1.基础语法

// 定义一个类HelloWorld,类名必须与文件名相同
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

javac HelloWorld.java     //编译

java HelloWorld           //运行

public class Variable {
    public static void main(String[] args){
        int i = 12;
        System.out.println(i);
        
        float f = 0.5f;
        System.out.println(f);
        
        char c = ‘B‘;
        System.out.println(c);
    }
}

public class DataType {
    public static void main(String[] args){
        int i = 5;
        byte b = 2;
        // byte类型比int小，做运算的时候会自动把byte类型转换成int类型
        int j = i + b;
        System.out.println(j);
    }
}

public class DataType {
    public static void main(String[] args){
        double i = 2.33;
        System.out.println(i);    //2.33
        //把double类型强制转换成int类型
        int j = (int) 2.33;
        System.out.println(j);    //2
    }
}

public class DemoIfElse {
    public static void main(String[] args){
        int score = 74;
    
        if(score<0 || score>100){
            System.out.println("数据错误");
        }else if(score>=80 && score<=100){
            System.out.println("优秀");
        }else if(score>=60 && score<80){
            System.out.println("及格");
        }else{
            System.out.println("不及格");
        }
    }
}

public class DemoSwitch {
    public static void main(String[] args){
        int i = 2;
        
        switch(i){
            case 1:
                System.out.println("星期一");
                break;
            case 2:
                System.out.println("星期二");
                break;
            case 3:
                System.out.println("星期三");
                break;
            default:
                System.out.println("数据错误");
                break;
        }
    }
}

public class DemoFor{
    public static void main(String[] args){
        //for循环
        for(int i=1;i<=10;i++){
            System.out.println(i);
        }
        //while循环
        int j = 1;
        while(j<=10){
            System.out.println(j);
            j++;
        }
        
        //do..while循环
        int a = 1;
        do{
            System.out.println(a);
            a++;
        }while(a<=10);
    }
}