public class Foo {
public static void main(String[] args) {
float f;
System.out.println(f);
}
}
The print statement causes the following compile-time error,
The local variable f may not have been initialized
If primitives in Java already have a default value (float = 0.0f), why am I required to define one?
Edit:
So, this works
public class Foo {
float f;
public static void main(String[] args) {
System.out.println(new Foo().f);
}
}
Thanks, everyone!
Because it's a local variable. This is why nothing is assigned to it :
Local variables are slightly different; the compiler never assigns a
default value to an uninitialized local variable. If you cannot
initialize your local variable where it is declared, make sure to
assign it a value before you attempt to use it. Accessing an
uninitialized local variable will result in a compile-time error.
Edit: Why does Java raise this compilation error ?
If we look at the IdentifierExpression.java class file, we will find this block :
...
if (field.isLocal()) {
LocalMember local = (LocalMember)field;
if (local.scopeNumber < ctx.frameNumber && !local.isFinal()) {
env.error(where, "invalid.uplevel", id);
}
if (!vset.testVar(local.number)) {
env.error(where, "var.not.initialized", id);
vset.addVar(local.number);
}
local.readcount++;
}
...
As stated (if (!vset.testVar(local.number)) {), the JDK checks (with testVar) if the variable is assigned (Vset's source code where we can find testVar code). If not, it raises the error var.not.initialized from a properties file :
...
javac.err.var.not.initialized=\
Variable {0} may not have been initialized.
...
Source
In fact, the compiler does not assign a default value to your float f, because in this case it is a local variable -- and not a field:
Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.
Class fields (non-final ones anyway) are initialized to default values. Local variables are not.
It's not always necessary to assign a value when a field is declared. Fields that are declared but not initialized will be set to a reasonable default by the compiler.
So a (non-final) field like f in
class C {
float f;
}
will be initialized to 0f but the local variable f in
void myMethod() {
float f;
}
will not be.
Local variables are treated differently from fields by the language. Local variables have a well-scoped lifetime, so any use before initialization is probably an error. Fields do not so the default initialization is often convenient.
Actually local variables are stored in stack.Hence there is a chance of taking any old value
present for the local variable.It is a big challenge for security reason..Hence java says you have to initialise a local varible before use.
Hi guys solution is simple.
the values that are stored on the heap memory are initialized by the compiler based datatype but local variables are stored on stack memory so we have to inialize it explictly.
Related
Every type in Java has a primitive value when declared. The article Primitive Data Types contains a description for primitive data types. Knowing this, why does Eclipse show an error telling me the variable may not have been initialized?
If I have, for example,
int x;
x++;
From the reference:
Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error.
From the Java Language Specification, Java SE 8 Edition, 4.12.5 Initial Values of Variables:
A local variable (§14.4, §14.14) must be explicitly given a value before it is used, by either initialization (§14.4) or assignment (§15.26), in a way that can be verified using the rules for definite assignment (§16 (Definite Assignment)).
Local variables don't get initialized.
This is a local variable:
void aaa() {
int x;
}
This is an instance variable. These do get initialized automatically:
class X {
int x;
}
Data Type Default Value (for fields)
byte 0
short 0
int 0
long 0L
float 0.0f
double 0.0d
char ‘u0000’
String (or any object) null
boolean false
From the Primitive Data Types reference provided by you:
"Local variables are slightly different; the compiler never assigns a default value to an uninitialized local variable. If you cannot initialize your local variable where it is declared, make sure to assign it a value before you attempt to use it. Accessing an uninitialized local variable will result in a compile-time error".
What you see is not an error, but your Eclipse preference. You can change it to ignore uninitialized variables in Eclipse preference.
I have one question: in java we declare int,long,double etc.,(primitive data)
or non primitive (object data), not initialized with default values, but at run
time it will take default values. Now my question is which one assigns
default values: java compiler or Java Virtual Machine (JVM)?
For Example:
int x;
System.out.println(x) //Result is 0;
There are three different types of declared variables in Java. They are instance, class and local variables.
Instance Variables
Instance variables are the non-static fields of your class, often referred to simply as fields.
