Why this error of "variable might not have been initialized"?

[mandy]

why am I getting this error "varible temp1 might not have been
initialized" and "varible temp2 might not have been initialized"

Here is the method tht returns a boolean comparison result:

// call in driver as t1.isLess(t2)
public boolean isLess(HW_413_7_Temperature Temperature2)
{
float temp1, temp2;
char scale1 = this.charScale;
char scale2 = Temperature2.charScale;

if( (scale1 == 'c' || scale1 == 'C') && (scale2 == 'f' || scale2 ==
'F') )
{
temp1 = this.temperature;
temp2 = Float.parseFloat(Temperature2.getCelsiusTemperature()
);
// need to parse back to float in order to compare
}

if( (scale1 == 'f' || scale1 == 'F') && (scale2 == 'c' || scale2 ==
'C') )
{
temp1 = Float.parseFloat(this.getCelsiusTemperature() );
// need to parse back to float in order to compare
temp2 = Temperature2.temperature;
}

if ( ( (scale1 == 'f' || scale1 == 'F') && (scale2 == 'f' || scale2
== 'F') ) || ( (scale1 == 'c' || scale1 == 'C') && (scale2 == 'c' ||
scale2 == 'C') ) )
{
temp1 = this.temperature;
temp2 = Temperature2.temperature;
}

return (temp1<=temp2);

}

 

[mandy]

Never mind.

 

[Mike Schilling]

why am I getting this error "varible temp1 might not have been
initialized" and "varible temp2 might not have been initialized"

Here is the method tht returns a boolean comparison result:

// call in driver as t1.isLess(t2)
public boolean isLess(HW_413_7_Temperature Temperature2)
{
float temp1, temp2;
char scale1 = this.charScale;
char scale2 = Temperature2.charScale;

if( (scale1 == 'c' || scale1 == 'C') && (scale2 == 'f' || scale2 ==
'F') )
{
temp1 = this.temperature;
temp2 = Float.parseFloat(Temperature2.getCelsiusTemperature()
);
// need to parse back to float in order to compare
}

if( (scale1 == 'f' || scale1 == 'F') && (scale2 == 'c' || scale2 ==
'C') )
{
temp1 = Float.parseFloat(this.getCelsiusTemperature() );
// need to parse back to float in order to compare
temp2 = Temperature2.temperature;
}

if ( ( (scale1 == 'f' || scale1 == 'F') && (scale2 == 'f' || scale2
== 'F') ) || ( (scale1 == 'c' || scale1 == 'C') && (scale2 == 'c' ||
scale2 == 'C') ) )
{
temp1 = this.temperature;
temp2 = Temperature2.temperature;
}

return (temp1<=temp2);

}

If none of the if blocks are executed, neither temp1 nor temp2 will have
been set. Perhaps you know that both scales are set to either 'f' or 'c',
but the compiler doesn't.

 

[Rhino]

why am I getting this error "varible temp1 might not have been
initialized" and "varible temp2 might not have been initialized"

Here is the method tht returns a boolean comparison result:

// call in driver as t1.isLess(t2)
public boolean isLess(HW_413_7_Temperature Temperature2)
{
float temp1, temp2;
char scale1 = this.charScale;
char scale2 = Temperature2.charScale;

if( (scale1 == 'c' || scale1 == 'C') && (scale2 == 'f' || scale2 ==
'F') )
{
temp1 = this.temperature;
temp2 = Float.parseFloat(Temperature2.getCelsiusTemperature()
);
// need to parse back to float in order to compare
}

if( (scale1 == 'f' || scale1 == 'F') && (scale2 == 'c' || scale2 ==
'C') )
{
temp1 = Float.parseFloat(this.getCelsiusTemperature() );
// need to parse back to float in order to compare
temp2 = Temperature2.temperature;
}

if ( ( (scale1 == 'f' || scale1 == 'F') && (scale2 == 'f' || scale2
== 'F') ) || ( (scale1 == 'c' || scale1 == 'C') && (scale2 == 'c' ||
scale2 == 'C') ) )
{
temp1 = this.temperature;
temp2 = Temperature2.temperature;
}

return (temp1<=temp2);

