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javasizeof
Wei Shi
Wei Shi

Reputation: 5055

Best practice for getting datatype size(sizeof) in Java

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?

Upvotes: 31

Views: 74536

Answers (7)

Ed Staub
Ed Staub

Reputation: 15690

See @Frank Kusters' answer, below!

(My original answer here was for Java versions < 8.)

Upvotes: 49

azis.mrazish
azis.mrazish

Reputation: 105

Automated and abstract solution is to write the sample to DataOutput and see the resulted size.

Upvotes: 0

Artur Mkrtchyan
Artur Mkrtchyan

Reputation: 981

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

Upvotes: 1

Frank Kusters
Frank Kusters

Reputation: 2634

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

Upvotes: 39

drewjd
drewjd

Reputation: 51

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.

Upvotes: 5

Martijn Courteaux
Martijn Courteaux

Reputation: 68907

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);

Upvotes: 9

Peter Lawrey
Peter Lawrey

Reputation: 533790

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.

Upvotes: 3

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