In this article, we explain what happens when Java attempts to assign a double/float value to an int type, a float value to a double variable and vice versa. We also discuss how expressions containing different numeric types (double and int, say) are evaluated in Java.

- slide 1 of 4
### Assignment of double/float to integer types

In the article on Java Integer Arithmetic, we showed how Java performs arithmetic with integer values. We now show how it is done with floating-point numbers.

In Java,

**double**to**int**(and int to double) conversions are sometimes necessary. For example, we may calculate a percentage but are interested only in the whole number part of the result. For instance, the result may be 64.37% but we don't care about the fraction. Our natural instinct is to assign the**double**value to an**int**variable.However, Java does

*not*let us assign a floating-point value (float or double) directly to an integer variable (byte, short, int or long).For example, consider:

int n = 987.654321; //cannot assign double to int

long m = 3.24f; //cannot assign float to long

None of these statements is valid in Java. We can use a cast to make them valid, as in:

int n = (int) 987.654321;

long m = (long) 3.24f;

Now, 987 is assigned to

**n**and 3 is assigned to**m**.When we assign a floating-point value to an integer type, the fractional part, if any, is dropped (not rounded) and the resulting integer value is assigned. It is up to us to ensure that the integer obtained is small enough to fit in the integer type. If not, the resulting value is unpredictable.

For example, the largest byte value is 127. The statement

byte b = (byte) 300.75;

is valid and will attempt to store the value 300 in the byte variable

**b**. Since this is too big to fit in**b**, the value assigned is unpredictable. In this case, the value assigned is 44.As another example, the statement

short s = (short) 65585.36;

will store 49 in

**s**; 65585 is too big to fit in a short variable. Here, the low-order 16 bits of 65585 (which happens to be 49) is stored in**s**.Suppose the double variable

**d**has the value 987.654321. If we want the*rounded*value of**d**(988) stored in the**int**variable**n**, we could do this withn = (int) (d + 0.5);

If the first digit after the point in

**d**is 5 or more, adding 0.5 would add 1 to the whole number part. If the first digit after the point is less than 5, adding 0.5 would not change the whole number part.For example, if

**d**is 245.75, adding 0.5 would give 246.25 and 246 would be assigned to**n**. But if d were 245.49, adding 0.5 would give 245.99 and 245 would be assigned to**n**. - slide 2 of 4
### Assignment between double and float

As expected, you can store a

**float**value in a**float**variable and a**double**value in a**double**variable. Since**float**is smaller than**double**, Java allows you to store a**float**value in a**double**variable without any problems. However, if you try to assign a**double**value to a**float**variable, Java will complain. Consider:double d = 987.654321;

float x = d; //this is wrong; can’t assign double to float

The second statement will give an error message since Java will recognize that precision could be lost when assigning a “bigger" value (

**double**) to a “smaller" variable (**float**). However, if we are sure that that is what we want to do, we can use a*cast*to tell Java that we know what we are doing and are aware of the risk. We must write the statement as:float x = (float) d;

The construct

**(float)**is called a*cast*and it forces the**double**value**d**to be converted to**float**. A cast consists of a type name enclosed in brackets.However, since a

**float**variable allows only about 6 or 7 digits of precision, we should expect that the value of**d**may not be assigned precisely to**x**. Indeed, when run using one compiler, the value 987.654297 was assigned to**x**. When**d**was changed to 987654321.12345, the value assigned was 987654336.000000. In both cases, about 6 or 7 digits of precision were retained.As an exercise, see what values would be printed using your compiler.

Recall (from the article Floating-point Numbers and Expressions) that Java automatically treats a floating-point constant as a

**double**type, unless it is followed by**f**or**F**. We must be careful with statements such as:float x = 3.64; //wrong

Since 3.64 is a

**double**constant, Java will not allow us to assign it directly to a**float**variable. We can make the assignment valid by using either of the following:x = (float) 3.64; //use a cast to convert to float

x = 3.64f; //write 3.64 as a float constant

- slide 3 of 4
### Expressions with integer and floating-point values

It is quite common to use expressions involving both integer and floating-point values, for example,

**a / 3**where**a**is**float****n * 0.25**where**n**is**int**In Java, the rule for such expressions is:

If either operand of an arithmetic operator is floating-point, the calculation is done in floating-point arithmetic.

In the first example above, the integer 3 is converted to

**float**and the calculation is done in**float**. In the second example,**n**is converted to**double**(since 0.25 is**double**) and the calculation is done in**double**.If the expression contains floating-point values only, the calculation is done in

**double**unless*both*operands are**float**, in which case the calculation is done in**float**. This can be summarized as follows (click image for larger view):float float float

float double double

double float double

double double double

For example, if

**g**is**float**, the expression**g/36.0**is done in**double**since 36.0 is a**double**constant.How do we get the exact value of an integer division, 19/5, say? We can force a double precision calculation by writing one or both constants as

**double**, thus: 19/5.0, 19.0/5 or 19.0/5.0. We can also use a cast, as in(double) 19 / 5

A cast allows us to force the conversion of one type to another. Here, the

**int**19 is cast to**double**, forcing 5 to be converted to**double**and a double precision division is performed.However, we must be careful with a construct like

(double) (19 / 5)

since it may not do what we think. This does NOT do a floating-point division. Since both constants are integer, the expression inside the brackets is evaluated as an integer division (which discards the remainder), giving 3;

*this*value is converted to**double**, giving 3.0. - slide 4 of 4
### References

Java Programming – A Beginner’s Course

Related programming references

Article: Java double to int

#### Introduction to Java Programming

- Introduction to Java Programming - An Overview
- Java - Data, Variable and Algorithm Explained To A Beginner
- Java Example: Algorithm and Program For Area of Square
- Java Programming For Beginners - Test, Debug, Document, Maintain
- JDK Java Compiler: The Java Development Kit
- Java Programming For Beginners - How To Compile And Run Java Programs
- Data Types, Constants And Variables
- Java Programming For Beginners - Characters and printf
- Java Programming For Beginners - Part 9
- Java Programming For Beginners - Part 10
- Java Programming For Beginners - Part 11
- Java Programming For Beginners - Part 12
- Java Programming For Beginners - Part 13
- Java Programming For Beginners - Part 14
- Java Programming For Beginners - Integer Data Types
- Java Programming for Beginners - Part 16
- Java Integer Arithmetic For Beginners
- Java Programming For Beginners - Part 18
- Java Programming For Beginners - Part 19
- Java double to int and Other Conversions