[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});

数组作为函数的返回值

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;
}

以上实例中result数组作为函数的返回值。

Arrays 类

java.util.Arrays类能方便地操作数组，它提供的所有方法都是静态的。具有以下功能：

给数组赋值：通过fill方法。

对数组排序：通过sort方法,按升序。

比较数组：通过equals方法比较数组中元素值是否相等。

查找数组元素：通过binarySearch方法能对排序好的数组进行二分查找法操作。

具体说明请查看下表：

序号

方法和说明

1    public static int binarySearch(Object[] a, Object key)
用二分查找算法在给定数组中搜索给定值的对象(Byte,Int,double等)。数组在调用前必须排序好的。如果查找值包含在数组中，则返回搜索键的索引；否则返回 (-(插入点) - 1)。    

2    public static boolean equals(long[] a, long[] a2)
如果两个指定的 long 型数组彼此相等，则返回 true。如果两个数组包含相同数量的元素，并且两个数组中的所有相应元素对都是相等的，则认为这两个数组是相等的。换句话说，如果两个数组以相同顺序包含相同的元素，则两个数组是相等的。同样的方法适用于所有的其他基本数据类型（Byte，short，Int等）。    

3    public static void fill(int[] a, int val)
将指定的 int 值分配给指定 int 型数组指定范围中的每个元素。同样的方法适用于所有的其他基本数据类型（Byte，short，Int等）。    

4    public static void sort(Object[] a)
对指定对象数组根据其元素的自然顺序进行升序排列。同样的方法适用于所有的其他基本数据类型（Byte，short，Int等）。    

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