For (int a: num) ( for (int i: a) ( System.out.println(i); ) )

Type variableName = new Type; Type variableName = (comma-delimited values); Type variableName = new Type; Type variableName = (comma-delimited values);

is also valid, but I prefer parentheses after the type because it's easier to see that the variable's type is actually an array.

The following shows an array declaration, but the array is not initialized:

Int myIntArray = new int;

Below is the declaration as well as the initialization of the array:

Int myIntArray = (1,2,3);

Now the following also shows the declaration as well as the initialization of the array:

Int myIntArray = new int(1,2,3);

But this third one shows the anonymous creation property of an array-object, which is indicated by the reference variable "myIntArray", so if we only write "new int(1,2,3);" then it can be an anonymous array object.

If we just write:

Int myIntArray;

This is not an array declaration, but the following statement makes the following expression complete:

MyIntArray=new int;

I find it helpful if you understand each part:

Type name = new Type;

Type is the type of a variable called name ("name" is called an identifier). The literal "Type" is the base type, and the parentheses mean it is the array type of that base. Array types, in turn, are their own, which allows you to create multidimensional arrays of type Type ( array type Type ). The new keyword refers to the allocation of memory for a new array. The number between the bracket tells how big the new array will be and how much memory will be allocated. For example, if Java knows that the underlying Type is 32 bytes and you want an array of size 5, it needs to internally allocate 32 * 5 = 160 bytes.

You can also create arrays with already existing values, such as

Int name = (1, 2, 3, 4, 5);

which not only creates empty space, but also fills it with these values. Java can tell that the primitives are integers and that there are 5 of them, so the size of the array can be determined implicitly.

Also, if you want something more dynamic, there is the List interface. This won't work, but is more flexible:

List listOfString = new ArrayList (); listOfString.add("foo"); listOfString.add("bar"); String value = listOfString.get(0); assertEquals(value, "foo");

There are two main ways to create an array:

This one, for an empty array:

Int array = new int[n]; // "n" being the number of spaces to allocate in the array

And this one, for an initialized array:

Int array = (1,2,3,4 ...);

You can also create multidimensional arrays, for example:

Int array2d = new int[x][y]; // "x" and "y" specify the dimensions int array2d = ( (1,2,3 ...), (4,5,6 ...) ...);

Take the primitive type int for example. There are several ways to declare an int array:

Int i = new int; int i = new int (value1, value2, value3, etc); int i = (value1, value2, value3, etc);

where in all these cases you can use int i instead of int i .

With reflection you can use (Type) Array.newInstance(Type.class, capacity);

Note that the method parameters... display variable arguments . Basically, any number of parameters is fine. It's easier to explain with code:

Public static void varargs(int fixed1, String fixed2, int... varargs) (...) ... varargs(0, "", 100); // fixed1 = 0, fixed2 = "", varargs = (100) varargs(0, "", 100, 200); // fixed1 = 0, fixed2 = "", varargs = (100, 200);

Inside the method, varargs is treated like a normal int. Type... can only be used in method parameters, so int... i = new int() will not compile.

Note that when passing an int to a method (or any other Type), you cannot use the third way. In the statement int i = *(a, b, c, d, etc)*, the compiler assumes that (...) means int . But that's because you're declaring a variable. When passing an array to a method, the declaration must be either new Type or new Type (...) .

Multidimensional arrays

Multidimensional arrays are much more difficult to handle. Essentially, a 2D array is an array of arrays. int means array of int s. The key is that if an int is declared as int[x][y] , the maximum index is i . Essentially, a rectangle int is equal to:

Declaring an array of object references:

Class Animal () class Horse extends Animal ( public static void main(String args) ( /* * Array of Animal can hold Animal and Horse (all subtypes of Animal allowed) */ Animal a1 = new Animal; a1 = new Animal() ; a1 = new Horse(); /* * Array of Animal and all subtype of Horse */ Animal a2 = new Animal(); of Horse can hold only Horse and its subtype (if any) and not allowed supertype of Horse nor other subtype of Animal */ Horse h1 = new Horse; // Not allowed h1 = new Horse(); ; /* * This can not be declared. */ Horse h2 = new Animal;

