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1. Why use arrays and the definition of arrays 1.1 Why use arrays

Question 1:

When declaring a variable, each individual variable corresponds to a variable name, but now when dealing with a set of data of the same type, if you want to represent the age of 100 people in the class, you definitely do not want to define 100 variables to represent the age of each person. What are we going to do? Take a look at the following examples.

Int age = 17 position / indicates an age

Question 2:

Find the sum of two numbers, need a method, find the sum of five numbers, need to overload a method, find the sum of 1000 numbers, the sum of 10000 numbers, the parameter list of the method will be very long, and there must be many methods. And you have to remember which method is two parameters and which method is three parameters. This always feels very uncomfortable, careful analysis of this function, in fact, is to find the sum of a set of values, this method does not care about the specific number of additions, it only cares about which numbers need to be added.

Master's advice: when defining the parameters of a method, it is best not to exceed 5.

1.2 what is an array

To put it simply, it is a set of data, a pile of data. The so-called array is a kind of data form in which several variables of the same type are organized in an orderly form for convenience in programming. These collections of data of the same type in a certain order are called arrays. Each data in the array is called an array element, and the elements in the array are indexed to indicate where they are stored. The index starts at 0 and the step size is 1, which is a bit like the row number of an Excel table increasing row by row.

1.3 definition of array

Method 1 (recommended): array element type [] array name; eg:int [] ages

Think of int [] as a data type, an array type of int type.

Mode 2: type array name of array element []; eg: int ages []

Note: arrays must be initialized before they can be used. Because initialization means allocating space in memory.

2. Initialization of the array

Arrays in Java must be initialized before they can be used. Initialization means allocating memory to array elements and assigning initial values to each element.

There are two ways to initialize an array: static initialization and dynamic initialization; either way, once the array is initialized, the length of the array is fixed unless it is reinitialized. In other words, the array is of fixed length.

The array is fixed in length: once the array is initialized, the number of elements in the array is fixed and cannot be changed. If you need to change it, you can only reinitialize it.

2.1 static initialization of array

It is up to us to set the initialization value for each array element, and the length of the array is determined by the JVM.

Syntax:

Array element type [] array name = new array element type [] {element 1, element 2, element 3.}

For example:

Int [] nums = new int [] {1pm 3pm 5pm 7pm 9}

To put it simply, it must be initialized immediately after it is declared, not declared first and then initialized; int [] nums = {1meme3jue 5jin7jue 9}

Graphical array static initialization operation and reassignment operation

2.2 dynamic initialization of array

It is up to us to set the number of elements of the array (array length), and the initial value of each array element is determined by the system.

Syntax:

Array element type [] array name = new array element type [length]

For example:

Int [] ages = new int [100]

Note: int [] nums = new int [5] {1, 3, 5, 5, 7, 9}; / / the way it is written is wrong. You cannot use both static initialization and dynamic initialization.

2.3 when to use static initialization and when to use dynamic initialization?

When we know in advance which data to store, we choose static initialization.

When we don't know in advance what data to store, we can only use dynamic initialization.

Initial values are set for data types in Java, as shown in the following figure:

Data type

Initial value

Byte 、 short 、 int

0

Long

0L

Float

0F

Double

0.0D

Boolean

False

Char

'\ u0000' (for empty)

Reference data type

Null

3. Basic operation of array (one-dimensional array)

3.1 basic operation of array:

Get element: element type variable = array name [index]

Setting element: array name [index] = value

Traversing array elements: a for loop is recommended because the for loop knows the number of cycles in advance.

Array length: int len = array name.length; length is an attribute, not a method.

Index range: start at 0 and increase one by one. [0, array name .length-1]

3.2 Common exceptions to Operand arrays:

NullPointerException: null pointer exception (null reference).

The reason for this exception: manipulate the array directly when the array is not initialized

Such as the following code:

String [] bs = null;System.out.println (bs.length)

ArrayIndexOutOfBoundsException: the index of the array is out of bounds.

The reason for this exception: when fetching the data element according to the index, a value outside the range of the array index was entered.

The code is as follows:

Int [] nums = {1int result 3je 7je 9}; int a = nums [4]; 3.3.get the maximum and minimum elements of the array / * * find the maximum value of the array * * @ param nums * @ return * / public static int getMax (int [] nums) {int result = 0; for (int I = 0; I)

< nums.length; i++) { int num = nums[i]; if (result < num) { result = num; } } return result;}/** * 求数据最小值 * * @param nums * @return */public static int getMin(int[] nums) { int result = 0; for (int i = 0; i < nums.length; i++) { int num = nums[i]; if (i == 0) { result = num; } if (result >

Num) {result = num;}} return result;} 3.4 print array elements

When we print an array directly using System.out.println (), we print it as a hashcode value, such as

Int [] nums = new int [] {1,3,5,7,9}; System.out.println (nums)

We don't like it. When we want to print an array, we print out the elements of the array, and then we need to loop through the print.

Int [] nums = new int [] {1,3,5,7,9}; for (int I = 0; I

< nums.length; i++) { System.out.print(nums[i] + " ");} 但是呢，每次想打印数据中的元素都还要循环遍历一遍，好麻烦啊！ 有没有更好的方式呢？其实不用着急，Java前辈们已经帮我们想到了这一点了，我们只需要调用Arrays.toString()方法就能够对数组进行打印。 3.5 逆序排列数组元素 例子：原数组[A, B, C, D, E]，要求对该数组进行逆序操作得到新数组:[E, D, C, B, A]。 public static String[] reversedOrder(String[] nums) { String[] result = new String[nums.length]; int index = 0; for (int i = nums.length - 1; i >

= 0; iMel -) {result [index] = nums [I]; index++;} return result;} 3.6 Linear search: element occurrence index (first / last)

The linear search of an array means traversing one by one, looking for the same elements in the array as key, and if you can't find it, you can return-1 (convention, custom), with an efficiency of O (n).

