Question about sizeof. I want to reverse bits in number.

Question

What will be equivalent of this in Java?

for (i = (sizeof(num)*8-1); i; i--)  

num is given number, not array. I want to reverse bits in integer.

Answer 1

This loop is likely iterating over an array in reverse order. In this case, it is an array of 'num' objects, and there are 8 elements in the array (the '-1' is necessary because an array of 8 elements has valid indices 0...7).

To do that in Java, the equivalent would be:

for(int i = array.length-1; i >= 0; --i)

Answer 2

Java does not have sizeof. Arrays have the length property, and many collections have size() and similar things like that, but a linguistic sizeof for any arbitrary object is both not supported and not needed.

Related questions

• Is there any sizeof-like method in Java?
• In Java, what is the best way to determine the size of an object?

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Getting bits of an integer in LSB-MSB order

To get the bits of an integer from its least significant bit to its most significant bit, you can do something like this:

    int num = 0xABCD1234;
    System.out.println(Integer.toBinaryString(num));
    for (int i = 0; i < Integer.SIZE; i++) {
        System.out.print((num >> i) & 1);
    }

This prints:

10101011110011010001001000110100    // MSB-LSB order from toBinaryString
00101100010010001011001111010101    // LSB-MSB order from the loop

So in this specific case, the sizeof * 8 translates to Integer.SIZE, "the number of bits used to represent an int value in two's complement binary form". In Java, this is fixed at 32.

JLS 4.2.1 Integral types and values

For int, from -2147483648 to 2147483647, inclusive

Answer 3

in C/C++, the sizeof operator tells you how many bytes a variable or type takes on the current target platform. That is to say, it depends on the target platform, and therefore there is a keyword for discovering it.

Java targets a virtual machine, and the size of types is constant.

If num is an int, it is 4 bytes (32-bits). If it is long, it is 8 bytes (64 bits).

Furthermore, you cannot treat a variable as an array of bytes. You have to use bitwise operators (shifts, and, or etc) to manipulate the bits in a primitive like an int or long.

Answer 4

There isn't a direct equivalent. The sizeof returns the size of a type or the type of the expression in bytes, and this information is not available in Java.

It's not required as the sizes in bytes of the built-in types are fixed, lengths of arrays are obtained using the .length psuedo-field, and memory for objects is allocated using new, so the object size is not required.

If you tell use what the type of num is, then it can be translated.

Answer 5

In addition to polygenelubricants' answer, there's another way to reverse the bits of an integer in Java:

int reversed = Integer.reverse(input);

Easy!

It's worth checkout the source code for Integer.reverse, it's rather nifty (and extremely scary).