It is possible to import just one static method, or member in extend? I know that it is possible to import all static methods by *, like in this example ...
import static example.Types.*
But I need only one.
Yes it is possible.
import static System.out;
You can then merely write
out.println("Doobedoo");
Which will save you from typing the System.out part several times over but is potntially confusing for anyone trying to follow you code.
Use it sparingly and generally only for constants ( static final ).
If what you're looking for is something like the using keyword from C# or C++ java doesn't have that feature.
Java 5 added an import static option that allows static variables (typically constants) to be referenced without qualifying them with a class name.
For example, after
import static java.awt.Color;
It would then be possible to write
Color background = RED;
instead of
Color background = Color.RED;
Example above have been taken from: Java: packages and Import.
The site used for the example seems to be out of date, sorry any trouble this can cause
See How java import works for more detail.
Related
I'm mostly a c/C++/objective-C programmer, presently working in Java on an android application. My question is simple: I want a utility function, preferably not associated with any class that I can invoke from anywhere in my project (#include of some sort necessary?).
I know I can make a public static function of a class and invoke it as Class.someFunction();. I would like to just have someFunction(); I'm not sure if this is possible in java, or what the syntax for it is.
You can achieve the same "effect" by using a static import, by adding the following import in each file that you want to use it in:
import static x.y.Class.someFunction; // or x.y.Class.*;
...
// some code somewhere in the same file
someFunction();
However, all methods in Java must be part of a class. Static imports just let you pretend (briefly) otherwise.
P.S. This also works for static fields.
You could use a static import:
import static com.example.MyUtilityClass.*; // makes all class methods available
// or
import static com.example.MyUtilityClass.myMethod; // makes specified method available
You don't see this used very often because, if overused, it causes harder-to-debug code (see the last paragraph at the link above).
Here's a related question about when it's advisable to use this.
Also, following the programming best practices, You should define all such common, frequently used functionality in some utility class where you can define your functions or fields(probably constants- i.e. static and final attributes) that is going to be used/called at different places within the API.
Although, you still need to import the Utility class.
Else define such functionality in the top most parent class in your API hierarchy structure, that way you even don't have to import the class.
Hope this helps.
thanks....!!!
Yeap import static..
For instance:
import static java.lang.Math.max; // Allowing to use max method anywhere in the source
class SomeClass {
int m = max( 1, 2 );// m now is 2 due t Math.max( int, int )
}
I have always had a question about java.lang.Math: (It might by very basic)
Why do I have to Math.abs(-100) and can't to abs(-100)?
I figure that Math is a class. And abs is a static method. But why can I not simply import java.lang.Math and use abs(-100)?
You can import all the methods in Math:
import static java.lang.Math.*;
or just the one method you want:
import static java.lang.Math.abs;
Normal imports just import classes, making that class available via its short name.
abs is a static method and in order the compiler knows where it's defined, you have to specify the class (in your case - Math).
Note that you could do a static import on Math.abs and then you'd be able to just do abs(-100) instead of Math.abs(-100). In this case you'll have to add an import statement like this one:
import static java.lang.Math.abs;
Note also that in Java, unlike JavaScript and PHP, there aren't any public functions, which is why import statements are important.
java.lang.Math is statically imported in every Java Class.
static import java.lang.Math;
Every class of the java.lang package is imported that way.
As you know everything in java is within the class. So their can be only two alternatives.
Static Function and
Non Static Function
And java.lang.Math is a utility library. Creating object of this is not worth for you. so Java guys created all the functions static in this library.
And for your question you can call a member function directly if and only if they are member of same class.
I have a need to share a variable between two classes in the same package. I would not like to debate the "why" I'm using a global variable. I avoid them normally at all cost.
My understanding is that I need to declare my variable as static, and that any variable declared in such a manner is available to all classes in the package. I am using a Java library called Lanterna that is used to create text-based GUIs. In order write characters to the screen buffer, I have to create an object (which I called screen) of the Screen type. The two declarations below are placed near the top of my entry class (not in the constructor).
public static Terminal terminal = TerminalFacade.createTerminal(System.in, System.out, Charset.forName("UTF8"));
public static Screen screen = new Screen(terminal);
The types Terminal and Screen are declared as import statements at the top of my program. I don't receive any errors in Eclipse with these statement. In the class where I attempt to access the screen object, I get an error saying Multiple Markers at this line, screen cannot be resolved.
If any additional information needs to be provided please let me know.
