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Strings="$$";
s=s.replaceAll("\\s+\\$\\$","$$");

throw exception

java.lang.IllegalArgumentException: Illegal group

Use"\\$\\$"in the second parameter:

Strings="$$";
s=s.replaceAll("\\s+\\$\\$","\\$\\$");

The$is group symbol in regex’s replacement parameter

So you need to escape it

the second parameter also need to escape!