}

I can tell you how to make this message go away: initialize the two
variables. In other words, replace:

float temp1, temp2;

with something like this:

float temp1 = 0.0f, temp2 = 0.0f;

The initialization values don't have to be 0.0f; in fact, for your
application you may want to avoid 0.0f as the initial value of the variables
because 0 is a temperature that can actually occur in nature. Someone might
think that you were actually storing a temperature of 0 F or 0 C when you
only meant to initialize your variables. Perhaps -500.0f would be a better
initialization value since neither -500 F or -500 C can actually occur since
they are below absolute zero, the lowest possible temperature.

I'm not entirely sure why Java generates this message in the first place but
I long ago realized that the message is avoided by initializing all
variables to _something_. I usually initialize Objects to null and numbers
to zero.

Someone more familiar with the internal workings of Java or the JVM may have
a better idea why Java generates the message in the first place if you are
concerned about that.

 

[mandy]

Mike Schilling wrote:
[..]

If none of the if blocks are executed, neither temp1 nor temp2 will have
been set. Perhaps you know that both scales are set to either 'f' or 'c',
but the compiler doesn't.

Well, compiler didn't complain about similar scenarios in 2 other
methods. Well, I assigned a value at declaration of local variable to
please the compiler.

 

[Roedy Green]

Well, compiler didn't complain about similar scenarios in 2 other
methods. Well, I assigned a value at declaration of local variable to
please the compiler.

Instances and statics get automatically initialised to 0/null. Locals
do not. If you cover all the cases with no even theoretically possibly
leaks with your variable initialisation code, then the compiler knows
the variable has to be initialised, and it won't complain.

Those few facts should be enough to explain why only sometimes the
compiler complains about uninitialised variables.

 

[Mike Schilling]

I can tell you how to make this message go away: initialize the two
variables. In other words, replace:

float temp1, temp2;

with something like this:

float temp1 = 0.0f, temp2 = 0.0f;

The initialization values don't have to be 0.0f; in fact, for your
application you may want to avoid 0.0f as the initial value of the
variables because 0 is a temperature that can actually occur in nature.
Someone might think that you were actually storing a temperature of 0 F or
0 C when you only meant to initialize your variables. Perhaps -500.0f
would be a better initialization value since neither -500 F or -500 C can
actually occur since they are below absolute zero, the lowest possible
temperature.

I'm not entirely sure why Java generates this message in the first place
but I long ago realized that the message is avoided by initializing all
variables to _something_. I usually initialize Objects to null and numbers
to zero.

Someone more familiar with the internal workings of Java or the JVM may
have a better idea why Java generates the message in the first place if
you are concerned about that.

It's not mysterious; the message is (for once) pretty clear. If scale1 were
'd' and scale2 were 'z', none of the if blocks would be executed, and when
it came time to execute the line

return (temp1<=temp2);

the values of temp1 and temp2 would not have been set. This causes an error,
because Java refuses to use a local variable which has not been explicitly
assigned a value. If it were guaranteed that they would be set, say by
chanigng the code to

if ( ) // f->c
{
// set both variables
}
else if () // c->f
{
// set both variables
}
else if () // no conversion
{
// set both variables
}
else // something is wrong
{
// throw exception
}
return (temp1<=temp2);

no warning would be issued.

 

[Rhino]

It's not mysterious; the message is (for once) pretty clear. If scale1
were 'd' and scale2 were 'z', none of the if blocks would be executed, and
when it came time to execute the line

return (temp1<=temp2);

the values of temp1 and temp2 would not have been set. This causes an
error, because Java refuses to use a local variable which has not been
explicitly assigned a value. If it were guaranteed that they would be
set, say by chanigng the code to

if ( ) // f->c
{
// set both variables
}
else if () // c->f
{
// set both variables
}
else if () // no conversion
{
// set both variables
}
else // something is wrong
{
// throw exception
}
return (temp1<=temp2);

no warning would be issued.

I've used other languages where numeric variables were automatically
initialized to zero if you didn't initialize them yourself. That was often
quite convenient. Why doesn't Java choose to do the same thing? Is it just
trying to avoid the occasional case where zero wouldn't be a good default
initialization?

