JAVA之DAY3

数组知识点

package day3;

public class Demo01 {

    public static void main(String[] args) {
        // int a[]; //1：声明数组
        // a = new int[5]; //2：分配空间 【数据类型[] 数组名 = new 数据类型[大小]】;
        // a[0] = 5; //3：赋值
        // a[0] *= 5; //4：对数组进行处理
        // System.out.println(a[0]);

        // 数组长度已经固定
        int score[] = { 1, 2, 3, 4, 5 };
        // score[5] = 6; //数组越界

        int score2[] = new int[] { 1, 2, 3, 4, 5 };

        score2[3] = 55;

        System.out.println(score2.length);
    }

}

package day3;

public class Demo02 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("本次活动特价商品有：");
        String good[] = { "nike", "adidas", "Kappa" };
        for (int i = 0; i < good.length; i++) {
            System.out.println(good[i]);
        }

    }

}

运行图

package day3;

import java.util.Scanner;

public class Demo03 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        double money[] = new double[5];
        double sum = 0.0;
        for (int i = 0; i < 5; i++) {
            System.out.println("请输入第" + (i + 1) + "笔购物金额：");
            money[i] = in.nextDouble();
            sum += money[i];
        }
        System.out.println("序号	金额（元）");
        for (int i = 0; i < money.length; i++) {
            System.out.println((i + 1) + "	" + money[i]);
        }
        System.out.println("金额总和为" + sum);
    }
}

运行图

依次输入1、2、3、4、5

练习、求平均值

package day3;

import java.util.Scanner;

public class Lianxi {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int scores[] = new int[5];
        int sum = 0;
        Scanner in = new Scanner(System.in);
        System.out.println("5个成绩");
        int num = scores.length;
        for(int i = 0;i<num;i++){
            scores[i] = in.nextInt();
            sum += scores[i];
        }
        System.out.println("平均分是"+(double)sum/scores.length);
    }

}

package day3;

import java.util.Scanner;

public class Demo04 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int price[] = new int[5];
        Scanner in = new Scanner(System.in);
        for (int i = 0; i < price.length; i++) {
            System.out.println("请输入第" + (i + 1) + "家手机价格");
            price[i] = in.nextInt();
        }
        int min = price[0];
        for (int i = 0; i < price.length; i++) {
            if (price[i] > min) {
                min = min;
            } else {
                min = price[i];
            }
        }
        System.out.println(min);
    }

}

运行图

依次输入1、2、3、4、5取得最小值

foreach循环

package day3;

public class Demo05 {

    public static void main(String[] args) {
        int arr[] = { 12, 34, 56, 99, 78 };
        // foreach循环 i为数组数据
        for (int i : arr) {
            System.out.println(i);
        }
    }
}

数据类型传值

package day3;

public class Demo06 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
//        基本数据类型传值
        /*int i = 6;
        int j = i;
        i = 7;
        System.out.println(j);*/
        
//        引用数据类型
        int zhangSan[] = {180,25};
        int liSi[] = zhangSan;
        zhangSan[0] = 170;
        
        System.out.println(liSi[0]);//结果是170而不是180 ，因为是数组，他们公用一个地址，调用的也是数组地址上的数据，
        
    }

}

sort升序排列

package day3;

import java.util.ArrayList;
import java.util.Arrays;

public class Demo07 {

    public static void main(String[] args) {
        int arr[] = {99,98,75,45,76,55};
        //sort：对数组升序排列
        Arrays.sort(arr);
        for (int i : arr) {
            System.out.println(i);
        }
    }

}

运行图

比较两个数组相等的equal函数

package day3;

import java.util.Arrays;

public class Demo08 {

    public static void main(String[] args) {
//        比较两个数组是否完全相等
        int arr1[] = {10,20,30,40};
        int arr2[] = {10,20,30,40};
        int arr3[] = {10,20,30};
        int arr4[] = {20,10,30,40};
//        数组的相等是指数组内元素大小、顺序和类型全部相等
        System.out.println(Arrays.equals(arr1, arr2));
        System.out.println(Arrays.equals(arr2, arr3));
        System.out.println(Arrays.equals(arr2, arr4));
    }

}

运行图

 toString函数：把数组转换成字符串

package day3;

import java.util.Arrays;

public class Demo09 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        int arr1[] = {10,50,40,30};
        //toString：用来吧数组转换成字符串
        System.out.println(Arrays.toString(arr1));
    }

}

fill：给源数组填充新数据

package day3;

import java.util.Arrays;

public class Demo10 {

    public static void main(String[] args) {
        int arr1[] = {10,50,40,30};
        //fill：用来给源数组填充新数据
        Arrays.fill(arr1, 30);
        //转换成字符串
        System.out.println(Arrays.toString(arr1));
    }

}

运行图

binarySearch:查找数组中指定数据的索引

package day3;

import java.util.Arrays;

public class Demo11 {
    public static void main(String[] args) {
        //binarySearch对升序排列的数组才有效
        //binarySearch:用来查找数组中指定数据的索引
        int arr[] = {10,20,50,40,30,55,20};
        Arrays.sort(arr);                                        //排序
        System.out.println(Arrays.toString(arr));
        System.out.println(Arrays.binarySearch(arr, 20));
    }
}

copyOf：复制数组

package day3;

import java.util.Arrays;

public class Demo12 {

    public static void main(String[] args) {
//        复制数组        
        int[] arr = {10,50,40,30};
        //copyOf：从数组头部开始，复制指定长度的数组
        //参数1：要复制的源数组
        //参数2：要复制的数组个数
        //注意如果要复制的数组元素数量大于源数组长度，多出的数组元素会补0
        int[] arr2 = Arrays.copyOf(arr, 3);
        System.out.println(Arrays.toString(arr2));
        
        int arr3[] = Arrays.copyOf(arr, 4);
        System.out.println(Arrays.toString(arr3));
        
        int arr4[] = Arrays.copyOf(arr, 6);
        System.out.println(Arrays.toString(arr4));
    }

}

运行图

copyOfRange：复制数组中的某一段数据

package day3;

import java.util.Arrays;

public class Demo13 {

    public static void main(String[] args) {
        //copyOfRange:复制数组中的某一段数据
        //参数1：要复制的源数组；
        //参数2：开始索引（包括）
        //参数3：结束索引（不包括）
        int arr[] = {10,20,33,44,66,77};
        int arr2[] = Arrays.copyOfRange(arr, 0, 3);
        //or
        //int arr2[] = Arrays.copyOfRange(arr, 0, arr.length-1);
        System.out.println(Arrays.toString(arr2));
    }

}

运行图

输出一段数组，并将数组中的元素升序排列、逆序排列

package day3;

import java.util.Arrays;

public class Demo14 {

    public static void main(String[] args) {
        char arr[] = {'a','c','u','b','f','c'};
        System.out.println(Arrays.toString(arr));
        Arrays.sort(arr);
        System.out.println(Arrays.toString(arr));
        
        char newArr[] = new char[arr.length];
        for (int i = 0; i < arr.length; i++) {
            newArr[i] = arr[arr.length-1-i];
        }
        System.out.println(Arrays.toString(newArr));
        
        
    }

}