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A Java program can be syntactically correct but still not compile because of errors related to the semantics of the language.

This group of errors results from code that is syntactically correct but violates the semantics of Java, in particular the rules relating to type checking.

variable ... might not have been initialized

An instance variable declared in a class has a default initial value. A class is a template that is used to create or instantiate instances called objects . Since memory is allocated separately for each object, these variables are called instance variables . However, a local variable declared within a method does not, so you are required to initialize it before use, either in its declaration or in anassignment statement:

void m(int n) {   // n is initialized from the actual parameter   int i, j;  i = 2;             // i is initialized by the assignment   int k = 1;         // k is initialized in its declaration  if (i == n)        // OK     k = j;           // Error, j is not initialized  else     j = k;}

The variable must be initialized on every path from declaration to use even if the semantics of the program ensure that the path cannot be taken:

void m(int n) {   int i;  if (n == n)           // Always true     i = n;  else     n = i;              // Error, although never executed!!}

Eclipse: The local variable ... may not have been initialized

Note: If the expression in the if -statement can be computed at compile-time:

if (true)                  // OK if ('n' == 'n')            // OK

the error will not occur.

... in ... cannot be applied to ...

This error message is very common and results from an incompatibility between a method call and the method's declaration:

void m(int i) { ... }  m(5.5);        // Error, the literal is of type double, not int

Check the declaration and call carefully. You can also get the error message cannot find symbol if the declaration of the method is in the API. The Application Programming Interface (API) describes how to use the library of classes supplied as part of the Java system. Byextension, it is also used as a name for the library itself.

Eclipse: The method ... in the type ... is not applicable for the arguments ...

operator ... cannot be applied to ...,...

Operators are only defined for certain types, although implicit type conversion is allowed between certain numeric types:

int a = 5; boolean b = true;int c = a + b;       // Error, can't add a boolean value double d = a + 1.4;  // OK, int is implicitly converted to double

Eclipse: The operator + is undefined for the argument type(s) int, boolean

possible loss of precision

This error arises when trying to assign a value of higher precision to a variable of lower precision without anexplicit type cast. Surprisingly, perhaps, floating point literals are of type double and you will get this message if you try to assign one to a variable of type float :

float sum = 0.0;          // Error, literal is not of type float

The correct way to do this is to use a literal of type float or an explicit type cast:

float sum = 0.0f;         // OK float sum = (float) 0.0;  // OK

Eclipse: Type mismatch: cannot convert from double to float

incompatible types

Java checks that the type of an expression is compatible with the type of the variable in an assignment statement and this error will result ifthey are incompatible:

boolean b = true; int a = b;              // Error, can't assign boolean to int

Eclipse: Type mismatch: cannot convert from boolean to int

Important note
In the C language it is (unfortunately!) legal to write if (a = b) using the assignment operator instead of if (a == b) using the equality operator. The meaning is to execute the assignment, convert the value to an integerand use its value to make the decision for the if -statement (zero is false and non-zero is true). In Java, the assignment is legal and results in a value,but (unless a is of type boolean !) this error message will result because the expression in an if -statement must be of type boolean , not int . Writing == instead of = becomes a simple compile-time error in Java, whereas in C this error leads to runtime errors that are extremelydifficult to find.

inconvertible types

Not every type conversion is legal:

boolean b = true; int x = (int) b;   // Error, can't convert boolean to int

Eclipse: Type mismatch: cannot convert from ... to ...

Questions & Answers

Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
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Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
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industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
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scanning tunneling microscope
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what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
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What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
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what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
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Introduction about quantum dots in nanotechnology
Praveena Reply
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Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Difference between extinct and extici spicies
Amanpreet Reply
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Source:  OpenStax, Compile and runtime errors in java. OpenStax CNX. Dec 28, 2009 Download for free at http://cnx.org/content/col10913/1.2
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