筆記Notes要點(diǎn)

• switch-case語(yǔ)句注意

• switch-case題目（switchTest5）

• 循環(huán)執(zhí)行順序

• if-else 實(shí)現(xiàn)3個(gè)整數(shù)排序

• String字符串比較

• 循環(huán)四要素

• 打印九九乘法表

• 特殊關(guān)鍵字continue、break

• switch(表達(dá)式) 表達(dá)式里僅支持 byte、short、char、int、枚舉、String（jdk1.7后）

switch(表達(dá)式){

??????? case 常量1：

????????????執(zhí)行語(yǔ)句1;

????????????break;

??????? case 常量2：

????????????執(zhí)行語(yǔ)句2;

????????????break;

????????......

????????default:

????????????執(zhí)行語(yǔ)句n;

????????????break;

????????}

• case 后面只能跟常量，default是可選的。如果都沒(méi)有匹配成功則執(zhí)行default中的執(zhí)行語(yǔ)句。default的位置是靈活的。

• switch-case和if-else可以替換，switch-case效率高一些

/*

從鍵盤分別輸入年、月、日，判斷這一天是當(dāng)年的第幾天

???注：判斷一年是否是閏年的標(biāo)準(zhǔn)：

???????1）可以被4整除，但不可被100整除

????或

???????2）可以被400整除

*/

import java.util.Scanner;

public class SwitchTest5{

????public static void main(String[] args){

????????//1.讀取數(shù)據(jù)

????????Scanner sc = new Scanner(System.in);

????????System.out.println("請(qǐng)輸入年份");

????????int year = sc.nextInt();

????????System.out.println("請(qǐng)輸入月份");

????????int month = sc.nextInt();

????????System.out.println("請(qǐng)輸入日期");

????????int day = sc.nextInt();

????????int sumDay = 0; //用來(lái)統(tǒng)計(jì)當(dāng)年的總天數(shù)

????????//2.統(tǒng)計(jì)天數(shù)

????????switch(month){

????????case 4:

????????????sumDay += 31;

????????case 3:

????????????//判斷當(dāng)年是否是閏年

????????????if((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)){

????????????????sumDay += 29;

????????????}else{

????????????????sumDay += 28;

????????????}

????????case 2:

????????????sumDay??+= 31;

????????case 1:

????????????sumDay += day;

????????}

????????//3.輸出

????????System.out.println(year + "年" + month + "月" + day + "日是當(dāng)年的第" + sumDay + "天");

????}

}

/*

????編寫(xiě)程序：由鍵盤輸入三個(gè)整數(shù)分別存入變量num1、num2、num3，

????對(duì)它們進(jìn)行排序(使用 if-else if-else),并且從小到大輸出。

*/

import java.util.Scanner;

public class IfTest

{

????public static void main(String[] args){

????????Scanner sc = new Scanner(System.in);

????????System.out.println("請(qǐng)輸入三個(gè)數(shù)字");

????????int num1,num2,num3;

????????num1 = sc.nextInt();

????????num2 = sc.nextInt();

????????num3 = sc.nextInt();

????????if(num1>num2){

????????????if(num3>num1){//num3最大

??????????????? System.out.println("小到大："+ num2 +" "+ num1 +" "+ num3);

????????????}else if(num3<num2){//num3最小

??????????????? System.out.print("小到大："+ num3 + " " + num2 + " " + num1);

????????????}else{

??????????????? System.out.println("小到大："+ num2 + " " + num3 + " " + num1);

????????????}

????????}else{//num1<num2

????????????if(num3>num2){//num3最大

??????????????? System.out.println("小到大："+ num1 + " "+ num2 + " "+ num3);

????????????}else if(num3<num1){//num2最大

??????????????? System.out.println("小到大："+ num3 + " " + num1 + " " + num2);

????????????}else{

??????????????? System.out.println("小到大："+ num1 + " " + num3 + " " + num2);

????????????}

????????}

????}

}

• • String字符串比較

String str = new String("帥");

????????//判斷帥不帥

????????System.out.println("帥" ==??str); //這樣不能比字符串的內(nèi)容

????????boolean handsome = "帥".equals(str); //比較的是字符串的內(nèi)容 - 如果內(nèi)容相同返回true否則返回false

• • switch使用小技巧

對(duì)學(xué)生成績(jī)大于60分的，輸出“合格”。低于60分的，輸出“不合格”

switch(score / 60){

????????

????????case 0:

????????????System.out.println("不合格");

????????????break;

????????case 1:

????????????System.out.println("合格");

????????????break;

• 編寫(xiě)程序：輸入月，日，輸出第幾天

/*

4. 編寫(xiě)程序：從鍵盤上輸入2019年的“month”和“day”，要求通過(guò)程序輸出輸入的日期為2019年的第幾天。

*/

import java.util.Scanner;

public class SwitchTest4{

????public static void main(String[] args){

????

