When you declare an import like this:
import com.microsoft.azure.storage.*;
Does that include everything in its subpackages too? For example, does it include this?
import com.microsoft.azure.storage.blob.*
If not, why not? (Edit: the "why" question is basically off topic. ignoring that bit when considering a correct answer.)
No it does not. It only imports everything in the package (i.e. the directory itself). Sub-directories are considered to be different packages, so you would need to:
import com.microsoft.azure.storage.*
import com.microsoft.azure.storage.blob.*
As to why the language designers chose to go this route, one can only guess, but the system that they decided to go with does allow for a more fine-grained approach.
yes you can import all the classes from an import but it does not make it possible to import multiple packages with similar names. For example import java.util*; does not also import java.util.prefs or .jar you have to import these all separately. I don't know if that answers your question, and to the why I am not really sure it just makes sense to do it this way. If you were to import similar packages that had the same static variables, but you only need two or three of the packages then you would get errors or code that does not run properly.
There's a name for these - type import on demand.
A type-import-on-demand declaration allows all accessible types of a named package or type to be imported as needed.
They're only also importing the package itself, and not any subpackages, as clarified by the example, emphasis mine:
import java.util.*;
causes the simple names of all public types declared in the package java.util to be available within the class and interface declarations of the compilation unit. Thus, the simple name Vector refers to the type Vector in the package java.util in all places in the compilation unit where that type declaration is not shadowed (§6.4.1) or obscured (§6.4.2).
does that include everything in / subdirectories too? including
something like this?
* stands for all the compilation units inside the package com.microsoft.azure.storage where sub packages are actually not compilation units and thus not fetched when you write myPack.*. Compilation unit includes class, interface, enum etc.
Related
Does Java support grouping the imports like following:
import package.{Class1,Class2}
I know that * operator imports sub packages but I want to import specific ones.
In Rust or some modern languages it is supported like following:
use package::{Class1, Class2};
Are there any alternative instead of writing each import line by line specifically like this?
import package.Class1;
import package.Class2;
Java Language Specification, section 7.5. Import Declarations, shows:
An import declaration allows a named type or a static member to be referred to by a simple name (§6.2) that consists of a single identifier.
[...]
A single-type-import declaration (§7.5.1) imports a single named type, by mentioning its canonical name (§6.7).
A type-import-on-demand declaration (§7.5.2) imports all the accessible types of a named type or named package as needed, by mentioning the canonical name of a type or package.
A single-static-import declaration (§7.5.3) imports all accessible static members with a given name from a type, by giving its canonical name.
A static-import-on-demand declaration (§7.5.4) imports all accessible static members of a named type as needed, by mentioning the canonical name of a type.
As you can see, it's either a single named type, or all accessible types. No syntax for a list of types.
Side note: I almost never look at the import statements of my code. I let Eclipse manage that for me (Source > Organize Imports... (Ctrl+Shift+O)), so I don't really care about having many single-type import statements. The section with the imports is collapsed anyway, so I don't even have to scroll past them. Oh the joy of using a good IDE.
In java, only supported ways to import multiple classes are as follows :
A - import individual classes
import package.Class1;
import package.Class2;
B - import all classes in a package or subpackage
import package.*;
import package.subpackage.*;
Refer Oracle doc for more information : https://docs.oracle.com/javase/tutorial/java/package/usepkgs.html
No. Java doesn't have a construct to import a set of select classes using one statement. You either import all types from the package, or you import them one by one.
Using * lets you import all classes from the same package (not to import sub-packages, as you mentioned):
import package.*; //Both Class1 and Class2 are imported
import static package.Class1.* //All static members of Class1 are imported
The first form import package.* is usually discouraged because of the increased potential for name clashes (same class names from different packages). This is probably where import package.{Class1,Class2} would have helped, but there's no such construct in Java.
Can any one clearly explain to me what exactly happens when we use the import statement in Java files? Does it increase the size of the file if we add more and more java classes? Why don't we use class loader for the same? And what are the restrictions for importing?
import declarations (not statements) are essentially short-hand enabler at the source code level: it allows you to refer to a type or a static member using a single identifier (e.g. List, min) as opposed to the fully qualified name (e.g. java.util.List, Math.min).
import declaration section is a compile-time element of the source codes, and has no presence at run-time. In JVM bytecodes, type names are always fully qualified, and unless you're using a poorly written compiler, the binary should only contain names for types that are actually being used.
