How to assign to String[] from StringBuffer in a loop?

[navrsalemile]

Hi all,


public class Test {
public String[] getSJ()
{
String[] jg;
String[] jig;

String[] result;

StringBuffer buf = new StringBuffer();
int i;
int j;
int k = -1;

jg = {"g1", "g2"};

for( i=0; i < jg.length; i++ )
{
jig = {"1", "2", "3"};
for( j=0; i < jig.length; j++, k++ )
{
buf.append( jg ).append( ":" );
buf.append( jig[j] ).append( ":" );
buf.append( Date() );
result[k] = buf.toString();
buf = null;
}
}
return result;
}
}



I want to add to result string array by assigning from buf which is StringBuffer in a loop. But what happens after buf = null??
Will the added String be gone?

Or should I just do:

buf = "";

and continue with the loop and the string objects will be preserved? But isn't result just an array of references?

Many thanks,

 

[sedj]

I'm confused here ...
Why are you bothering to set your StringBuffer to null ?
That will just cause a big NullPointerException.
Also, you use the variable 'k' to -1, and then assign a subscript of the 'result' array at position -1 - which will cause a big IndexOutOfBoundsException ...

Also, please post code, correctly indented, within the tags [ignore]

[/ignore] .


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[Diancecht]

And a performance tip:

If you set an initial size for the StringBuffer a little bigger than the expected final size, the concatenatios will be much quicker.

Maybe won't make a big difference for a three iterations loop, but can do if the loop grows.

Cheers,
Dian

 

[stefanwagner]

Defining all variables at the beginning of a function was c-Style in former times, wasn't it?
In java you introduce them just in time, to the smallest scope possible.

	public String [] getSJ ()
	{
		String [] result = new String [6];
		String [] jg = {"g1", "g2"};
		int k = 0;
		for (int i=0; i < jg.length; i++ )
		{
			String [] jig = {"1", "2", "3"};

			for (int j=0; j < jig.length; j++, k++ )
			{
				StringBuffer buf = new StringBuffer ();
				buf.append ( jg [i] ).append ( ":" );
				buf.append ( jig [j] ).append ( ":" );
				buf.append ( new Date () );
				result [k] = buf.toString ();
			}
		}
		return result;
	}

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[Diancecht]

I don't agree. You should not declare variables inside a loop, that would need an objet allocation per iteration.

I'd go this way:

    public String [] getSJ ()
    {
        String [] result = new String [6];
        String [] jg = {"g1", "g2"};
        StringBuffer buf = null;
        String [] jig = {"1", "2", "3"};

        for (int i=0; i < jg.length; i++ )
        {
            for (int j=0; j < jig.length; j++)
            {
                buf = new StringBuffer (100);
                buf.append ( jg [i] ).append ( ":" );
                buf.append ( jig [j] ).append ( ":" );
                buf.append ( new Date () );
                result [3*i+j] = buf.toString ();
            }
        }
        return result;
    }

Cheers,
Dian

 

[navrsalemile]

As the matter of fact this appears to be correct solution (in the example I use limited number of items but it should be dynamically allocated):

import java.util.*;

public class Test {

	public void Test() {
	}

public String[] getSJ()
    {
        String[] jg = {"g1", "g2"};    //can be unlimited
        String[] jig = {"1", "2", "3"};//ditto
		
	String[]     result = new String[0];

	ArrayList al = new ArrayList();

        StringBuffer buf = new StringBuffer();
        int i;
        int j;
        int k = 0;

for( i=0; i < jg.length; i++ )
        {
         for( j=0; j < jig.length; j++, k++ )
            {
               buf.append( jg[i] )
                  .append( ":" )
                  .append( jig[j] )
                  .append( ":" )
                  .append( new Date() );
               
	       al.add( buf.toString() );
               buf.setLength(0);
            }
    }
    return( (String[])al.toArray(result) );
  }

 public static void main( String[] args ) {
    Test   t = new Test();
    String[] res = t.getSJ();
    for( int i=0; i<res.length; i++ ) {
	System.out.println( res[i] );
    }
 }
}

Does anybody know how to do it without help of containers, i.e. just using Array? I read somewhere about varargs in Java but forgot how to do it

 

[stefanwagner]

@diancecht:

I don't agree. You should not declare variables inside a loop, that would need an objet allocation per iteration.

I can't agree.
A declaration doesn't leed to object allocation, but calling<i> new</i> does.

You don't save anything in the case of 'buf'.
You do save the time for creating jig.
In a loop which is run twice.
I'm impressed.

But you allocate a StringBuffer of size 100 instead of 34.

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[Diancecht]

@stefan

You're right, I was thinking in another thing: the declaration of the buf variable is irrelevant, but

- Since you cannot know neither how many times the loop will be executed nor the size of the jig array, it's ok to declare it outside.
- Defining a size for the StringBuffer bigger than the expected amount of data saves time on concatenating, since toy don't need to expand the capacity of the buffer again and again
- Glad you're impressed, I'll write it down in my diary.

@salemile.

You could define the result array as jig.size()*jg.size() but I see nothing wrong on using Arraylist.

Is this for an assignment?

Cheers,
Dian

 

[navrsalemile]

No, but I am sure I saw varargs example in Bruce Eckel book Thinking in Java. If I correctly remember in the initializer he puts comma character after the last item which is same as varargs in C/C++

 

[Diancecht]

Ok, just curious.

Cheers,
Dian