An array is a sequential list of elements

Int item = value; int one_dimensional_array = ( value, value, value, .., value ); int two_dimensional_array = ( ( value, value, value, .. value ), ( value, value, value, .. value ), .. .. .. .. ( value, value, value, .. value ) );

If this is an object, then this is the same concept

Object item = new Object(); Object one_dimensional_array = ( new Object(), new Object(), .. new Object() ); Object two_dimensional_array = ( ( new Object(), new Object(), .. new Object() ), ( new Object(), new Object(), .. new Object() ), .. .. .. ( new Object(), new Object(), .. new Object() ) );

In case of objects you need to either assign it to null for initialization using new Type(..) , classes like String and Integer are special cases that will be treated as follows

String a = ("hello", "world" ); // is equivalent to String a = ( new String(("h","e","l","l","o")), new String(("w","o","r" ,"l","d")) ); Integer b = ( 1234, 5678 ); // is equivalent to Integer b = ( new Integer(1234), new Integer(5678) );

In general you can create arrays that are M dimensional

Int .. array = // ^ M times brackets ((..( // ^ M times ( bracket // this is array.. // ^ M times ))..) // ^ M times ) bracket ;

It is worth noting that creating a dimensional array M is expensive in terms of Space. Because when you create an array M with N in all dimensions, the total size of the array is greater than N^M , since each array has a link, and in M ​​dimension there is an (M -1)-dimensional array of links. The overall size is as follows

Array initializers and unnamed arrays

Copying arrays

What is an array?

An array in Java is a collection of elements of the same type that can be accessed by index.

Array elements in Java are located one after another in the computer's memory. Below is an example of an array in Java.

Declaring an Array in Java

Let's declare an array to store elements of type int:

Here the variable arr is declared, which is an array. To use this variable you need to define it.

Array Definition in Java

To define an array in Java, you must specify its length, i.e. number of elements that can be stored in it:

Our array will store 5 elements.

An array is a collection of elements. Each element of the array can be referred to by its number. The number is usually called an index. The numbering of array elements in Java starts from zero.

How to load elements into an array?

Let's assign a value to the first element of the array, and the first element has index zero:

Let's assign a value to the second element of the array, and the second element has index one:

for(int inn = 0; inn< 5; inn++)
{
arr = inn;
}

When declaring an array, you can immediately load values ​​into it:

int arr = (0, 1, 2, 3, 4);

the number of elements here is 5, i.e. There is no need to specify the number of elements, it will be determined automatically.

How to get elements from an array?

Each element of the array can be referred to by its number. To get an element of an array, you need to specify the name of the array and the index of the element:

This is the first element of the array, because the first element has index zero.

Let's assign the value of the third element of the array to the variable int a:

Let's output all the elements of the array in a loop (we'll iterate over the array):

For(int inn = 0; inn< 5; inn++) { System.out.println("arr[" + inn + "] = " + arr); }

A simplified version of the loop for outputting an array is as follows:

For(int inn: arr) ( System.out.println("arr[" + inn + "] = " + arr); )

How to delete an array in Java?

You can delete an array in Java like this:

How to get the length of an array in Java?

We get the length of an array in Java like this:

int arrLength = arr.length;

How to get the first element of an array in Java?

int firstElem = arr;

How to get the half-last element of an array in Java?

int lastElem = arr;

How to define a variable length array in Java?

How to define a variable length array in Java? No way. When you define an array, you then set its length; it cannot be changed later. In such cases, collections are used, for example: Vector, ArrayList, etc.

So, the array length cannot be variable. But you can use a variable when defining an array. If so:

int cd;
int ab = new int;//Error.

then we get an error: the length of the array cannot be a variable.

