Develop Reference

Why do you need to import java.util.* to use Arrays.toString()

The Arrays.toString() would not work without import java.util.* for some reason. Here is a simple java code that deletes an element from an array by the length of the string that the user inputs.
import java.util.*;
public class conjunction {
public static String func(String x[], int input) {
String temp[] = new String[x.length - 1];
String temp2[] = new String[x.length - 1];
for (int c = 0; c < x.length; c++) {
if (x[c].length() == input & c + 1 < x.length) {
temp2[c] = x[c + 1];
x[c + 1] = x[c];
x[c] = temp2[c];
} else if (x[c].length() != input) {
temp[c] = x[c];
}
}
return Arrays.toString(temp);
}
}
It says it cannot resolve symbol Arrays.

That's because the Arrays class lives in package java.util, so you either need to import it explicitly (with import java.util.Arrays;) or you import everything from that package (with the wildcard import java.util.*;)
More generally, everything in Java lives in a package and needs to be imported, with the exception of the java.lang package, where classes like String live, which is imported by default.

An import is never required in Java, and in fact it does not exist at the byte-code level. Your code will work if you remove the import statement and change your return to
return java.util.Arrays.toString(temp);
An import reduces the typing required at development time (it is a programmer convenience).

import java.util.Arrays; also works

using import statement is vary beginner OOP concept in JAVA.. according to your question toString(temp) is a static method of Arrays class. And the Arrays class belongs to package java.util, so using that static method you need to import java.util.Arrays. point to be noted every java class by default import java.lang.*
here i am sharing some thought about package.
Every thing in JAVA belong to package. Package are used in Java, in-order to avoid name conflicts and to control access of class, interface and enumeration etc. A package can be defined as a group of similar types of classes, interface, enumeration or sub-package. Using package it becomes easier to locate the related classes and it also provides a good structure for projects with hundreds of classes and other files.
Mainly Java project support two Types of Packages: Built-in and User defined
Using import keyword
import keyword is used to import built-in and user-defined packages into your java source file so that your class can refer to a class that is in another package by directly using its name.
There are 3 different ways to refer to any class that is present in a different package:
Using fully qualified name (But this is not a good practice.): Like return java.util.Arrays.toString(temp);This is generally used when two packages have classes with same names. For example: java.util and java.sql packages contain Date class.
To import only the class/classes you want to use : Like import java.util.Arrays;. then only the class with name Arrays in the package with name util will be available for use.
To import all the classes from a particular package: Like import java.util.*. it make all the classes and interfaces of this package (util package) will be accessible but the classes and interface inside the subpackages will not be available for use.
Points to remember
When a package name is not specified, the classes are defined into the default package and the package itself is given no name.
Java.lang.* package in Java: Provides classes that are fundamental to the design of the Java programming language.

toString() is a method inside Arrays class(static) and this class is in the package java.util.The source file for this class is in package named util which is in a package named java.
Also we can use package qualifiers instead of import.
The import statement is not always required.Class reference can be made with explicit qualification with "." operator.
eg java.util.ArrayList<String> str //fully qualified
There is one situation where qualification in necessary- when two classes have same name but are in different packages.
egjava.util.Timer and java.swing.Timer
Here the name Timer is ambiguous and can't be used without qualification.

Why does Java allow nested method calls without importing the class? [duplicate]

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

A wildcard is said to import all classes in a package. "It doesn't import child packages, fields, or methods." What does this mean?

In the Sybex book, OCA Oracle Certified Associate Java SE 8 Programmer I - Study Guide, page 10 of Chapter 1 states the following:
The * is a wildcard that matches all classes in the package. Every
class in the java.util package is available to this program when Java
compiles it. It doesn’t import child packages, fields, or methods; it
imports only classes. (Okay, it’s only classes for now, but there’s a
special type of import called the “static import” that imports other
types. You’ll learn more about that in Chapter 4.)
It was my naive understanding that since a class contains members (fields and methods), it is implied that those are imported, as well. However, according to the author of this book, it appears that the situation is more caveated.
If you are importing a class, and you don't have access to the members of that class, then what is the point of importing that class?

what is the point of importing that class?
Imagine that you do not import classes inside java.util. If you want to create a Map you type:
java.util.Map<String, Integer> myMap = new java.util.HashMap<>();
If you import the class in that package like import java.util.*;:
Map<String, Integer> myMap = new HashMap<>();
If you are importing a class, and you don't have access to the members
of that class
Imports do nothing with access, they are all about readability and convenience. You don't have to type that much and the second declaration is much more readable, but you can use the same methods of myMap in both cases.
The static import that the book mentions (from the doc):
The static import declaration is analogous to the normal import
declaration. Where the normal import declaration imports classes from
packages, allowing them to be used without package qualification, the
static import declaration imports static members from classes,
allowing them to be used without class qualification.
A good example for static import is the usage of Mockito in unit tests. Without any imports you can verify some behavior like:
org.mockito.Mockito.verify(mock, org.mockito.Mockito.never()).someMethod();
If you use normal import import org.mockito.Mockito;:
Mockito.verify(mock, Mockito.never()).someMethod();
and with static import import static org.mockito.Mockito.*; you can type
verify(mock, never()).someMethod();
Here the verify and never static methods can be used even without specifying their class (Mockito).

If you are importing a class, and you don't have access to the members of that class, then what is the point of importing that class?
Suppose you have a package com.ebony.maw.utils, and that package has a class MyUtilities, and the class has a static method findFizgigs(). If you say
import com.ebony.maw.utils.*;
you can now say
MyUtilities.findFizgigs("thingy-002");
instead of having to say
com.ebony.maw.utils.MyUtilities.findFizgigs("thingy-002");
But you still can't say
findFizgigs("thingy-002");
without the class name. That's what they mean by importing the class, but not importing the method. It just means you can use the class name without having to supply a package name prefix. But you can still use the method--you just have to supply the class name as a prefix.

Does the star import include subpackages in Java?

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.

Meaning of the import statement in a Java file

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