[java tutorial] Java array

Java Arrays

Arrays are one of the important data structures for every editing language. Of course, different languages implement and process arrays differently.

Arrays provided in the Java language are used to store fixed-size elements of the same type.

You can declare an array variable, such as numbers[100] instead of directly declaring 100 independent variables number0, number1, ...., number99.

This tutorial will introduce you to the declaration, creation and initialization of Java arrays, and give you the corresponding code.

Declare array variables

Array variables must first be declared before the array can be used in the program. The following is the syntax for declaring array variables:

dataType[] arrayRefVar; // 首选的方法 或 dataType arrayRefVar[]; // 效果相同，但不是首选方法

Note: It is recommended to use the declaration style of dataType[] arrayRefVar to declare array variables. The dataType arrayRefVar[] style comes from the C/C++ language and is adopted in Java to allow C/C++ programs to quickly understand the java language.

Examples

The following are code examples of these two syntaxes:

double[] myList; // 首选的方法 或 double myList[]; // 效果相同，但不是首选方法

Create arrays

Java language uses the new operator to create arrays. The syntax is as follows :

arrayRefVar = new dataType[arraySize];

The above syntax statement does two things:

1. Use dataType[arraySize] to create an array.

2. Assign the reference of the newly created array to the variable arrayRefVar.

The declaration of array variables and the creation of an array can be completed in one statement, as shown below:

dataType[] arrayRefVar = new dataType[arraySize];

In addition, you can also create an array in the following way.

dataType[] arrayRefVar = {value0, value1, ..., valuek};

The elements of an array are accessed by index. Array indexing starts at 0, so index values range from 0 to arrayRefVar.length-1.

Example

The following statement first declares an array variable myList, then creates an array containing 10 double type elements, and assigns its reference to the myList variable.

double[] myList = new double[10];

The picture below depicts the array myList. Here, there are 10 double elements in the myList array, and their subscripts range from 0 to 9.

Processing Arrays

The element type of the array and the size of the array are determined, so when processing array elements, we usually use a basic loop or foreach cycle.

Example

This example completely demonstrates how to create, initialize and manipulate an array:

public class TestArray { public static void main(String[] args) { double[] myList = {1.9, 2.9, 3.4, 3.5}; // 打印所有数组元素 for (int i = 0; i < myList.length; i++) { System.out.println(myList[i] + " "); } // 计算所有元素的总和 double total = 0; for (int i = 0; i < myList.length; i++) { total += myList[i]; } System.out.println("Total is " + total); // 查找最大元素 double max = myList[0]; for (int i = 1; i < myList.length; i++) { if (myList[i] > max) max = myList[i]; } System.out.println("Max is " + max); } }

The above example compiles and runs and the results are as follows:

1.9 2.9 3.4 3.5 Total is 11.7 Max is 3.5

foreach loop

JDK 1.5 introduces a new type of loop, called a foreach loop or enhanced loop, which can traverse an array without using subscripts.

Example

This example is used to display all elements in the array myList:

public class TestArray { public static void main(String[] args) { double[] myList = {1.9, 2.9, 3.4, 3.5}; // 打印所有数组元素 for (double element: myList) { System.out.println(element); } } }

The compilation and running results of the above example are as follows:

1.9 2.9 3.4 3.5

数组作为函数的参数

数组可以作为参数传递给方法。例如，下面的例子就是一个打印int数组中元素的方法。

public static void printArray(int[] array) { for (int i = 0; i < array.length; i++) { System.out.print(array[i] + " "); } }

下面例子调用printArray方法打印出 3，1，2，6，4和2：

printArray(new int[]{3, 1, 2, 6, 4, 2});

public static int[] reverse(int[] list) { int[] result = new int[list.length]; for (int i = 0, j = result.length - 1; i < list.length; i++, j--) { result[j] = list[i]; } return result; }