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 d
		d= 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");
			}
		}


	}
}