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Java-前置基础

news 来源：原创 2025/6/7 9:35:50

前言

基础基础

package org.example;public class Main {int a =10;String s1 = "你好";public static void main(String[] args) {System.out.println(a);System.out.println(s1);}
}

发现报错位置

public class Main {static int a =10;static String s1 = "你好";public static void main(String[] args) {System.out.println(a);System.out.println(s1);}
}

在前面加上静态 static即可

一些常见的数据类型

整数类型

//整数类型
public class Main {public static void main(String[] args) {byte a =5;short b =10 ;int c =15;long d =20;System.out.println(a);System.out.println(b);System.out.println(c);System.out.println(d);}
}

但是添加一下这个就有问题，原因是超出范围，所以就有问题，因为Java都是默认从int变量开始的

解决方法，末尾加L,并输出

浮点类型

解决方案加上f即可

//浮点类型public class Main {public static void main(String[] args) {float a = 3.14f;double b = 3.15;System.out.println(a);System.out.println(b);}
}

不过一般float用的少，一般用double比较多

数据转换

布尔类型不参与数据转换

流程框架图

//基本数据类型的类型转换public class Main {public static void main(String[] args) {char ch = 'a';int i =ch;System.out.println(i);}
}

发现这个强制转换这个可能会有精度的损失

//基本数据类型的类型转换public class Main {public static void main(String[] args) {byte b = (byte)128;System.out.println(b);}
}

//基本数据类型的类型转换public class Main {public static void main(String[] args) {float f =(float)3.14567891234;System.out.println(f);}
}

发现其实也损失转的也不算太多

算术运算符

//算术运算符public class Main {public static void main(String[] args) {int a =10;int b =3;System.out.println(a+b);System.out.println(a-b);System.out.println(a * b);System.out.println(a / b);System.out.println(a % b);}
}

//算术运算符public class Main {public static void main(String[] args) {int a =10;int b =3;System.out.println(a+b);System.out.println(a-b);System.out.println(a * b);System.out.println(a / b);System.out.println(a % b);System.out.println(a / 3.0);}
}

赋值运算符

//赋值运算符public class Main {public static void main(String[] args) {int a =5;a =a+5;int b =5;b+=5;System.out.println(b);System.out.println(a);}
}

自增运算符

//赋值运算符public class Main {public static void main(String[] args) {int a =10;int b =++a;System.out.println(a);System.out.println(b);}
}

//赋值运算符public class Main {public static void main(String[] args) {int a =10;int b =++a;int c =a++;System.out.println(a);System.out.println(b);System.out.println(c);}
}

if语句

//赋值运算符public class Main {public static void main(String[] args) {double score = 61;if (score>=60){System.out.println("及格");}}
}

//赋值运算符public class Main {public static void main(String[] args) {double score = 59;if (score>=60){System.out.println("及格");}else {System.out.println("不及格");}}
}

//赋值运算符
import java.util.Scanner;//引用数据类型
public class Main {public static void main(String[] args) {Scanner s = new Scanner(System.in);System.out.println("请输入成绩");double score = s.nextDouble();if (score>=60){System.out.println("及格");}else {System.out.println("不及格");}}
}

//赋值运算符
import java.util.Scanner;//引用数据类型
public class Main {public static void main(String[] args) {Scanner s = new Scanner(System.in);System.out.println("请输入成绩");double score = s.nextDouble();if (score>=90){System.out.println("优秀");}else if(score>=70) {System.out.println("良好");}else if (score>=60) {System.out.println("及格");}else {System.out.println("不及格");}}
}

for循环

public class Main {public static void main(String[] args) {for(int i =1;i<=10;i++){System.out.println("嗨嗨嗨");}}
}

public class Main {public static void main(String[] args) {for(int i =1;i<=10;i++){//括号里的都很灵活System.out.println("我正在跑第"+i+"圈");}}
}

计算1-100的和

//计算1-100的和
public class Main {public static void main(String[] args) {int sum =0;for(int i =1;i<=100;i++){//括号里的都很灵活sum+= i;}System.out.println(sum);}
}

//计算1-100的所有偶数
public class Main {public static void main(String[] args) {for(int i =1;i<=100;i++){//括号里的都很灵活if(i%2==0){System.out.println(i);}}}
}

continue 跳过本次循环

//计算1-100的所有奇数
public class Main {public static void main(String[] args) {for(int i =1;i<=100;i++){//括号里的都很灵活if(i%2==0){continue;}System.out.println(i);}}
}

//输出1-100之间的前23个偶数
public class Main {public static void main(String[] args) {int count =0 ;for(int i =1;i<=100;i++){//括号里的都很灵活if(i%2==0){System.out.println(i);count++;}if (count ==3){break;}}}
}

while循环

//用while()实现计算1-100的和
public class Main {public static void main(String[] args) {int i =1;int sum =0;while (i<=100){sum += i;i++;}System.out.println(sum);}}

do..while循环

//用do...while()实现计算1-100的和
public class Main {public static void main(String[] args) {int i =1;int sum =0;do {sum+=i;i++;}while (i<=100);System.out.println(sum);}}

switch语句

//switch语句
import java.util.Scanner;
public class Main {public static void main(String[] args) {Scanner s =new Scanner(System.in);System.out.println("这里有1234号任务，请选择一个任务号码:");int num =s.nextInt();switch (num){case 1:System.out.println("这是1");case 2:System.out.println("这是2");case 3:System.out.println("这是3");case 4:System.out.println("这是4");default:System.out.println("moren");}}}