Java基础3--条件选择结构

条件选择结构

数字交换

直接看代码：

整形a和double型b交换

public class Test1{public static void main(String[] args){int a =3;double b = 3.9;int c;double d;c = a;d = (double)a;//强制交换a赋值给double dd= b;b = (double)c;System.out.println("a is\t"+d);System.out.println("b is\t"+b);}}

boolean数据类型和关系运算符

数据类型：int double  String  char  现在增加一个布尔类型：boolean

boolean类型，数值只有：true、false

关系运算符：

<   >   <=   >=    ==    !=

代码实例：

public class Test{public static void main(String[] args){double a =9.0;int b= 4;boolean c = a>b;System.out.println(c);}}               //结果是true

if判断语句

if  (boolean)   {

boolean = true,执行大括号里面的。

}

else{

false 执行这里的

}

import java.util.*;public class Test{public static void main(String[] args){Scanner in;in = new Scanner(System.in);System.out.println("input your score\t:");int score = in.nextInt();if(score >= 90){System.out.println("good!!");}else{System.out.println(" your score not good ;");}}}

逻辑运算符号

• &&  和，两个条件为true结果为true
• ||    或，只要一个条件是true结果就是true
• !    取反，条件true，结果false

import java.util.*;public class Test{public static void main(String[] args){Scanner in = new Scanner(System.in);System.out.println("input a value:");int a = in.nextInt();System.out.println("input b value:");int b = in.nextInt();if(!(a>50)&&b >50){  //逻辑运算符用于boolean数据类型的，用在if条件里面System.out.println("a、b >50");}else{System.out.println("00000000000000");}}}

运算符的优先级3

。。。

多重if结构和if嵌套结构

if(){}else if(){}...else{}

import java.util.*;public class Test1{public static void main(String[] args){Scanner in;in = new Scanner(System.in);System.out.println("input score:");double score = in.nextDouble();if(type(score)==int||double||float){  //Java中判断数据类型的函数是？Python是type()if(score>=90){System.out.println("your are good,your score is "+score);}else if(score>=80){System.out.println("lianghao");}else if(score>=60){System.out.println("jige");}else{System.out.println("bad score");}}else{System.out.println("input number please!!!!");}}}

if嵌套

import java.util.*;public class Test2{public static void main(String[] args){Scanner in = new Scanner(System.in);System.out.println("input your sex(boy or girl)");String sex = in.next();if(sex.equals("boy")){  //字符串作比较，判断字符串是否相同；==>需要比较的字符串.equals("String")不能使用比较运算符 “==”System.out.println("how old are you?");int age = in.nextInt();if(age >= 20){System.out.println("old boy!!!");}else{System.out.println("little boy .........");}}else{System.out.println("hi mei nv!");}}}

字符串作比较：

//字符串作比较，判断字符串是否相同；==>! 需要比较的字符串.equals("String")不能使用比较运算符 “==”

switch判断

语法：

switch(int/char类型){  //switch结构中小括号只能出现int类型和char类型case 常量://case的个数可以随便定义，但后面一定要跟常量执行语句;break;case 常量:执行语句;break;default:执行语句}

实例、：

import java.util.*;public class switch1{public static void main(String[] args){Scanner in = new Scanner(System.in);System.out.println("what is your mingci");int mingci = in.nextInt();switch(mingci){case 1:System.out.println("9999");break;case 2:System.out.println("8888");break;default:System.out.println("ooooooooooooooo");}}}

取随机数方法

//取随机数两种方法import java.util.Random;  //注意大写public class Test5{public static void main(String[] args){Random n = new Random(); //和Java的Scanner一样，需要先声明int a = n.nextInt(100); //取0~99的随机数，结果是整数System.out.println(a);}}//可以支持多线程public class Test5{public static void main(String[] args){double a = Math.random()*100; //注意小写。取0~99的随机数，结果是double的需要强制转换就能输出int了System.out.println((int)a);}}

猜拳游戏

和计算机比较猜拳游戏：

import java.util.*;public class game{public static void main(String[] args){Scanner in = new Scanner(System.in);System.out.println("-----gameStart------");System.out.println("input one of 3 number:1 scissor  2 stone 3 paper");int person = in.nextInt(); //接收用户输入的整数，这里只能输入整数，输入其他类型，接收不了会报错；//String a = in.next();//double b = in.nextDouble();String mark1 = "test"; //标记if(person != 1&& person != 2&& person != 3)  //判断用户输入的数，只能在是1 2 3{System.out.println("pleas send number in 1  2  3");}else{switch(person){case 1:mark1 = "scissor";break;case 2:mark1 = "stone";break;case 3:mark1 = "paper";break;}int computer = (int)(Math.random()*3) + 1;String mark2 = "test2";//标记switch(computer){case 1:mark2 = "scissor";break;case 2:mark2 = "stone";break;case 3:mark2 = "paper";break;}if(person==computer)  //平局情况{System.out.println("you is "+mark1+" computer is "+ mark2+" piju =_=!!");}else if(person==1&&computer==2||person==2&&computer==3||person==3&&computer==1)//计算机赢了的情况{System.out.println("you is "+mark1+" computer is "+ mark2+" computer win,OoO");}else //人赢了的情况{System.out.println("you is "+ mark1+" computer is "+mark2+" you win haha");}}}}