Following code snippet throws
Boolean booleanFlag = null;
System.out.println("Why this boolean flag code is executed?");
The Java compiler does provide errors based on any data flow analysis (beyond checking whether variables have been initialized, and in this case you've initialized it to
null). It is entirely unaware that the value of
booleanFlag is necessarily
null at that point in the execution.
As for why, I can only speculate that the language authors knew a feature like that would add too much complexity to the language and too much performance cost to the compiler.