跳过部分基础，写三个小练习

src/class12/test1.java

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+package class12;
+
+import java.util.Scanner;
+
+public class test1 {
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+        System.out.println("请输入第一个数字：");
+        double num1 = sc.nextDouble();
+        System.out.println("请输入第二个数字：");
+        double num2 = sc.nextDouble();
+        System.out.println("请输入运算符：(+,-,*,/)");
+        String str = sc.next();
+        double result = getResult(num1,num2,str);
+        System.out.println("结果是："+result);
+    }
+    public static double getResult(double num1,double num2,String str){
+        return switch (str) {
+            case "+" -> num1 + num2;
+            case "-" -> num1 - num2;
+            case "*" -> num1 * num2;
+            case "/" -> num1 / num2;
+            default -> {
+                System.out.println("输入的运算符有误");
+                yield 0;
+            }
+        };
+    }
+}

src/class12/test2.java

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+package class12;
+
+import java.util.Scanner;
+
+public class test2 {
+    public static void main(String[] args) {
+        //guess
+        int a = (int)(Math.random()*100);
+        Scanner sc = new Scanner(System.in);
+        while(true){
+            int b = sc.nextInt();
+            if(big_or_small(a,b)){
+                break;
+            }
+        }
+        System.out.println("猜对了！");
+
+    }
+    public static boolean big_or_small(int a,int b){
+//        错误返0，正确返回1
+        if(b>a){
+            System.out.println("猜大了");
+            return false;
+        }else if(b<a){
+            System.out.println("猜小了");
+            return false;
+        }else {
+            return true;
+        }
+    }
+}

src/class12/test3.java

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+package class12;
+
+public class test3 {
+    public static void main(String[] args) {
+        System.out.println("验证码已生成：");
+        System.out.println(getCode(4));
+        System.out.println(getCode(6));
+        System.out.println(getCode2(4));
+        System.out.println(getCode2(6));
+
+    }
+    public static String getCode(int len){
+        StringBuilder code= new StringBuilder();
+        for (int i = 0; i < len; i++) {
+            int type = (int)(Math.random()*3);
+            switch (type) {
+                case 0:
+                    code.append((char) (Math.random() * 26 + 'a'));
+                    break;
+                case 1:
+                    code.append((char) (Math.random() * 26 + 'A'));
+                    break;
+                case 2:
+                    code.append((int) (Math.random() * 10));
+                    break;
+            }
+        }
+        return code.toString();
+    }
+    public static String getCode2(int len) {
+        String dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+        StringBuilder code = new StringBuilder();
+        for (int i = 0; i < len; i++) {
+            int index = (int)(Math.random() * dict.length());
+            code.append(dict.charAt(index));
+        }
+        return code.toString();
+    }
+}