 3.14

Remove tailing ".0" from prices

How I can remove ".0" from my Price number from my android studio app?
MyApp save orders in database, then user can place order, after that server app get the orders placed.
I'm also using Retrofit2.
I have double prices 2000.0.
I want them without tailing 0, just 2000.
There are other sections with this problem, which I guess some guide about how to fix this part can help me to fix other parts.

There are 2 ways you can solve this.
(Adding to Andreas comment)
Either use int instead of double altogether.
Or you can always cast your double value in int.
double x = 1000.324;
System.out.println("double x = "+x);
System.out.println("int x = "+(int) x);
prints:
double x = 1000.324
int x = 1000

You could use :
new DecimalFormat("#").format(2000.0);
The result is "2000" without decimals.

There are a few things you can do do remove the numbers after the decimal and it depends on what you are trying to achieve.
You can cast them to an Int or use new DecimalFormat("#").format(value) which will remove anything after the .
You can use the Math.rint() or Math.round functions this will round to the closest integer 3.99 will become 4

Simply typecast your double value into int
double percentageValue=200.0;
System.out.println("remove value of after decimal point=="+(int)percentageValue);
and set value in android just like this:-
txt_product_price.setText(String.valueOf((int)percentageValue));

simply convert the Double value to Integer
Double x = 2000.0;
Integer y = (int) x;
Log.e("Integer format",String.valueOf(y));
and the Log is 2000

In Java how does one convert from declared int value then print out decimal approximations

This is to say that the int are put into a System.out.println(); method that divides them up. But the value is a whole number, how does one take what's outputted as a whole number and get a decimal value from the declare int values?
Thank you

Well there's a couple ways, but you just cast it:
int myInt = 42;
System.out.println((double) myInt);
//Formatter
System.out.printf("%.2f\n", (double) myInt); //control output, e.g. 2 decimal places

I believe the problem is that you have two integer values and you want to merge them to become one number, with one of the 2 integers in the decimal part?
Well, if that is your question, then you can solve it by a simple while loop.
double wholePart = (double) userInput;
double decimalPart = (double) userInput;
while(Math.abs(decimalPart)>=1)
decimalPart /= 10;
System.out.print(Math.sign(decimalPart * wholePart) (Math.abs(wholePart) + Math.abs(decimalPart)));
Edit2: Using absolute values

How to get java to produce decimal points while dividing

I'm making a basic calculator where you can plus, times, divide and minus as i was experimenting to see if it worked i noticed that instead of 5 divided by being equal to 1.25 it only displayed 1.
Here's the code i use to handle the math problems:
if (box.getSelectedItem().equals(divide)){
JOptionPane.showMessageDialog(null, Integer.parseInt(first.getText()) / Integer.parseInt(second.getText()), "Answer", -1);
main(args);
}
Is there code that displays the decimal points as well?

Since you are using Integer,it is happening.
Use Double to preserve decimals.
In your case,use
Double.parseDouble(first.getText()) / Double.parseDouble(second.getText())

Integer division will give you Integer. Try using Double or BigDecimal data type.

You need to do the casting
(double)parseInt(first.getText()) / (double)parseInt(second.getText())
Int/Int will give you an Integer. So you need to cast it to Double to get the result in decimal.
EDIT:
If you dont want to show decimal when the result is a whole number then you need to check it like this:
Double res = (double)parseInt(first.getText()) / (double)parseInt(second.getText())
Integer x;
if(res % 1 == 0)
{
x = (int)res
}

Can you replicate the Floor function found in Excel in Java?

I have searched the internet but have not found any solutions for my question.
I would like to be able to use the same/replicate the type of FLOOR function found in Excel in Java. In particular I would like to be able to provide a value (double or preferably BigDecimal) and round down to the nearest multiple of a significance I provide.
Examples 1:
Value = 24,519.30235
Significance = 0.01
Returned Value = 24,519.30
Example 2:
Value = 76.81485697
Significance = 1
Returned Value = 76
Example 3:
Value = 12,457,854
Significance = 100
Returned Value = 12,457,800
I am pretty new to java and was wondering if someone knew if an API already includes the function or if they would be kind enough to give me a solution to the above. I am aware of BigDecimal but I might have missed the correct function.
Many thanks

Yes you can.
Lets say given numbers are
76.21445
and
0.01
what you can do is multiply 76.21445 by 100 (or divide per 0.01)
round the result to nearest or lower integer (depending which one you want)
and than multiply it by the number again.
Note that it may not exactly print what you want if you will not go for the numbers with decimal precision. (The problem of numbers which in the binary format are not finite in extansion). Also in Math you have the round function taking doing pretty much what you want.
http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html you use it like this
round(200.3456, 2);
one Example Code could be
public static void main(String[] args) {
BigDecimal value = new BigDecimal("2.0");
BigDecimal significance = new BigDecimal("0.5");
for (int i = 1; i <= 10; i++) {
System.out.println(value + " --> " + floor(value, significance));
value = value.add(new BigDecimal("0.1"));
}
}
private static double floor(BigDecimal value, BigDecimal significance) {
double result = 0;
if (value != null) {
result = value.divide(significance).doubleValue();
result = Math.floor(result) * significance.doubleValue();
}
return result;
}

To round a BigDecimal, you can use setScale(). In your case, you want RoundingMode.FLOOR.
Now you need to determine the number of digits from the "significance". Use Math.log10(significance) for that. You'll probably have to round the result up.
If the result is negative, then you have a significance < 1. In this case, use setScale(-result, RoundingMode.FLOOR) to round to N digits.
If it's > 1, then use this code:
value
.divide(significance)
.setScale(0, RoundingMode.FLOOR)
.multiply(significance);
i.e. 1024 and 100 gives 10.24 -> 10 -> 1000.