Primitive numeric fields initialize to 0. This includes byte, short, int, long, float and double.
booleans initialize to false .
chars initialize to the null character \u0000.
Reference types initialize to null.
Class Variables
A class variable is a field within a class declared as static, often referred to as a static variable or static field. It is also same initialize as instance variable.
Local Variables
A local variable is a variable defined within a method, which includes any method
parameters.
Local variables must be initialized before use. They do not have a default value.
Initialization process is done by JVM when method is create.
The default values for fields are assigned by the JVM at runtime. From JLS 15.9.4 (emphasis mine):
The new object contains new instances of all the fields declared in the specified class type and all its superclasses. As each new field instance is created, it is initialized to its default value.
Of course, given that this behavior is standardized in the JLS, a compiler could conceivably take advantage of that to perform certain optimizations based on the assumption that uninitialized fields start with their default value.
Fields are initialized to the equivalent of 0 in whatever type they are (null for reference types). This article gives a nice list:
Data Type: Default Value:
boolean false
char \u0000
int,short,byte / long 0 / 0L
float / double 0.0f / 0.0d
any reference type null
Local variables are not given an initial value, and it is a compiler error to use them if they are not assigned a value through all possible code paths prior to use.
Note that array elements are automatically initialized to default values as well when a new array is created (e.g. each element of new int[100] will be initialized to 0). This applies to both field and local array variables.
I have eclipse, and when I try to see if an uninitialized object equals null, it won't let me, it comes up with a "x might not have been intialized" error and I know it should work.
example of what I mean:
Object obj;
System.out.println(obj==null ? "no value":"has a value");
it would not compile and it would say 'obj might not have been initialized' How can I change my compiler settings in eclipse to fix this?
How can I change my compiler settings in eclipse to fix this?
You can't. The Java language specification requires any conformant Java compiler to treat this as a compilation error.
There is no Eclipse compiler setting to cause it to break this rule.
Even (hypothetically) if there was such a setting, I think that the bytecode file would fail verification when the JVM attempted to load it. (If you could somehow trick the JVM into using the value of an uninitialized local variable, that would undermine runtime type security, leading to JVM crashes ... and worse.)
If obj was an instance variable rather than a local variable, it would be default initialized to null and you wouldn't get a compilation error. But local variables are not default initialized.
You don't change the compiler settings in eclipse to fix this: you just initialize the variable obj.
Object obj = null;
System.out.println(obj==null ? "no value":"has a value");
From the Java Specification 4.12.5 - Initial Values of Variables:
A local variable (§14.4, §14.14) must be explicitly given a value before it is used, by either initialization (§14.4) or assignment (§15.26), in a way that can be verified using the rules for definite assignment (§16).
If you're really bent on not initializing obj you need to make it a member of a class instead of a local variable. Then it will have a default initial value: (again, refer to Java Specification 4.12.5 - Initial Values of Variables)
public class Example {
private static Object obj;
public static void main(String[] argv) throws Exception {
System.out.println(obj==null ? "no value":"has a value");
}
}
... but under the hood it's still getting initialized.
The compiler shows error because the rule is that all local variables must be initialized before they are first read. So first declare local variable without initializing it, initialize it later, and then use it:
Object obj = null;
System.out.println(obj==null ? "no value":"has a value");
That's a local variable, you need to initialise it with:
Object obj = null;
While certain fields of classes and objects, and arrays, may be implicitly initialised to useful defaults, that is not the case for local variables.
Section 16.2.4 (Local variable declaration statements) of JLS7 is the section to read if you want to understand this but it will take some time to understand, it's pretty obtuse :-)
You'd probably want to start at the top of section 16 (Definite Assignment) and work through from there. The first part of that has two paragraphs which are most pertinent here (my italics for emphasis):
For every access of a local variable or blank final field X, X must be definitely assigned before the access, or a compile-time error occurs.
Such an assignment is defined to occur if and only if either the simple name of the
variable (or, for a field, its simple name qualified by this) occurs on the left hand
side of an assignment operator.