Example: int [] arr = {10 arr 20, 30, 10, 50, 50, 50, 50, 50, 50, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10,

/ * get the index of the first occurrence of the specified element in the array * * @ param nums * @ param element * @ return * / public static int indexOf (int [] nums, int element) {for (int I = 0; I)

< nums.length; i++) { if (element == nums[i]) { return i; } } return -1;}/** * 获取数组中指定元素最后一次出现的索引 * * @param nums * @param element * @return */public static int lastIndexOf(int[] nums, int element) { for (int i = nums.length - 1; i >

= 0; iMel -) {if (element = = nums [I]) {return I;}} return-1;} 4, Multidimensional array

In the previous article, we mentioned that an array is actually a collection of multiple data. If there are multiple arrays now and I want to save multiple arrays in a collection, how should I do that? At this point, we need to introduce the concept of multi-dimensional array. A multidimensional array actually regards the whole array as an element and stores it in another array.

Syntax for multidimensional arrays:

Array element type [] array name

For example, define the format of a two-dimensional array as follows:

Int [] [] arr = new int [] [] {arr1, arr2,arr3}; int [] [] arr = new int [] [] {{1mem2, 3}, {4, 5}, {6}}; 4.1 the difference between a multidimensional array and an one-dimensional array

One-dimensional array: each element in the array is a value (values of base types and reference types)

Two-dimensional array: each element in the array is an one-digit array

Three-dimensional array: each element in the array is another two-dimensional array

Note: strictly speaking, there is no concept of multidimensional arrays in Java. In order to distinguish it from the C language, it is generally called an array in an array.

4.2 initialization of two-dimensional array

Static initialization:

Int [] [] arr = new int [] [] {

{1,2,3}

{4,5}

{6}

}

Dynamic initialization:

Int [] [] arr = new int [3] [5]; / / create a two-dimensional array of length 3, with each element (one-dimensional array) of length 5.

For N-dimensional arrays, N loops are required.

5. Java5's new syntax support for arrays

Java5's new syntax support for arrays is mainly to enhance the variable parameters of for loops (foreach) and methods.

5.1 enhanced for Loop-foreach

Previously, we used the for loop to print elements as follows

Int [] nums = new int [] {1,3,5,7,9}; for (int I = 0; I

< nums.length; i++) { System.out.println(nums[i]);} 其实我们在使用循环迭代数组的时候，往往是不关心迭代变量(数组的索引)。那在Java中有没有更好的方式，在迭代数组元素的时候就只操作数组元素，不去操作数组的索引呢？其实是有的。 从Java5开始(JDK1.5)开始，Java提供了一种新的语法：增强for循环(foreach)。 语法： for(数组元素类型 变量 : 数组名) { 循环体 } 我们通过反编译工具查看字节码，会发现foreach其实在底层依然是使用for循环+索引来操作数组的。我们把增强for循环称之为编译器的新特性---->

Grammatical sugar.

Note: the biggest advantage of syntax candy is that it allows developers to write less and simpler code to accomplish the same function. Foreach is preferred when we don't care about the index of the array when iterating over the array elements. Of course, foreach is not as simple as it is explained in this blog. Xing Zai will take you to explain foreach in depth when collecting the framework.

5.2 variable parameters of the method

Why add variable parameters when Java5? Let's take a look at the following requirements

Requirements: write a method to count the sum passed by the array.

Although it can be achieved, we are definitely upset, mainly because of the following points:

In order to find the sum of multiple numbers, we also have to create an array to store the data.

If multiple numbers are variable, for example, if the sum of three numbers becomes the sum of five numbers, you have to modify the definition array, but the array is of fixed length.

What should we do if we want to solve the problem? At this point, we need to introduce another new feature of Java5: variable parameters of the method (that is, the variable number of parameters).

Note:

The variable parameter of the method is actually a syntactic sugar, a new feature at the compiler level. The main purpose is to make it easier for developers to write code.

The underlying variable parameter of the method is an array type.

The variable parameter must be the last parameter of the method to avoid the ambiguity of the parameter.

A method has at most one variable parameter.

VI. Array element copy

Array copy: copies an array from the specified source array, starting at the specified location and ending at the specified location in the destination array.

A subsequence of the array component is copied from the source array referenced by src to the target array referenced by dest.

The number of the copied component is equal to the length parameter.

Components located between srcPos and srcPos+length-1 in the source array are copied to the destPos to destPos+length-1 locations in the target array, respectively.

Array copy operations are often used, and SUN stores the array copy operations directly in the System class in JDK.

Root class in the Object:Java language. Is the ancestor of all kinds. Object can represent any data type.

This method has no method body, and it uses the native modifier (native method). The bottom layer of this method is implemented by Cramble + language, and Java directly calls the functions written in other languages.

How to use the arraycopy method:

System.arraycopy (src, 2, dest, 5, 4)

Check out the API documentation (Java help documentation / like a dictionary), and you can find ways to do what you want in any class. I have the document in hand, I have the world!

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