Java Double到Int的类型转换

Question

我正在尝试将2147483648的双精度值转换为整数，进行类型转换后，我得到的输出为2147483647，该值减少1。我知道这是由于溢出而发生的，但是有一种方法可以转换它转换为int类型而不降低其精度？

Answer 1

As aforementioned in previous answers, you can use the long primitive data type, or the BigInteger reference type. Using integral data types rather than floating points would be better for representing integers exactly.

As a side note, since Java SE 8, one can use the Integer class to use int for unsigned arithmetic. Per the Oracle doc - "Use the Integer class to use int data type as an unsigned integer". Unsigned integers have a greater maximum value than int. This question can be of use: Declaring an unsigned int in Java .

This is my first answer, hope you solved your problem! :)

Answer 2

I can think of 2 options.

Long

Unlike int , which is a 32-bit signed integer data type, long is a 64-bit signed integer data type. This means that the largest value it can store is 9223372036854775807 . The number that you are trying to store, 2147483648, is well inside that range.

BigInteger

This is a reference type that represents an "immutable arbitrary-precision integer". You can create a BigInteger instance that represents 2147483648 by doing

new BigInteger("2147483648")

Learn more about it here: https://docs.oracle.com/javase/7/docs/api/java/math/BigInteger.html

Answer 3

Max Values

The maximum integer, as indicated in the Integer class by the static final int MAX_VALUE , is 2^31-1 (2,147,483,647). This value is the maximum integer as it is the largest 32-bit signed integer.

The maximum double, as indicated in the Double class by the static final int MAX_VALUE , is (2-2^(-52))(2^1023). The double data type follows the double-precision 64-bit IEEE 754 floating point format to express a wide range of dynamic numerical values.

Narrowing Primitive Conversion

In you're observation, you have a double with a value 2,147,483,648 which you attempt to convert to an integer by type casting.

double d = 2147483648;
int i = (int) d;

Casting between primitive types allows your to convert the value of one primitive type to another. Converting from a double to an integer is known as a Narrowing Primitive Conversion , wherein you:

"may lose information about the overall magnitude of a numeric value and may also lose precision and range."

The narrowing conversion of a floating-point number to an int is as follows:

• If the floating-point number is NaN, the result is an int of 0.
• Otherwise, if the floating-point number is not an infinity, the floating-point number is rounded to an integer value V, rounding toward zero using IEEE 754 round-toward-zero mode. If this integer value can be represented as an int, then the result is V.
• Otherwise, one of the following two cases must be true: a. The value must be too small and the result is the smallest representable value of type int or b. The value must be too large and the result is the largest representable value of type int.

With the integer value of the double value larger than the Integer.MAX_VALUE, in order to allow for representation as an integer the value of Integer.MAX_VALUE is used.

Avoid Loss

To avoid the loss of precision you will need to either cast to a primitive type or a numeric Object wherein the maximum value is greater than 2147483648 (and resolution allows for accuracy to be maintained).

The long primitive type has a maximum value of 2^63-1 (9.223372e+18), which would be a good choice if you want to use numbers within the numeric integer space. Note that while the Long.MAX_VALUE is very large, Double.MAX_VALUE is MUCH larger due to the floating-point format.

double d = 2147483648;
long i = (long) d;

Answer 4

Converting from a double to an int will lose precision for numbers greater than Integer.MAX_VALUE or less that Integer.MIN_VALUE . There is no way to represent numbers outside that range as an int . It is mathematically impossible.

Converting from a double to an long will also lose precision. This will occur for all integers outside of Long.MIN_VALUE through Long.MAX_VALUE . But a second problem is that double itself is not able to represent all integers in that range ... so there will be loss of precision before the conversion ¹ .

Moral:

Don't use floating point numbers if you need to represent integers precisely. Use an integral type ( byte , short , char , int or long ) ... or BigInteger .

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^{1 - A double is a 64 bit IEE floating point number, which has 52 bits of precision and a sign bit. By contrast, a long has 64 bits of precision (including sign).}