You need to set the cd value:

int cd = 10;
int ab = new int;

It's okay now. If you change the cd variable after defining the array, this will not affect the array, i.e. its length will not change. Example:

Int cd = 10; int ab = new int; cd = 12;// This is possible arrLength = ab.length; System.out.println("ab array length = " + arrLength); //Outputs: ab array length = 10 ab=4;// And here is the error

We get an error:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 11

The maximum index of our array is 9. Changing the value of the cd variable does not affect the array, because it is already defined and its length is constant.

Variables can be used to access array elements:

Int var = 1;
int elem = arr;
var = 2;
elem = arr;

Array of characters in Java

An example of a character array in Java and its output:

Char charArr = ("S", "B", "P"); for(int inn = 0; inn< charArr.length; inn++) { System.out.println("charArr[" + inn + "] = " + charArr); }

How to fill an array in Java?

You can fill an array using the static fill method.

An array is a collection of variables of the same type that are referred to by a common name. Arrays can be created from elements of any type, and they can have one or more dimensions. A specific element in an array is accessed by its index (number). In this note we will look at processing one-dimensional and two-dimensional arrays.

One-dimensional arrays in Java

A one-dimensional array is essentially a list of variables of the same type. To create an array, you must first create an array variable of the desired type. The general format for declaring a one-dimensional array is:
type var-name ;
Here type declares the underlying type of the array; var-name is the name of the array variable. The base type determines the data type of each array element. For example, a declaration of a one-dimensional array of int components named month_days looks like:
int month_days ;
Although this declaration establishes the fact that month_days is an array variable, no array actually exists. In fact, month_days is set to null (null pointer), which represents an array with no value. To associate month_days with an actual, physical array of integers, you need to allocate memory for it using the new operation and assign it to the month_days array; new is a special operation that allocates memory.

The general format of new as applied to one-dimensional arrays is:
array-var = new type ;
where type is the type of data being distributed, size is the number of elements in the array, array-var is a variable that is associated with the array. To use new to allocate memory for an array, you need to specify the type and number of array elements. Elements in the array allocated with the new operation will be automatically initialized to zero. The following example allocates memory for a 12-element array of integers and associates it with the month_days variable.
month_days = new int;
After this statement executes, month_days will refer to an array of twelve integers. Then all elements in the array will be initialized to zero.
The process of obtaining an array involves two steps. First, you must declare an array variable of the desired type. Second, you need to allocate the memory that will contain the array using the new operation and assign it to an array variable. Thus, in Java all arrays are dynamically allocated.

Once you have allocated memory for an array, you can access a specific element in it by specifying an index in square brackets. The numbering of array elements starts from zero. Array names are references.

A combination of declaring an array variable and allocating memory to the array directly in the declaration is possible:
int month_days = new int;

Consider the code of a program that replaces negative elements of an array with the maximum element:

Public class FindReplace ( public static void main(String args) ( int myArray; // declaration without initialization int mySecond = new int; /* memory allocation with initialization to default values ​​*/ int a = (5, 10, 0, -5 , 16, -2); // declaration with initialization int max = a; for (int i = 0; i< a.length; i++) { if (a[i]<0) a[i] = max; mySecond[i] = a[i]; System.out.println("a[" + i + "]=" + a[i]); } myArray = a; // установка ссылки на массив a } }

The result of execution will be:

>java FindReplace a=5 a=10 a=0 a=5 a=16 a=5

Assigning mySecond[i] = a[i] will cause part of the elements of the array mySecond , namely six, to be assigned the values ​​of the elements of the array a . The remaining elements of mySecond will retain the values ​​obtained during initialization, that is, zeros. If the assignment is organized in the form mySecond = a or myArray = a, then both arrays participating in the assignment will receive a reference to the array a, that is, both will contain six elements and refer to the same memory location.
Arrays can be initialized at the time they are declared. The process is much the same as that used to initialize simple types. An array initializer is a comma-separated list of expressions surrounded by curly braces. The array will automatically be created large enough to contain as many elements as you specify in the array initializer. There is no need to use the new operation. For example, to store the number of days in each month, the following code creates an initialized array of integers:

Public class MonthDays ( public static void main(String args) ( int month_days = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31); System.out.println("April contains " + month_days + " days."); ) )

As a result of executing the program, the following will be displayed on the screen:

April contains 30 days.

