Best practice for getting datatype size(sizeof) in Java

问题

I want store a list of doubles and ints to a ByteBuffer, which asks for a size to allocate. I'd like to write something like C's syntax

int size=numDouble*sizeof(double)+numInt*sizeof(int);

But there is no sizeof in Java. What is the best practice to calculate the size in byte? Should I hardcode it?

回答1:

(If you're using Java 8 or beyond, be sure to look at @Frank Kusters' answer!)

All of the primitive wrappers have a SIZE constant, which is in bits, not bytes.

So in the example given, it would be:

int size=(numDouble*Double.SIZE+numInt*Integer.SIZE) / Byte.SIZE;

Or, if you wanted to avoid the division:

int size=numDouble*(Double.SIZE/Byte.SIZE)+numInt*(Integer.SIZE/Byte.SIZE);

(Because the division is of two constants, it's done at compile-time.)

回答2:

Since Java 8, all wrapper classes of primitive types (except Boolean) have a BYTES field. So in your case:

int size = numDouble * Double.BYTES + numInt * Integer.BYTES;

Documentation: http://docs.oracle.com/javase/8/docs/api/java/lang/Integer.html

回答3:

Write your own method. In Java the datatypes are platform independent always the same size:

public static int sizeof(Class dataType)
{
    if (dataType == null) throw new NullPointerException();

    if (dataType == int.class    || dataType == Integer.class)   return 4;
    if (dataType == short.class  || dataType == Short.class)     return 2;
    if (dataType == byte.class   || dataType == Byte.class)      return 1;
    if (dataType == char.class   || dataType == Character.class) return 2;
    if (dataType == long.class   || dataType == Long.class)      return 8;
    if (dataType == float.class  || dataType == Float.class)     return 4;
    if (dataType == double.class || dataType == Double.class)    return 8;

    return 4; // 32-bit memory pointer... 
              // (I'm not sure how this works on a 64-bit OS)
}

Usage:

int size = numDouble * sizeof(double.class) + numInt * sizeof(int.class);

回答4:

A better solution might be to not emulate C syntax and use an ObjectOutputStream with a nested ByteArrayOutputStream to generate a byte array which can then be written to your ByteBuffer.

回答5:

The size in Java is always the same. You can hardcode it but you only need to do this because you are working with bytes in a ByteBuffer. If you use double[] or DoubleBuffer you don't need these.

回答6:

You can also use the sizeof4j library to get the sizeof the double you just need SizeOf.doubleSize()

来源：https://stackoverflow.com/questions/6766343/best-practice-for-getting-datatype-sizesizeof-in-java

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