Question

I am trying to parse an expression to find out some reserved operators/strings and replace them with corresponding numeric values , which are stored in a hashmap, and display the resultant expression which has numeric values instead of the reserved strings. This is what I am trying to do:

String expression = "SUM_ALL(4:5,5:6)>MAX(6:7)";
Map<String, Double> functionOperators = new HashMap<String,Double>();
functionOperators.put("SUM_ALL(4:5,5:6)", 4.0);
functionOperators.put("MAX(6:7)",5.0);

expression = expression.replaceAll("(SUM_ALL|MAX)\\s*\\(\\s*([\\d:,]+)\\s*\\)",
            Double.toString(functionOperators.get(String.format("%s(%s)","$1","$2"))));
System.out.println(expression);

“$1” and “$2” are used to back reference the groups in the expression.

Here is the output of the same:

Exception in thread "main" java.lang.NullPointerException

whereas when I remove the fetching part (from the hashmap) and instead limit it to just a string change using the back-referenced groups, it works just fine.

expression = expression.replaceAll("(SUM_ALL|MAX)\\s*\\(\\s*([\\d:,]+)\\s*\\)",
            String.format("%s|%s|","$1","$2"));

Output:

SUM_ALL|4:5,5:6|>MAX|6:7|

I suspect that while fetching the value from the hashmap, the “$1” and “$2” variables have still not been resolved by back-referencing and hence, it is not able to locate a key corresponding to the same.

If this is true, then what method do you suggest to implement my use case where I need the replacement value from a predefined hashmap.

Answer 1

I dont know what volume of dat you have but it seems the problem is replaceAll that is not working properly because I’ve tried with replace and it works awesome.

String expression = "SUM_ALL(4:5,5:6)>MAX(6:7)<SUM_ALL(4:5,5:6)";
String mat = "";
Map<String, Double> functionOperators = new HashMap<String,Double>();
functionOperators.put("SUM_ALL(4:5,5:6)", 4.0);
functionOperators.put("MAX(6:7)",5.0);
Matcher m = 
    Pattern.compile(
            "(?:(SUM_ALL|MAX|MIN)\\(((?:\\d+:\\d+,?){1,2})\\)[+-><\\*/]?)")
            .matcher(expression);
while (m.find()) {
    mat = String.format("%s(%s)", m.group(1), m.group(2));
    expression = expression.replace(mat, functionOperators.get(mat).toString());
}
System.out.println(expression);

Was taken directly from your earlier example (yesterday)