Class loaders are used for an entirely different concept, and has nothing to do with import feature at all.
JLS 7.5 Import Declarations
An import declaration allows a static member or a named type to be referred to by a simple name that consists of a single identifier. Without the use of an appropriate import declaration, the only way to refer to a type declared in another package, or a static member of another type, is to use a fully qualified name.
A single-type-import declaration imports a single named type, by mentioning its canonical name.
A type-import-on-demand declaration imports all the accessible types of a named type or package as needed. It is a compile time error to import a type from the unnamed package.
A single static import declaration imports all accessible static members with a given name from a type, by giving its canonical name.
A static-import-on-demand declaration imports all accessible static members of a named type as needed.
References
JLS 7.5.1 Single-Type-Import Declaration
JLS 7.5.2 Type-Import-on-Demand Declaration
JLS 7.5.3 Single Static Import Declaration
JLS 7.5.4 Static-Import-on-Demand Declaration
See also
Java Tutorials/Using package members
Java Language Guide/static import
Various import related questions
On the grammatical role of import:
What is an import called? - it's a declaration, not a statement
On on-demand vs single-type:
Import package.* vs import package.SpecificType
Why is using a wild card with a Java import statement bad?
What’s the difference between import java.util.*; and import java.util.Date;?
On import static:
What does the static modifier after import mean?
What is a good use case for static import of methods?
Should I use static import?
Performance-related issues:
Does importing of packages change visibility of classes? - ABSOLUTELY NOT!
Do multiple import statements in a program affect performance? - NOPE!
Any reason to clean up unused imports in Java, other than reducing clutter?
Packages consist of classes, classes in a package consist of methods, variables etc etc.
A class has a full name which comprises of the package name and the class name. If you need to use a class in your code,you need to give the compiler the full name of the class.So, you use an import statement OR you can type the fully qualified name every place you use that class in your code.
For example, if you need an AraryList in your code, you use the import statement import java.util.ArrayList; instead of typing the fully qualified class name every place you need an Arraylist.
For more detailed info, see JLS.
The imports in java are only hints for the compiler. It doesn't affect the size of the binary class file at all. You can either use an imports once or write the full name of the Class every time you use it.
Imports are just a concession to readability and the laziness of the developer.
import statements say to the compiler: if you have a function that cannot be found in this class look in the list of imports.
This way you can refer to functions in other packages without having to copy the definition (like C(++) .h files) to your own package.
The import statement in Java allows to refer to classes which are declared in other packages to be accessed without referring to the full package name. You do not need any import statement if you are willing to always refer to java.util.List by its full name, and so on for all other classes. But if you want to refer to it as List, you need to import it, so that the compiler knows which List you are referring to.
Classes from the java.lang package are automatically imported, so you do not need to explicitly do this, to refer to String, for example.
Read more: http://wiki.answers.com/Q/Why_import_statement_is_needed_in_Java_program#ixzz1zDh2ZBhE
This question already has answers here:
Meaning of the import statement in a Java file
(5 answers)
Closed 6 years ago.
What does java.util.*; do? Why do we include it at the beginning of our files?
The statement java.util.*; imports all of the java.util package members so that you don't have to use a package member's fully qualified name. According to the JavaDocs here the package java.util
Contains the collections framework, legacy collection classes, event
model, date and time facilities, internationalization, and
miscellaneous utility classes (a string tokenizer, a random-number
generator, and a bit array)
Although this approach can seem more convenient and is sometimes appropriate you shouldn't always be including an import java.util.*; statement at the beginning of all of your files unless you are using a substantial amount of the members contained in the java.util package. Only include the members that you use like so:
import java.util.ArrayList;
import java.util.LinkedList;
By doing so it helps make you more familiar with each package member that you're using instead of blindly importing the whole package. The most important reason is that by using the wildcard character(*) you have a greater chance of coming across name ambiguities which can lead to errors.
import java.awt.*;
import java.util.*;
In the above code example, the class List becomes ambiguous because both packages have a List class.
