A DESCRIPTION OF THE PROBLEM :

As per the API specification of DecimalFormat "A DecimalFormat pattern contains a positive and negative subpattern, for example, "#,##0.00;(#,##0.00)". Each subpattern has a prefix, numeric part, and suffix.", but it does not throw any exception when an invalid pattern is used, such as "(#(#)" (numeric part / special character after suffix). Below are some examples of similar invalid patterns and the output given by DecimalFormat

1. formatting -3456.349347 with pattern (#(#) (invalid positive sub-pattern) returns -(3456()
2. formatting -3456.349347 with pattern #,##0.0#;(#(#) (invalid negative sub-pattern) returns (3,456.35()

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :

Execute below code snippet
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DecimalFormat format = new DecimalFormat("(#(#)");
double number = -3456.349347;
System.out.println(format.format(number));
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Expected
Throw Exception

Actual
Returns unpredictable results
-(3456() for (#(#) and (3,456.35() for #,##0.0#;(#(#)

REPRODUCIBILITY :
This bug can be reproduced always.