what is the size of an object?

can anyone tell me the size of an object
as premitives got the size why dont objects have some constant value(size)?

You mean the number of bytes that an object takes up in memory?

There's no way to find that out in the standard Java API. Java does not have a "sizeof" operator, like you have in C and C++.

Note that the size of an object in memory depends on the particular implementation of the JVM you are using. It might be different for the Sun JVM on Windows and the HP JVM on HP-UX, for example.

If you really are desperate to find this out in your Java application, I think there is a way to find out how many bytes an object takes up via the debugger API, but that API is not easy to use.

How can objects have predefined size as it really depends on the number and type of instance variable it holds.
[ August 23, 2006: Message edited by: Ela Jha ]

Originally posted by saikrishna cinux:
can anyone tell me the size of an object
as premitives got the size why dont objects have some constant value(size)?

Dear Saikrishna,

You can use the code below:

/**
* Measures the approximate size of an object in memory, given a Class which
* has a no-argument constructor.
*/
public final class ObjectSizer {
 
/**
* First and only argument is the package-qualified name of a class
* which has a no-argument constructor.
*/
public static void main(String...aArguments) {
Class theClass = null;
try {
theClass = Class.forName(aArguments[0]);
} catch (Exception ex) {
System.err.println("Cannot build a Class object: " + aArguments[0]);
System.err.println(
"Use a package-qualified name, and check classpath.");
}
long size = ObjectSizer.getObjectSize(theClass);
System.out.println(
"Approximate size of " + theClass + " objects :" + size);
}
 
/**
* Return the approximate size in bytes, and return zero if the class
* has no default constructor.
*
* @param aClass refers to a class which has a no-argument constructor.
*/
public static long getObjectSize(Class aClass) {
long result = 0;
 
//if the class does not have a no-argument constructor, then
//inform the user and return 0.
try {
aClass.getConstructor(new Class[] {
});
} catch (NoSuchMethodException ex) {
System.err.println(
aClass + " does not have a no-argument constructor.");
return result;
}
 
//this array will simply hold a bunch of references, such that
//the objects cannot be garbage-collected
Object[] objects = new Object[fSAMPLE_SIZE];
 
//build a bunch of identical objects
try {
Object throwAway = aClass.newInstance();
 
long startMemoryUse = getMemoryUse();
for (int idx = 0; idx < objects.length; ++idx) {
objects[idx] = aClass.newInstance();
}
long endMemoryUse = getMemoryUse();
 
float approximateSize = (endMemoryUse - startMemoryUse) / 100f;
result = Math.round(approximateSize);
} catch (Exception ex) {
System.err.println("Cannot create object using " + aClass);
}
return result;
}
 
// PRIVATE //
private static int fSAMPLE_SIZE = 100;
private static long fSLEEP_INTERVAL = 100;
 
private static long getMemoryUse() {
putOutTheGarbage();
long totalMemory = Runtime.getRuntime().totalMemory();
 
putOutTheGarbage();
long freeMemory = Runtime.getRuntime().freeMemory();
 
return (totalMemory - freeMemory);
}
 
private static void putOutTheGarbage() {
collectGarbage();
collectGarbage();
}
 
private static void collectGarbage() {
try {
System.gc();
Thread.currentThread().sleep(fSLEEP_INTERVAL);
System.runFinalization();
Thread.currentThread().sleep(fSLEEP_INTERVAL);
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
}

Here is the Interpreter class :

import java.util.*;
import java.text.MessageFormat;
 
/**
* Given a package-qualified class name, return the approximate size of
* the object in bytes.
*/
public final class ObjectSizeInterpreter implements Interpreter {
 
/**
* @param aLine is a non-null, package-qualified name of a class.
* @param aResult is a non-null, empty List which acts as an "out"
* parameter; when returned, aResult must contain a non-null, non-empty
* List containing a description of the size of the object.
*
* @return true only if the user has requested to quit the Interpreter.
* @exception IllegalArgumentException if a param does not comply.
*/
public boolean parseInput(String aLine, final List aResult) {
if (aResult == null) {
throw new IllegalArgumentException("Result param cannot be null.");
}
if (!aResult.isEmpty()) {
throw new IllegalArgumentException("Result param must be empty.");
}
if (aLine == null) {
throw new IllegalArgumentException("Line must not be null.");
}
 
boolean hasRequestedQuit =
aLine.trim().equalsIgnoreCase(fQUIT)
|| aLine.trim().equalsIgnoreCase(fEXIT);
if (hasRequestedQuit) {
//display only a blank line
aResult.add(fNEW_LINE);
} else {
try {
Class theClass = Class.forName(aLine);
long size = ObjectSizer.getObjectSize(theClass);
if (size > 0) {
Object[] insertedData = { theClass, new Long(size)};
MessageFormat sizeMessage = new MessageFormat(fPATTERN);
String message = sizeMessage.format(insertedData);
aResult.add(message);
aResult.add(fNEW_LINE);
}
aResult.add(fDEFAULT_PROMPT);
} catch (ClassNotFoundException ex) {
//recover by asking the user for corrected input
aResult.clear();
aResult.add(fERROR_PROMPT);
}
}
 
if (aResult.isEmpty()) {
throw new IllegalStateException("Result must be non-empty.");
}
return hasRequestedQuit;
}
 
/**
* Return the text to be displayed upon start-up of the Interpreter.
*/
public String getHelloPrompt() {
return fHELLO_PROMPT;
}
 
/// PRIVATE /////
private static final String fHELLO_PROMPT = "Please enter a class name>";
private static final String fDEFAULT_PROMPT = "Please enter a class name>";
private static final String fERROR_PROMPT =
"Invalid.  Example:\"java.lang.String\">";
private static final String fPATTERN =
"Approximate size of {0} objects in bytes: {1}";
private static final String fQUIT = "quit";
private static final String fEXIT = "exit";
private static final String fNEW_LINE =
System.getProperty("line.separator");
}

Now you compile both class and try to run like following

Hope this helps you out.

Alternatevely check this link ..

This may be usefull for you.

Do you know your data size?

thank's a lot for your code
but if any one ask me about the object size what should tell them?

Tell them the question does not have a simple answer.

Bill