Utilizing max memory using strings

Question

Is there any way to utilize maximum memory of the system using strings? I'm using runtime to display the free memory. I've tried with this code:

class Mem{
       public static void main(String[] args) {
              System.out.println(Runtime.getRuntime().maxMemory());
              System.out.println(Runtime.getRuntime().totalMemory());
              System.out.println(Runtime.getRuntime().freeMemory());
              String str=new String("Hi");
              for(long i=0;i<1000000;i++){
                     str+="aa";
                     //System.out.println(i);
              }      
              System.out.println(Runtime.getRuntime().freeMemory());
       }
}

However, the garbage collector comes into action for every few iterations and frees up the memory. Is is possible to make it utilize maximum memory and display the free memory before gc frees it up?

Peter Lawrey · Accepted Answer · 2013-01-26 11:04:15Z

4

A way to allocate and hold a large amount of memory quickly is to do

List<byte[]> bytes = new ArrayList<>();
for(int i = 0; i < 1000; i++)
    bytes.add(new byte[10000000]); // 10 MB

answered Jan 26, 2013 at 11:04

Peter Lawrey

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4 Comments

Víctor Herraiz Over a year ago

Hehe, nice way to waste resources in a fragmented way, +1 from me.

Víctor Herraiz Over a year ago

you could catch the firsts OutOfMemory and reduce the size of the array geometrically to scratch the last bits ...

Peter Lawrey Over a year ago

There arrays will go straight into tenured space continuously. i.e. there shouldn't be much fragmentation.

Víctor Herraiz Over a year ago

I see, then could a linked list be a better option ?

NPE · Accepted Answer · 2013-01-26 10:19:54Z

3

At the end, your string only contains 2 million characters. If you want to make your code run out of memory, change

        str += "aa";

to

        str += str;

That'll make the string grow exponentially, and no amount of garbage collection will help even with a modest number of iterations.

edited Jan 26, 2013 at 10:19

answered Jan 26, 2013 at 10:06

NPE

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2 Comments

assylias Over a year ago

That should go faster indeed!

vidya sagar Over a year ago

yeah, of course!! str+=str did it.why didn't i think of that? But its hell lot slower than I thought. I included the try/catch block as suggested by assylias and caught the OutOfMemoryException even for as few iterations as 50!!

assylias · Accepted Answer · 2013-01-26 10:06:02Z

3

str+="aa"; creates a new string each time and reassigns str so the old string is eligible to garbage collection.

However it will run out of memory at some stage if you iterate enough.

You should put the for loop in a try/catch block, catch OutOfMemoryError and include your print statement in the catch block.

answered Jan 26, 2013 at 10:06

assylias

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Comments

Evgeniy Dorofeev · Accepted Answer · 2013-01-26 10:33:13Z

3

try something like

    System.out.println(Runtime.getRuntime().maxMemory());
    System.out.println(Runtime.getRuntime().totalMemory());
    System.out.println(Runtime.getRuntime().freeMemory());
    String str = new String(new char[32_000_000]);
    System.out.println(Runtime.getRuntime().totalMemory());
    System.out.println(Runtime.getRuntime().freeMemory());

note that str += "aa" is so slow that you may make wrong conclusions. GC cannot free no memory in your case

edited Jan 26, 2013 at 10:33

answered Jan 26, 2013 at 10:13

Evgeniy Dorofeev

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