Is There Any Sizeof-Like Method in Java

Is there any sizeof-like method in Java?

No. There is no such method in the standard Java SE class library.

The designers' view is that it is not needed in Java, since the language removes the need for an application¹ to know about how much space needs to be reserved for a primitive value, an object or an array with a given number of elements.

You might think that a sizeof operator would be useful for people that need to know how much space their data structures take. However you can also get this information and more, simply and reliably using a Java memory profiler, so there is no need for a sizeof method.

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Previous commenters made the point that sizeof(someType) would be more readable than 4. If you accept that readability argument, then the remedy is in your hands. Simply define a class like this ...

public class PrimitiveSizes {
    public static int sizeof(byte b) { return 1; } 
    public static int sizeof(short s) { return 2; }
    // etcetera
}

... and statically import it ...

import static PrimitiveSizes.*;

Or define some named constants; e.g.

public static final int SIZE_OF_INT = 4;

Or (Java 8 and later) use the Integer.BYTES constant, and so on.

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Why haven't the Java designers implemented this in standard libraries? My guess is that:

• they don't think there is a need for it,
• they don't think there is sufficient demand for it, and
• they don't think it is worth the effort.

There is also the issue that the next demand would be for a sizeof(Object o) method, which is fraught with technical difficulties.

The key word in the above is "they"!

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^{1 - A programmer may need to know in order to design space efficient data structures. However, I can't imagine why that information would be needed in application code at runtime via a method call.}

Best practice for getting datatype size(sizeof) in Java

See @Frank Kusters' answer, below!

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

sizeof Java object

The question is not meaningful, at least not without further context.

The notion of "size" in Java is only reasonably well defined for primitives: A byte is 8 bit (unsurprisingly) an int is 32 bit, a long 64bit, etc. (see e.g. http://download.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html for a full list).

For object instances, it's more complicated, because:

• Object instances can (and usually will) contain references to other instances internally, so you must decide whether to count these dependent instances, and how. What if several instances share a dependency?
• Sometimes, object instances may be reused (e.g. interning of java.lang.String, see http://en.wikipedia.org/wiki/String_interning ). So if you use x objects of size y, the total size may be smaller than x*y
• The JVM has a lot of leeway about how to implement objects and instances internally.
  It may use different techniques for different instances (e.g. sharing internal data structures), so there may not even be a meaningful "size" to assign to a single object.

Maybe you could explain why you are interested in object sizes.

There are some rules of thumb for estimating the heap memory used by instances (e.g. in the Sun JVM, a java.lang.Object instance uses 8 byte), but these will depend on the JVM you use.

Generally, if you want to know about your heap usage, use a memory / heap profiler.

Edit:

Well, there is (as of JDK 6) a way to get an approximation of the amount of memory used by an object: http://download.oracle.com/javase/6/docs/api/java/lang/instrument/Instrumentation.html#getObjectSize%28java.lang.Object%29

It's still only an approximation...

Is there a simple way to get the size of a java object?

There is an opensource java.SizeOf project that determines size of any of your Java object in memory.

How big is an Object? Why is there no sizeof?

You've kind of answered your own question, in c you manage memory in java the jvm does.

In c you're directly allocating the memory for the data structures you're storing (malloc) and so you often need to know the sizes of these things.

In java the memory system is largely abstracted away so you don't (usually) care you just call new and it'll do whatever it does and if different jvms do things differently the memory used by the classes you've declared may vary.

Sometimes it is useful to know how much memory your classes are taking up (say you're trying to reduce your memory footprint in a tightly constrained environment) but it's pretty rare that you'd need that sort of information at runtime.

In Java, what is the best way to determine the size of an object?

You can use the java.lang.instrument package.

Compile and put this class in a JAR:

import java.lang.instrument.Instrumentation;

public class ObjectSizeFetcher {
    private static Instrumentation instrumentation;

    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
    }

    public static long getObjectSize(Object o) {
        return instrumentation.getObjectSize(o);
    }
}

Add the following to your MANIFEST.MF:

Premain-Class: ObjectSizeFetcher

Use the getObjectSize() method:

public class C {
    private int x;
    private int y;

    public static void main(String [] args) {
        System.out.println(ObjectSizeFetcher.getObjectSize(new C()));
    }
}

Invoke with:

java -javaagent:ObjectSizeFetcherAgent.jar C

Find the Size (or Length) of custom datatype

Update: I can't use size() because when I write it, it will gives warning cannot resolve method 'size()'

Ensure you are invoking the size() method from the correct instance. This is an example of how you can get the size from an arraylist:

ArrayList<Card> hisCards = new ArrayList<Card>();
int numOfCards = hisCards.size();    //get number of elements from hisCards

Edit: You are invoking size() on a Card object instead of an arraylist !

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Change your method to:

public static int checkColumn(ArrayList<Card> cardArray)

Java String array: is there a size of method?

Yes, .length (property-like, not a method):

String[] array = new String[10];
int size = array.length;

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