Java limitaion

[Pojahn_M]

The only limitation java have(that I know of) is OutOfMemeoryException heap space.
What is the max number of characters you can put in a string before running out of heap space?

[stchman]

The only limitation java have(that I know of) is OutOfMemeoryException heap space.
What is the max number of characters you can put in a string before running out of heap space?

I would gather that would depend on how much RAM the computer has.

[Pojahn_M]

but doesnt heap space have a default value for all computers?

[sunde887]

I believe you can change the amount of heap space. I'd suggest writing an experiment, get a stringbuilder and continuously concatenating random characters to it.

[Pojahn_M]

doing it right now(with a string).
Right now, I am at 17000 characters. I report back when I reach the limit.

[Pojahn_M]

I canceled the execution after the string was 2 million characters long. It was so slow appending character to such a long string.

[stchman]

I wrote this small program to see how many items I could add to an ArrayList to see how big it could get before I exceeded the amount of memory.

import java.util.ArrayList;

public class MemoryLimit {
    public static void main( String[] args ) {
        MemoryLimit m = new MemoryLimit();
        m.memoryLimit();
    }
    
    public void memoryLimit() {
        ArrayList<Character> list = new ArrayList<Character>();
        
        int i = 0;

        try {
            while( true ) {
                list.add( 'a' );
                i++;
            }
        } catch( OutOfMemoryError e ) {
            System.out.printf( "Number of one byte characters added to an ArrayList = %d\n", i );
        }        
    }
}

The output is as follows:

Number of one byte characters added to an ArrayList = 25764983

It seems I was able to add nearly 26 million characters to the ArrayList before running out of memory.

The PC I used was a Core 2 Quad machine running Windows XP with 4GB RAM.

I was wondering if someone more knowledgeable about computer or OS memory architecture could explain this.

Thanks.

[sunde887]

I can't vouch for the accuracy, but here is a link which may be helpful: [JavaSpecialists 029] - Determining Memory Usage in Java

[sunde887]

The memory tester from that article and the following Object Factory,

import java.util.*;

public class ListFactory implements ObjectFactory{
	public Object makeObject(){
		ArrayList<Character> charList = new ArrayList<Character>();
		for(int i = 0; i < 25764982; ++i)
			charList.add((char)33);
		return charList;
	}
}

produces this:

ListFactory produced java.util.ArrayList which took 103059968 bytes

103,059,968

The problem was that a character takes up more than a byte so using the counter directly as you did will cause some problems.

[stchman]

I just ran the same program on my Core2Duo laptop running Ubuntu 10.04 64bit with 4GB RAM and I got the following:

Number of one byte characters added to an ArrayList = 86956820

[Fabken]

I ran the same problem on my six core amd phenom 2 1055t computer with 6 gigs of ram and I got 25764983 as output.

[Tolls]

Maximum heap size is (currently) 1/4 of your physical memory or 1Gb, whichever is smaller.
This can be changed (as said earlier) by using Xmx.

[stchman]

I wonder do the makers of the JRE set these heap sizes based on OS? The Linux 64 bit OS has a little over 3X the heap space. I don't know what OS Fabken is running, but I would guess Windows 7 64 bit, but I could be wrong.

[Tolls]

Not according to the documentation...of course it all depends if you are both using Oracle JVMs.

besides, I wouldn't rely on the above code...I'd use the Runtime.maxMemory() method, which will tell you what Xmx is set to, pretty much.

[stchman]

I ran the following code on my Windows XP machine:

public void memoryUsage() {
    long mem = Runtime.getRuntime().maxMemory();
    System.out.printf( "Number of bytes the JRE will attempt to use = %d bytes.\n", mem );
}

I got the following output:

Number of bytes the JRE will attempt to use = 259522560 bytes.

[Tolls]

Which implies a 1/4 Gb (ish) Xmx by default.
Which seems a bit odd.
Does it have dual processors (or dual core) and only about 1Gb of memory?

[stchman]

I ran the program on my Core 2 Quad running Ubuntu 10.04 64bit with 6GB RAM

import java.util.ArrayList;

public class MemoryLimit {
    public static void main( String[] args ) {
        MemoryLimit m = new MemoryLimit();
        m.memoryLimit();
        m.memoryUsage();
    }
    
    public void memoryLimit() {
        ArrayList<Character> list = new ArrayList<Character>();
        
        int i = 0;

        try {
            while( true ) {
                list.add( 'a' );
                i++;
            }
        } catch( OutOfMemoryError e ) {
            System.out.printf( "Number of one byte characters added to an ArrayList = %d\n", i );
        }        
    }
    
    public void memoryUsage() {
        long mem = Runtime.getRuntime().maxMemory();
        System.out.printf( "Number of bytes the JRE will attempt to use = %d bytes.\n", mem );
    }
}

I got the following output:

Number of one byte characters added to an ArrayList = 130435231
Number of bytes the JRE will attempt to use = 1394409472 bytes.

[Tolls]

The maxmem is about correct.
Dual core counts as 2 processors for the calculation, so it uses about a quarter.
Exactly what counts as available memory is probably down to what the OS considers telling the JVM is available.
In this case about half a gig short of the 6 gig.