I modified it to:

		try
		{
		String command = "cmd /c start javap java.lang.String"; 
                Process process = Runtime.getRuntime().exec(command);
		StringBuilder s=new StringBuilder();
		StringBuilder t=new StringBuilder();
		IOUtils.collectProcessStreams(process,s,t);
		System.out.println(s.toString());
		System.out.println(t.toString());
		}catch(Exception e)
		{
			System.out.println(e);
		}

and all I get are two blank lines after I run the program

although it creates the write.txt file,it did that before also. But the write.txt file is blank,as it was before.And the program keeps throwing a NullPointerException.
and also,in the corrections that you have made,at line 15,we can't re-declare temp,can we? Because we have already done that. And I was using

 temp = br.readLine(); 

so why do it again?

still doesn't work mate..

try
{
	String command = "cmd /c start javap java.lang.String"; 
        Process process = Runtime.getRuntime().exec(command);
	StringBuilder s=new StringBuilder();
	StringBuilder t=new StringBuilder();
	IOUtils.collectProcessStreams(process,s,t);
}catch(Exception e)
{
	System.out.println(e);
}

the command prompt opens,and the command is executed,and then that window closes automatically.

so like:

Process process = Runtime.getRuntime().exec("cmd /c start javap java.lang.String");
IOUtils.collectProcessStreams(process,_____,____);

what do I pass for the other two parameters?

@immeraufdemhund
from what I read on that link,I tried something on those grounds,and even that didn't work. Here's a piece of code on what I tried:

public static void main(String args[])
{
        try
	{
		Runtime RT = Runtime.getRuntime();
		String command = "cmd /c start javap java.lang.String"; 
		File file = new File("write.txt");
		Writer output = new BufferedWriter(new FileWriter(file));
                BufferedReader br = new(BufferedReader(newInputStreamReader(RT.exec(command).getInputStream()));
		String temp = br.readLine();
		while(!temp.equals(null))
		{
			output.write(temp);
			output.close();
			temp = br.readLine();
		}
		RT.exec("exit");
	}
	catch(Exception e)
	{
		System.out.println(e);
	}
}

@g00se
couldn't understand much on that page. If you could just explain a bit?