Comment: Java makes strict checks to make sure you don't accidentally try to store or read values ​​outside of the array's storage area. Executive system Java also makes careful checks to ensure that all array indices are in the correct range. (In this regard Java differs significantly from languages C/C++, which do not provide run-time bounds checking).

Multidimensional Arrays in Java

IN Java multidimensional arrays are, in fact, arrays of arrays. They look and act like regular multidimensional arrays. However, there are a couple of subtle differences. To declare a multidimensional array variable, define each additional index using a different set of square brackets. For example, the following statement declares a two-dimensional array variable named twoD:
int twoD = new int;
It allocates memory for a 4x5 array and assigns it to the twoD variable. Internally, this matrix is ​​implemented as an array of arrays of integers of type int .
Multidimensional arrays can be initialized. To do this, simply include each dimension's initializer within its own set of curly braces.
The following program creates and initializes arrays of arrays of equal length (matrices), and performs the product of one matrix by another:

Public class Matrix ( private int a; Matrix(int ​​n, int m) ( // creation and filling with random a = new int[n][m]; for (int i = 0; i< n; ++i) for (int j = 0; j < m; ++j) a[i][j] = (int) (Math.random()*5); show(); } public Matrix(int n, int m, int k) { // создание и заполнение с random a = new int[n][m]; for (int i = 0; i < n; ++i) for (int j = 0; j < m; ++j) a[i][j] = k; if (k != 0) show(); } public void show() { System.out.println("Матрица:" + a.length + " на " + a.length); for (int i = 0; i < a.length; ++i) { for (int j = 0; j < a.length; ++j) System.out.print(a[i][j] + " "); System.out.println(); } } public static void main(String args) { int n = 2, m = 3, z = 4; Matrix p = new Matrix(n, m); Matrix q = new Matrix(m, z); Matrix r = new Matrix(n, z, 0); for (int i = 0; i < p.a.length; ++i) for (int j = 0; j < q.a.length; ++j) for (int k = 0; k < p.a[i].length; ++k) r.a[i][j] += p.a[i][k]*q.a[k][j]; System.out.println("Произведение матриц: "); r.show(); } }

Since values ​​are assigned to array elements using the random() method, one of the options for executing the code could be the following:

> javac Matrix.java > java Matrix Matrix:2 by 3 3 2 0 3 3 1 Matrix:3 by 4 1 2 2 3 3 2 3 2 1 2 3 2 Product of matrices: Matrix:2 by 4 9 10 12 13 13 14 18 17

The following example demonstrates copying an array:

Public class ArrayCopyDemo ( public static void main(String args) ( int mas1 = (1,2,3), mas2 = (4,5,6,7,8,9); System.out.print("mas1: " ); show(mas1); System.out.print("mas2: "); // copying the mas1 array to mas2 System.arraycopy(mas1, 0, mas2, 2, 3); mas1 is copied starting from the zero element * 2 - the element from which the replacement begins * 3 - the number of elements to be copied */ System.out.println("\n after arraycopy(): "); System.out.print("mas1: " ); show(mas1); System.out.print("\nmas2: "); private static void show(int mas) ( for (int i = 0; i< mas.length; ++i) System.out.print(" " + mas[i]); } }

Result of program execution:

> javac ArrayCopyDemo.java > java ArrayCopyDemo mas1: 1 2 3mas2: 4 5 6 7 8 9 after arraycopy(): mas1: 1 2 3 mas2: 4 5 1 2 3 9

Alternative Array Declaration Syntax

There is another form that can be used to declare an array:
type var-name;
Here the square brackets follow the type specifier rather than the array variable name. For example, the following two declarations are equivalent:

Int al = new int; int a2 = new int;
The declarations presented here are also equivalent to:
char twodi = new char; char twod2 = new char;
This alternative form of announcement is included primarily for convenience.