Different rounding methods don't seem to work

Question

The following code:

    DecimalFormat df = new DecimalFormat();
    df.setMinimumFractionDigits(2);

    int zahl1 = 10;
    int zahl2 = 18;

    double double1 = zahl1 / zahl2;
    System.out.println("Double1 = " + df.format(double1));

    double double2 = Math.ceil(zahl1 / zahl2);
    System.out.println("Double2 = " + df.format(double2));

    double double3 = Math.round((zahl1 / zahl2) * 100) / 100;
    System.out.println("Double3 = " + df.format(double3));

    float float1 = Math.round((zahl1 / zahl2) * 100) / 100;
    System.out.println("Float1  = " + df.format(float1));

    long long1 = zahl1 / zahl2;
    System.out.println("long1 = " + df.format(long1));

    long long3 = Math.round((zahl1 / zahl2) * 100) / 100;
    System.out.println("long3 = " + df.format(long3));

renders the following output:

Double1 = 0,00
Double2 = 0,00
Double3 = 0,00
Float1  = 0,00
long1 = 0,00
long3 = 0,00

I read quite a bit now about floats and doubles and what not in Java, but I don't seem to fully understand how to get to my desired outcome of 0.6.

What am I doing wrong?

Answer 1

Since zahl1 and zahl2 are of type int, the following:

double double1 = zahl1 / zahl2;

uses integer division, the result of which is 0.

It does not matter that you're assigning the result to a variable of type double: the (integer) division happens first, and its result is then widened to a double.

To fix, change that line to:

double double1 = zahl1 / (double)zahl2;

Answer 2

You have to cast an Integer when dividing it by another Integer to float:

(float)zahl1 / (float)zahl2