Unreachable code compiler error [duplicate]

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Unreachable code is meaningless, so the compile-time error is helpful. The reason why it won’t be detected at the second example is, like you expect, for testing / debugging purposes. It’s explained in The Specification: 

if (false) { x=3; }

does not result in a compile-time error. An optimizing compiler may
realize that the statement x=3; will never be executed and may choose
to omit the code for that statement from the generated class file, but
the statement x=3; is not regarded as “unreachable” in the technical
sense specified here.

The rationale for this differing treatment is to allow programmers to
define “flag variables” such as:

static final boolean DEBUG = false;

and then write code such as:

if (DEBUG) { x=3; }

The idea is that it should be possible to change the value of DEBUG
from false to true or from true to false and then compile the code
correctly with no other changes to the program text.

Reference: http://docs.oracle.com/javase/specs/jls/se8/html/jls-14.html#jls-14.21

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