While terminal and screen are in-scope everywhere, they are not automatically imported, and you have to reference them by the class that contains them.
For example, if you declared them in class Myclass, you would access them by eg.
MyClass.terminal.frobnicate();
Alternatively, though this is not standard practice in most cases, you can import them like so:
import static myPackage.MyClass.terminal;
Then you will be able to simply reference terminal without clarifying that you refer to MyClass's terminal, and not some other class's static field called terminal.
Instead of import you need a static import (The Static Import Java guide says, in part, The static import construct allows unqualified access to static members without inheriting from the type containing the static members). Something like (obviously with your entry-class)
import static com.foo.EntryClass.terminal;
import static com.foo.EntryClass.screen;
While reading some java books, I came to know about static imports.
I have some doubts in my mind.
What is static imports.
When and why to use it.
Explaination with examples will be helpful.
One example is JUnit tests
import static org.junit.Assert.assertEquals;
...
assertEquals(x, y);
Imports are typing shortcuts. A "regular" import is a shortcut down to the class level...
import java.util.List
Let's you just use
List l;
Instead of
java.util.List l;
A static import is a shortcut down to the method level. The method must be static, since there is no instance to associate with it...
import static java.lang.Math.abs
Lets you just use
x = abs(y);
instead of
x = java.lang.Math.abs(y);
Imports do not effect your compiled output or running code in any way. Once something is compiled there's no way to tell if the original source had imports or not.
Recently i cam across a statements :
import static java.lang.System.out;
import static java.lang.System.exit;
I read these statements in some tutorial. Are these statements O.K ?
If the statements are alright what do they mean and should they be used regularly while writing code ?
They are called static imports. The effect is to allow you to use the names out and exit in your program as if they were defined in the current scope; so you can write exit(0) instead of System.exit(0).
Now, are they a good idea? Sometimes, when used sparingly, they are a good way to reduce clutter. But most of the time, they actually just make your code harder to understand. The reader will ask "Where is this out defined?" and "Where does exit() come from?" In general, you should avoid them.
But if you're writing a class that's all about processing SomeReallyLongName objects, and SomeReallyLongName defines a bunch of FINAL_CONSTANTS, importing them with static imports will save a lot of typing and a lot of clutter, and it will be pretty clear where those constants are coming from.
They are static imports. It allows you to do something like exit(0) instead of System.exit(0).
I do not recommend this for well known Java classes because it can be confusing to some. But sometimes it is useful for utility classes like Guava.
Iterables.filter(list, SomeClass.class)
is very verbose but you can make it easier to read with static imports: filter(list, SomeClass.class)
You should check with your team to see what they code guidelines are and try to be consistent.
Yes,it is perfectly alright .
This is known as static import.This allows members defined in class as static and
public to be used without specifying the class in which the field is defined.
This feature was defined in J2SE 5.0.
For example :
import static java.lang.Math.*;
import static java.lang.System.out;
// in main
out.print( max(100,200) ); // prints 200.You didn't have to use Math.max(.,.)
I think it may not be a good idea to use static imports as it'll make your code hard to read.
Yes, these statements are referred to as static imports and are perfectly valid. Take a look at the javase guide on static imports for more information.
With respect to usage, the guide states:
So when should you use static import? Very sparingly! Only use it when
you'd otherwise be tempted to declare local copies of constants, or to
abuse inheritance (the Constant Interface Antipattern). In other
words, use it when you require frequent access to static members from
one or two classes. If you overuse the static import feature, it can
make your program unreadable and unmaintainable, polluting its
namespace with all the static members you import. Readers of your code
(including you, a few months after you wrote it) will not know which
class a static member comes from. Importing all of the static members
from a class can be particularly harmful to readability; if you need
only one or two members, import them individually. Used appropriately,
static import can make your program more readable, by removing the
boilerplate of repetition of class names.
Static imports are a new feature added in Java 1.5
The static import construct allows unqualified access to static members without inheriting from the type containing the static members. Instead, the program imports the members, either individually
There is nothing wrong with your example if you want easy access to out and exit so that you can call them directly as out.println() for example. There is nothing syntactically incorrect about it nor from a style aspect though some may argue it is "confusing" and hard to figure out where out came from, but any modern IDE can help them figure that out.
These are static import concept.These are like simple imports but having different type of concept.See here you import one function exit() and one field out and both are static in their corresponding classes(in case both here Systen is their class).After this instead of writing System.out.println() you can simply write out.println().Similarly instead of System.exit(),you can write exit().