????????//1.讀取數(shù)據(jù)

????????Scanner sc = new Scanner(System.in);

????????System.out.println("請(qǐng)輸入月份");

????????int month = sc.nextInt();

????????System.out.println("請(qǐng)輸入日期");

????????int day = sc.nextInt();

????????int sumDay = 0; //用來(lái)統(tǒng)計(jì)當(dāng)年的總天數(shù)

????

????????//2.統(tǒng)計(jì)天數(shù)

????????switch(month){

????????/*

????????case 1:

????????????sumDay = day;

????????????break;

????????case 2:

????????????sumDay = day + 31;

????????????break;

????????case 3:

????????????sumDay = day + 31 + 28;

????????????break;

????????case 4:

????????????sumDay = day + 31 + 28 + 31;

????????????break;

????????*/

????????case 4:

????????????sumDay += 31;

????????????

????????case 3:

??????????? sumDay += 28; //賦值和+=完全不一樣。sumDay += 28 : 除了sumDay原來(lái)的天數(shù)再額外加上28

????????????

????????case 2:

????????????sumDay??+= 31; // sumDay = sumDay + 31

????????????

????????case 1:

????????????sumDay += day;

????

????????}

????????//3.輸出

????????System.out.println("2019年" + month + "月" + day + "日是當(dāng)年的第" + sumDay + "天");

????}

}

• 編寫(xiě)程序：輸入年，月，日，輸出第幾天

/*

從鍵盤分別輸入年、月、日，判斷這一天是當(dāng)年的第幾天

???注：判斷一年是否是閏年的標(biāo)準(zhǔn)：

???????1）可以被4整除，但不可被100整除

????或

???????2）可以被400整除

*/

import java.util.Scanner;

public class SwitchTest4

{

????public static void main(String[] args){

????????Scanner sc = new Scanner(System.in);

??????? System.out.println("請(qǐng)輸入年份：");

????????int year = sc.nextInt();

??????? System.out.println("請(qǐng)輸入月份：");

????????int month = sc.nextInt();

??????? System.out.println("請(qǐng)輸入天數(shù)：");

????????int day = sc.nextInt();

????????int sumDay = 0;

????????switch(month){

????????????case 12:

????????????????sumDay += 30;

????????????case 11:

????????????????sumDay += 31;

????????????case 10:

????????????????sumDay += 30;

????????????case 9:

????????????????sumDay += 31;

????????????case 8:

????????????????sumDay += 31;

????????????case 7:

????????????????sumDay += 30;

????????????case 6:

????????????????sumDay += 31;

????????????case 5:

????????????????sumDay += 30;

????????????case 4:

????????????????sumDay += 31;

????????????case 3:

????????????????// 28或 29

????????????????if((year%4==0&&year%100!=0)||year % 400 ==0){//閏年

????????????????????sumDay += 29;

????????????????}else{

????????????????????sumDay += 28;

????????????????}

????????????case 2:

????????????????sumDay += 31;

????????????case 1:

????????????????sumDay += day;

????????????????break;

????????}

????????System.out.println(year+"年" + month + "月" + day + "日是當(dāng)年的第" + sumDay + "天");

????}

}

• 循環(huán)四要素

循環(huán)的四個(gè)部分：

????????1.初始化條件

????????2.循環(huán)條件

????????3.循環(huán)體

????????4.迭代條件

• 打印九九乘法表

/*

1*1=1

1*2=2 2*2=4

1*3=3 2*3=6 3*3=9

打印九九乘法表

*/

public class c99

{

????public static void main(String[] args){

????????int i,j;

????????for(i=1;i<=9;i++){//9行

????????????for(j=1;j<=i;j++){

????????????????System.out.print(j + "*" +i+" = "+ i*j +" ");

????????????}

????????????System.out.println();

????????}

????}

}

• 特殊關(guān)鍵字continue、break

  • break用于 switch-case語(yǔ)句與循環(huán)結(jié)構(gòu)中，表示結(jié)束當(dāng)前case語(yǔ)句，結(jié)束循環(huán)語(yǔ)句

  • continue，用于 循環(huán)結(jié)構(gòu)? 表示結(jié)束當(dāng)此循環(huán)。

  • 在嵌套循環(huán)中break結(jié)束的包含該語(yǔ)句的循環(huán)語(yǔ)句，對(duì)外層循環(huán)無(wú)作用

  • 在嵌套循環(huán)中continue跳出的是包含他的循環(huán)語(yǔ)句的當(dāng)次循環(huán)，不跳出

• 嵌套循環(huán)中如何使用內(nèi)部循環(huán)的break結(jié)束外層循環(huán)

可以給循環(huán)命名，break + 循環(huán)名字

總結(jié)Summary

• if-else實(shí)現(xiàn)三個(gè)整數(shù)排序

• 打印九九乘法表

• break，continue使用注意事項(xiàng)