 

[Roedy Green]

Why doesn't Java choose to do the same thing? Is it just
trying to avoid the occasional case where zero wouldn't be a good default
initialization?

It is simply that 0/null is a very common value for instance
variables, and that initialisation has to be done anyway to avoid
random effects from uninitialised variables. Further it is too hard to
track if you actually initialised a static or instance variable.

For locals, 0/null is much more rarely the value you want. Nearly
always you want some function of other variables. Auto zero
initialisation then would just mask bugs. It is also possible to track
local init to ensure it is done.

 

[Mike Schilling]

I've used other languages where numeric variables were automatically
initialized to zero if you didn't initialize them yourself. That was often
quite convenient. Why doesn't Java choose to do the same thing? Is it just
trying to avoid the occasional case where zero wouldn't be a good default
initialization?

Java does initialize fields in objects and classes to 0/false/null, but not
local variables. I personally like this, since it forces the programmer to
think about when a variable should be initialized and to what value, but I
don't know of any definitive answer about why the Java designers chose it.

 

[Stefan Ram]

I don't know of any definitive answer about why the Java
designers chose it.

It follows the rules of C++, which follow the role model of C.
Why did the C designers chose it? I guess, because it would
take time, which would be wasted if an initialization is not
needed.

Java also has inherited the behavior of int-storage locations
to silently wrap instead of signaling an overflow. Possibly
for similar reasons. (In C, it is faster to just pass through
the implementation of the hardware processor than adding
overflow checks).

The question remains whether the reasons that were valid
for design choices of C in the 1970s are still valid for
Java today.

 

[Alex Hunsley]

I can tell you how to make this message go away: initialize the two
variables. In other words, replace:

float temp1, temp2;

with something like this:

float temp1 = 0.0f, temp2 = 0.0f;

The initialization values don't have to be 0.0f; in fact, for your
application you may want to avoid 0.0f as the initial value of the variables
because 0 is a temperature that can actually occur in nature. Someone might
think that you were actually storing a temperature of 0 F or 0 C when you
only meant to initialize your variables. Perhaps -500.0f would be a better
initialization value since neither -500 F or -500 C can actually occur since
they are below absolute zero, the lowest possible temperature.

It's very rarely a good idea to choose arbitrary out-of-domain values to
represent 'not a real value' or 'value was not set'. (A notable
exception is the result of functions like indexOf that return -1 to mean
'not found', which is a well recognised standard). You can usually avoid
this situation by careful structuring of your code and/or using
supplemental variable(s) with obvious meanings: e.g. a boolean variable
called 'temperatureIsSet'. If you ever find yourself putting an
arbitrary value into your code (with some constraint such as "is less
that absolute zero in degrees kelvin") please take it as a sign that
there is a better way to do things - and it is worth finding that better
way!

 

[Owen Jacobson]

Java does initialize fields in objects and classes to 0/false/null, but not
local variables. I personally like this, since it forces the programmer to
think about when a variable should be initialized and to what value, but I
don't know of any definitive answer about why the Java designers chose it.

Whether this was intended or not, Java's approach to local initialization
and validity does a decent job of discouraging the C-originated habit of
declaring every single local variable at the top of a function in favour
of declaring variables exactly where they're needed. I've found this
leads to clearer code, provided you don't fight with it by using dummy
initializers.

 

[Roedy Green]

Whether this was intended or not, Java's approach to local initialization
and validity does a decent job of discouraging the C-originated habit of
declaring every single local variable at the top of a function in favour
of declaring variables exactly where they're needed. I've found this
leads to clear

Narrowing the scope of a variable is a gift to your future self.

 

[Mike Schilling]

Mike Schilling wrote:
[..]

If none of the if blocks are executed, neither temp1 nor temp2 will have
been set. Perhaps you know that both scales are set to either 'f' or
'c',
but the compiler doesn't.

Well, compiler didn't complain about similar scenarios in 2 other
methods. Well, I assigned a value at declaration of local variable to
please the compiler.

Yes, that was the difference: the variable *did* have a value.