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 can hold Animal and Horse and all subtype of Horse */ Animal a2 = new Horse; a2 = new Animal(); a2 = new Horse(); /* * Array 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; h1 = new Animal(); // Not allowed h1 = new Horse() ; /* * This can not be declared. */ Horse h2 = new Animal; // Not allowed ) )

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's worth noting that creating a dimensional array M is expensive from a Space perspective. 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

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.

Java arrays

Last update: 11/09/2018

An array represents a set of values ​​of the same type. Declaring an array is similar to declaring a regular variable that stores a single value, and there are two ways to declare an array:

data_type array_name; // either data_type array_name;

For example, let's define an array of numbers:

Int nums; int nums2;

After declaring the array, we can initialize it:

Int nums; nums = new int; // array of 4 numbers

An array is created using the following construction: new data_type[number of elements] , where new is a keyword that allocates memory for the number of elements specified in parentheses. For example, nums = new int; - this expression creates an array of four int elements, and each element will have a default value of 0.

You can also immediately initialize an array when declaring it:

Int nums = new int; // array of 4 numbers int nums2 = new int; // array of 5 numbers

With such initialization, all array elements have a default value. For numeric types (including the char type) this is the number 0, for the boolean type this is false , and for other objects this is null . For example, for the int type the default value is 0, so the nums array defined above will consist of four zeros.

However, you can also set specific values ​​for the elements of an array when you create it:

// these two methods are equivalent int nums = new int ( 1, 2, 3, 5 ); int nums2 = ( 1, 2, 3, 5 );

It is worth noting that in this case the square brackets do not indicate the size of the array, since it is calculated by the number of elements in the curly braces.

After creating an array, we can access any of its elements by index, which is passed in square brackets after the name of the array variable:

Int nums = new int; // set the values ​​of the array elements nums = 1; nums = 2; nums = 4; nums = 100; // get the value of the third element of the array System.out.println(nums); // 4

Indexing of array elements starts at 0, so in this case, to access the fourth element in the array, we need to use the nums expression.

And since our array is defined for only 4 elements, we cannot access, for example, the sixth element: nums = 5; . If we try to do this, we will get an error.

Array length

The most important property that arrays have is the length property, which returns the length of the array, that is, the number of its elements:

Int nums = (1, 2, 3, 4, 5); int length = nums.length; // 5

It's not uncommon for the last index to be unknown, and to get the last element of an array, we can use this property:

Int last = nums;

Multidimensional arrays

Previously, we looked at one-dimensional arrays, which can be represented as a chain or string of values ​​of the same type. But in addition to one-dimensional arrays, there are also multi-dimensional ones. The most famous multidimensional array is a table representing a two-dimensional array:

Int nums1 = new int ( 0, 1, 2, 3, 4, 5 ); int nums2 = ( ( 0, 1, 2 ), ( 3, 4, 5 ) );

Visually, both arrays can be represented as follows:

One-dimensional array nums1

Two-dimensional array nums2

Since the nums2 array is two-dimensional, it is a simple table. It could also be created like this: int nums2 = new int; . The number of square brackets indicates the size of the array. And the numbers in brackets indicate the number of rows and columns. And also, using indexes, we can use array elements in a program:

// set the element of the first column of the second row nums2=44; System.out.println(nums2);

A three-dimensional array declaration might look like this:

Int nums3 = new int;

Jagged array

Multidimensional arrays can also be represented as "jagged arrays". In the example above, the two-dimensional array had 3 rows and three columns, so we had a flat table. But we can assign each element in a two-dimensional array a separate array with a different number of elements:

Int nums = new int; nums = new int; nums = new int; nums = new int;

foreach

A special version of the for loop is designed to iterate over elements in sets of elements, such as arrays and collections. It is similar to the foreach loop found in other programming languages. Its formal announcement:

For (data_type variable_name: container)( // actions)

For example:

Int array = new int ( 1, 2, 3, 4, 5 ); for (int i: array)( System.out.println(i); )

In this case, the container is an array of data of type int . Then a variable of type int is declared

The same thing could be done with the regular version of for:

Int array = new int ( 1, 2, 3, 4, 5 ); for (int i = 0; i< array.length; i++){ System.out.println(array[i]); }

At the same time, this version of the for loop is more flexible than for (int i: array) . In particular, in this version we can change the elements:

Int array = new int ( 1, 2, 3, 4, 5 ); for (int i=0; i

Looping through multidimensional arrays

First, a loop is created to iterate over the rows, and then within the first loop, an inner loop is created to iterate over the columns of a particular row. In a similar way, you can iterate over three-dimensional arrays and sets with a large number of dimensions.