Think of it like a library of methods that you now have access to. You are basically importing more functionality into your project
When you do
import java.util.*
you now have the ability to do things like create arrays, manipulate dates and so on... (https://docs.oracle.com/javase/7/docs/api/java/util/package-summary.html)
The .* means you are importing all of the util functions like java.util.Arrays or java.util.Date...
Is it possible to make a method in java that can import things? If so how is it done? If you are wondering why I am asking this, it is because I am making a library in java to make your code smaller.
This is not possible in Java. You have to import the resources (classes, interfaces, enums, static imports, etc.) in each file. At most, you can use less code by importing all the public classes and interfaces that belong to a package by using * wildcard. Example:
//Importing all classes and interfaces declared ad java.util package
import java.util.*;
Can any one clearly explain to me what exactly happens when we use the import statement in Java files? Does it increase the size of the file if we add more and more java classes? Why don't we use class loader for the same? And what are the restrictions for importing?
import declarations (not statements) are essentially short-hand enabler at the source code level: it allows you to refer to a type or a static member using a single identifier (e.g. List, min) as opposed to the fully qualified name (e.g. java.util.List, Math.min).
import declaration section is a compile-time element of the source codes, and has no presence at run-time. In JVM bytecodes, type names are always fully qualified, and unless you're using a poorly written compiler, the binary should only contain names for types that are actually being used.
Class loaders are used for an entirely different concept, and has nothing to do with import feature at all.
JLS 7.5 Import Declarations
An import declaration allows a static member or a named type to be referred to by a simple name that consists of a single identifier. Without the use of an appropriate import declaration, the only way to refer to a type declared in another package, or a static member of another type, is to use a fully qualified name.
A single-type-import declaration imports a single named type, by mentioning its canonical name.
A type-import-on-demand declaration imports all the accessible types of a named type or package as needed. It is a compile time error to import a type from the unnamed package.
A single static import declaration imports all accessible static members with a given name from a type, by giving its canonical name.
A static-import-on-demand declaration imports all accessible static members of a named type as needed.
References
JLS 7.5.1 Single-Type-Import Declaration
JLS 7.5.2 Type-Import-on-Demand Declaration
JLS 7.5.3 Single Static Import Declaration
JLS 7.5.4 Static-Import-on-Demand Declaration
See also
Java Tutorials/Using package members
Java Language Guide/static import
Various import related questions
On the grammatical role of import:
What is an import called? - it's a declaration, not a statement
On on-demand vs single-type:
Import package.* vs import package.SpecificType
Why is using a wild card with a Java import statement bad?
What’s the difference between import java.util.*; and import java.util.Date;?
On import static:
What does the static modifier after import mean?
What is a good use case for static import of methods?
Should I use static import?
Performance-related issues:
Does importing of packages change visibility of classes? - ABSOLUTELY NOT!
Do multiple import statements in a program affect performance? - NOPE!
Any reason to clean up unused imports in Java, other than reducing clutter?
Packages consist of classes, classes in a package consist of methods, variables etc etc.
A class has a full name which comprises of the package name and the class name. If you need to use a class in your code,you need to give the compiler the full name of the class.So, you use an import statement OR you can type the fully qualified name every place you use that class in your code.
For example, if you need an AraryList in your code, you use the import statement import java.util.ArrayList; instead of typing the fully qualified class name every place you need an Arraylist.
For more detailed info, see JLS.
The imports in java are only hints for the compiler. It doesn't affect the size of the binary class file at all. You can either use an imports once or write the full name of the Class every time you use it.
Imports are just a concession to readability and the laziness of the developer.
import statements say to the compiler: if you have a function that cannot be found in this class look in the list of imports.
This way you can refer to functions in other packages without having to copy the definition (like C(++) .h files) to your own package.
The import statement in Java allows to refer to classes which are declared in other packages to be accessed without referring to the full package name. You do not need any import statement if you are willing to always refer to java.util.List by its full name, and so on for all other classes. But if you want to refer to it as List, you need to import it, so that the compiler knows which List you are referring to.
Classes from the java.lang package are automatically imported, so you do not need to explicitly do this, to refer to String, for example.
Read more: http://wiki.answers.com/Q/Why_import_statement_is_needed_in_Java_program#ixzz1zDh2ZBhE