char[] str2char (String string) {
 char[] chara;
 for (i=0; i< string.length(); i++)
 {
   chara[i] = string.charAt(i);
 }
 return chara;
}

I just had a bit of a run-around trying to work out why this function intialisation wouldn't compile.
If I use String[] instead, It will cause a "maybe missing a semicolon"
crazy~~

Thanks, JR
but I copied only these lines into a new sketch and ran it.
What makes me confusing is using a String[],  the message is different!

The function is really simple, It converts a string into a character array.
Why choose char[] is because I can use a "new String(chara)" to merge them back into a string after operation.  simple and clean.

Btw, about the DOT error : it's because you have no setup() method in your sketch. Add void setup() {} somewhere after your method, and it will work (there are other errors in your code, though, that you will be warned of).

Thanks for antiplastik's method, it works. but there are still some syntax error exist.  

Code:

void setup() {
  size(200,200);
String stringA = "   aaaa    aaa aaa";
String stringB = "xxxx xxxx   xxxxxxxxxxx    xxx";
replace(stringA,stringB);

}

void  replace(String stringA,String stringB) {
  if ((stringA != null) || (stringB != null)) {
    String outString;
    char[] charA = stringA.toCharArray();
    char[] charB = stringB.toCharArray();
    int maxium = max (charA.length,charB.length);    
    charA = expand(charA,maxium);
    charB = expand(charB,maxium);
    for (int i=0; i < maxium ; i++) {
	if (charB[i] !=' ') {
	  charA[i] = charB[i];
	}
    }  
    outString = new String(charA);
    println(charA);
    println(outString);
  //  return outString;
  }
} 

this can be complied, and works well.
but what expecting is return the String back from this function.

Code:

 void setup() {
  size(200,200);
String stringA = "   aaaa    aaa aaa";
String stringB = "xxxx xxxx   xxxxxxxxxxx    xxx";
String x = replace(stringA,stringB);
Println (x);

}

String  replace(String stringA,String stringB) {
  if ((stringA != null) || (stringB != null)) {
    String outString;
    char[] charA = stringA.toCharArray();
    char[] charB = stringB.toCharArray();
    int maxium = max (charA.length,charB.length);    
    charA = expand(charA,maxium);
    charB = expand(charB,maxium);
    for (int i=0; i < maxium ; i++) {
	if (charB[i] !=' ') {
	  charA[i] = charB[i];
	}
    }  
    outString = new String(charA);
    //println(charA);
    // println(outString);
    return outString;
  }
}
 

I get a " This method must return a result of type String"  

This coding is a part of practice of string operation.
what I want is using these codes animating some ascii images.  the characters on the static background can be replaced by the dynamic ones if overlapping happen.  

In other words, add a return null (or return "") line just before the end of the method.

Maybe I could use a Matrix to deal with all characters in one time.  but I were faced this problem some day.