数组的定义

1. 数组是相同类型数据的有序集合
2. 按照一定的先后次序排列组合而成
3. 每个数据称为数组的元素，可以用下标来访问

数组的声明和创建

int[] array1 = {1,2,3};  //静态初始化
//动态初始化
int[] array;    //声明
array = new int[3];    //创建
//赋值
array[0] = 1;
array[1] = 2;
array[2] = 3;
//lenth属性
int i = array.lenth;
System.out.println(array[3]);  //报错java.lang.ArrayIndexOutOfBoundsException: 3

java.lang.ArrayIndexOutOfBoundsException: 3
报错：数组索引超出边界异常

三种初始化

1. 静态初始化
2. 动态初始化
3. 默认初始化

数组与对象都在堆中

数组的使用

public class Demo02 {
    public static void main(String[] args) {
        int[] arrays = {1,2,3,4,5};
        int[] reverse = reverse(arrays);
        printArray(reverse);
    }
    //打印数组
    public static void printArray(int[] arrays){
        for (int i = 0; i < arrays.length; i++) {
            System.out.print(arrays[i]+" ");
        }
    }
    //反转数组
    public static int[] reverse(int[] arrays){
        int[] reverse = new int[arrays.length];
        for (int i = 0,j = arrays.length-1; i < arrays.length; i++,j--) {
            reverse[j] = arrays[i];
        }
        return reverse;
    }
}

多维数组

public class Demo03 {
    public static void main(String[] args) {
        int[][] arrays = {{1,2},{3,4},{5,6}};
        System.out.println(arrays.length);   //3
        System.out.println(arrays[2].length);  //2
    }
}

Arrarys类

java.util.Arrays

文档地址：https://www.yiibai.com/java/util/java_util_arrays.html

public class Demo04 {
    public static void main(String[] args) {
        int[] arrays = {1,2,10,4,5,6,7,8,9,3};
        System.out.println(arrays);  //[I@4554617c
        //打印数组元素  [1, 2, 10, 4, 5, 6, 7, 8, 9, 3]
        System.out.println(Arrays.toString(arrays));
        //排序        [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
        Arrays.sort(arrays);
        System.out.println(Arrays.toString(arrays));
        //数组填充    [10, 10, 10, 10, 10, 10, 10, 10, 10, 10]
        Arrays.fill(arrays,10);
        System.out.println(Arrays.toString(arrays));
    }
}

冒泡排序

import java.util.Arrays;

public class Demo05 {
    public static void main(String[] args) {
        //冒泡排序:比较相邻的两个数组，若第一个大于第二个则交换位置
        int[] arrays  = {1,2,45,46,67,4,21,64,76,123,100};
        System.out.println(Arrays.toString(arrays));//[1, 2, 45, 46, 67, 4, 21, 64, 76, 123, 100]
        int[] sort = sort(arrays);
        System.out.println(Arrays.toString(sort));//[1, 2, 4, 21, 45, 46, 64, 67, 76, 100, 123]
    }
    public static int[] sort(int[] arrays){
        for (int i = 0; i < arrays.length-1; i++) {
            int m = 0;
            for (int j = 0; j < arrays.length-1-i; j++) {
                if (arrays[j]>arrays[j+1]){
                    m = arrays[j];
                    arrays[j] = arrays[j+1];
                    arrays[j+1] = m;
                }
            }
        }
        return arrays;
    }
}

稀疏数组

当一个数组的大部分元素都是相同的数字时，可以使用稀疏数组来保存数组
稀疏数组的处理方式：
记录数组有几行几列，有多少个不同的值
把不同的值的元素和行列及值的记录保存在小规模的数组中，从而缩小程序的规模
如下图所示：左为原始数组，右为稀疏数组

Snipaste_2021-03-30_17-43-16.png

需求：编写一个五子棋游戏,

package com.array;

public class Demo06 {
    public static void main(String[] args) {
        //创建一个二维数组【11】【11】   0：没有棋子，1：白色棋子，2：黑色棋子
        int[][] arrays = new int[11][11];
        arrays[1][2] = 1;
        arrays[2][3] = 2;
        System.out.println("输出原始数组");
        for (int[] ints : arrays) {
            for (int anInt : ints) {
                System.out.print(anInt+"\t");
            }
            System.out.println();
        }
        //转化为稀疏数组保存
        //1.获取有效值 sum
        int sum = 0;
        for (int i = 0; i < 11; i++) {
            for (int j = 0; j < 11; j++) {
                if (arrays[i][j]!=0){
                    sum++;
                }
            }
        }
        System.out.println("有效值为："+sum);
        //2.创建一个稀疏数组的数组
        int[][] arrays2 = new int[sum+1][3];
        arrays2[0][0] = 11;
        arrays2[0][1] = 11;
        arrays2[0][2] = sum;
        //3.遍历二维数组，将非零的值，存放在稀疏数组中
        int count = 0;
        for (int i = 0; i < arrays.length; i++) {
            for (int j = 0; j < arrays[i].length; j++) {
                if (arrays[i][j]!=0){
                    count++;
                    arrays2[count][0] = i;
                    arrays2[count][1] = j;
                    arrays2[count][2] = arrays[i][j];
                }
            }
        }
        //4.输出稀疏数组
        System.out.println("输出稀疏数组：");
        for (int i = 0; i < arrays2.length; i++) {
            System.out.println(arrays2[i][0]+"\t"+arrays2[i][1]+"\t"+arrays2[i][2]);
        }
        //读取稀疏数组
        //1.创建一个二维数组
        int[][] arrays3 = new int[arrays2[0][0]][arrays2[0][1]];
        //2.给其中的元素还原他的值
        for (int i = 1; i < arrays2.length; i++) {
            arrays3[arrays2[i][0]][arrays2[i][1]] = arrays2[i][2];
        }
        //3.输出还原数组
        System.out.println("输出还原数组：");
        for (int[] ints : arrays3) {
            for (int anInt : ints) {
                System.out.print(anInt+"\t");
            }
            System.out.println();
        }
    }
}

程序结果为：

输出原始数组
0   0   0   0   0   0   0   0   0   0   0   
0   0   1   0   0   0   0   0   0   0   0   
0   0   0   2   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
有效值为：2
输出稀疏数组：
11  11  2
1   2   1
2   3   2
输出还原数组：
0   0   0   0   0   0   0   0   0   0   0   
0   0   1   0   0   0   0   0   0   0   0   
0   0   0   2   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   
0   0   0   0   0   0   0   0   0   0   0   

Process finished with exit code 0