How can I change my compiler settings in eclipse to fix this?
The solution is to fix your code. The purpose of this error is to detect and prevent bugs. Taking steps to allow broken code to compile is usually a very bad idea.
I need to use something similar to php's isset function. I know php and java are EXTREMELY different but php is my only basis of previous knowledge on something similar to programming. Is there some kind of method that would return a boolean value for whether or not an instance variable had been initialized or not. For example...
if(box.isset()) {
box.removeFromCanvas();
}
So far I've had this problem where I am getting a run-time error when my program is trying to hide or remove an object that hasn't been constructed yet.
Assuming you're interested in whether the variable has been explicitly assigned a value or not, the answer is "not really". There's absolutely no difference between a field (instance variable or class variable) which hasn't been explicitly assigned at all yet, and one which has been assigned its default value - 0, false, null etc.
Now if you know that once assigned, the value will never reassigned a value of null, you can use:
if (box != null) {
box.removeFromCanvas();
}
(and that also avoids a possible NullPointerException) but you need to be aware that "a field with a value of null" isn't the same as "a field which hasn't been explicitly assigned a value". Null is a perfectly valid variable value (for non-primitive variables, of course). Indeed, you may even want to change the above code to:
if (box != null) {
box.removeFromCanvas();
// Forget about the box - we don't want to try to remove it again
box = null;
}
The difference is also visible for local variables, which can't be read before they've been "definitely assigned" - but one of the values which they can be definitely assigned is null (for reference type variables):
// Won't compile
String x;
System.out.println(x);
// Will compile, prints null
String y = null;
System.out.println(y);
Instance variables or fields, along with static variables, are assigned default values based on the variable type:
int: 0
char: \u0000 or 0
double: 0.0
boolean: false
reference: null
Just want to clarify that local variables (ie. declared in block, eg. method, for loop, while loop, try-catch, etc.) are not initialized to default values and must be explicitly initialized.
To what value is a variable of the String type automatically initialized?
null
Unless it's inside a method (local variable), in which case it's not declared to anything.
Here's a summary of the answers posted by Martin v. Löwis and silky.
We can say the following about the initialization of a String object:
If the String is a local variable, it will not be initialized.
If the String is a class variable, instance variable, or an array component, then it will be initialized to null.
The reasoning is as follows:
As a variable with the type of String is a reference type, according to The Java Language Specification, Third Edition, Section 4.12.5: Initial Values of Variables says the following:
Every variable in a program must have
a value before its value is used
It goes on to say the following about the initialization of reference types:
Each class variable, instance variable, or array component is
initialized with a default value when
it is created (§15.9, §15.10):
[removed information on irrelevant information]
For all reference types (§4.3), the default value is null.
And finally, the follow about local variables:
A local variable (§14.4, §14.14) must
be explicitly given a value before it
is used, by either initialization
(§14.4) or assignment (§15.26), in a
way that can be verified by the
compiler using the rules for definite
assignment (§16).
If the variable is a class variable, instance variable, or array component, it is initialized to null (since the default value for a reference type is null)
If the variable is a local variable, then it must be given a value explicitly (i.e. it has no default value in this case).
A variable of type String is a reference variable. As an instance variable, it gets initialized to null, see the specification for the discussion of other cases.
It's null unless it's local, in which case it is technically uninitialized, but in fact you can't use it, for that reason, so the language is still type-safe. You can't deref a garbage pointer.
null
String str=null means that the str is a object of String class which is not pointing to anything...but when we talk abt memory allocation,memory will be allocated to str as soon as it comes into existence....u can check the amount of memory by using the profiling option in netbeans..
the string value should be NULL in default , no need to initialize it.
string class objects are NULL by default only if they are defined as class level atttributes, otherwise string objects don't have any default value they need to be explicity initialized.
If the variable of type String is within a method, it would not automatically initialise. Otherwise it would be initialised with null as a value.
Any variable that is declared within a class is automatically initialized.
Any variable that is declared within a method must be initialized otherwise it will generate an error.
Strings are initialized to null,ints to 0 and so on..
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