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How to convert long to int in Java vs. swift?

I tried converting this code to Java in swift 2.0

long checksumValue = 4253726258L; int liCrc32 = (int)checksumValue; System.out.println("liCrc32" + liCrc32)

LiCrc32 is -41241038

My quick version

 let int64: Int64 = 4253726258 let a:Int = Int(int64)

a returns me 4253726258

What I don’t understand, why is this negative value in Java?

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3 answers

You are using the wrong type: Swift Int does not necessarily correspond to Java Int because its size is platform dependent. Judging by your result, your platform uses 64-bit Int s. Here's how to confirm it:

 println(sizeof(Int)) // Should print 8 on your platform

Int32 corresponds to Java Int . Sorry, this change

 let int64: Int64 = 4253726258 let a:Int32 = Int32(int64)

may cause problems due to an error in the implementation of Swift.

If you need to sign an extended 32-bit value, you can do it as follows:

 let int64: Int64 = 4253726258 let a:Int = Int((int64 << 32) >> 32) println(a) // prints -41241038

Demo version

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From the Swift documentation

Int

In most cases, you do not need to select a specific size of integers to use in your code. Swift provides an additional integer type, Int, which is the same size as the current size of the native platform language:

  • On a 32-bit platform, Int is the same size as Int32 .

  • On a 64-bit platform, Int is the same size as Int64 .

If you don't need to work with a specific size of integers, always use Int for integer values ​​in your code. This helps to harmonize code and compatibility. Even on 32-bit platforms, Int can store any value between -2,147,483,648 and 2,147,483,647 and is large enough for many integer ranges.

Java Int , on the other hand, is always 4 bytes.

Integrated types byte , short , Int and long , the values ​​of which are 8-bit, 16-bit, 32-bit and 64-bit signed integers with two additions, respectively, [...]

Listing from long to Int narrows the primitive conversion

Narrowing a primitive conversion may lose information about the total value of a numerical value, and may also lose accuracy and range.

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int: By default, the int data type is a 32-bit signed integer with two values, which has a minimum value of -231 and a maximum value of 231-1. In Java SE 8 and later, you can use the int data type to represent a 32-bit unsigned integer that has a minimum value of 0 and a maximum value of 232-1. Use the Integer class to use int data, enter as an unsigned integer. See the “Number Classes” section for more information. Static methods such as compareUnsigned, divideUnsigned, etc. have been added to the Integer class to support operation arithmetic for unsigned integers.

long: The long data type is a 64-bit integer from two additions. The recorded long has a minimum value of -263 and a maximum value of 263-1. In Java SE 8 and later, you can use a long data type to represent an unsigned 64-bit long, which has a minimum value of 0 and a maximum value of 264-1. Use this data type when you need a range of values ​​than those provided by int. The Long class also contains methods such as compareUnsigned, divideUnsigned, etc. to support arithmetic operations for unsigned long.

The value that int in Java <J8 can be maximal:

 2.147.483.647

which is smaller than your int: 4.253.726.258

To do this, it cannot fit into int , you need to either stick to long , or upgrade Java to version 8 and make unsigned int.

What happens with the conversion is that after 2.147.483.647 the counter starts with the minimum number (-2.147.483.647) and finally gives you -41241038 , which is -2.147.483.647+ (4.253.726.258 - 2.147.483.647)

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