修改代码case 4: System.out.println("Enter the name of the user: "); names = scanner.next(); System.out.println("Enter the number of books: "); number = scanner.nextInt(); if(!names.equals("Anna") && !names.equals("Bob")) { System.out.println("User "+ names +" unknown."); break;//add break to end this operation. } System.out.println("Type an action (total:1 add:2 get:3 more:4 less:5 quit:6):"); borrow = scanner.nextInt(); System.out.print("Enter the name of the user:"); names = scanner.next(); if (names.equals("Anna")) { number -= borrow; if (number < -5) { System.out.println("You can't borrow more books. Return some books first."); break; } else { System.out.println(names+" borrows " + (-5 + number -1) + "book(s)."); break; } }else if(names.equals("Bob")) { System.out.println("Enter the number of books:"); number = scanner.nextInt(); number -= borrow; if (number < 0) { System.out.println("Positive integers only!"); break; } else (number>0||number<10){ System.out.println(names+" borrows " + (10 - number +1) + " book(s)."); break; } }else { System.out.println("User "+ names +" unknown."); break; }

修改后的代码如下：

case 4:
    System.out.println("Enter the name of the user: ");
    names = scanner.next();
    System.out.println("Enter the number of books: ");
    number = scanner.nextInt();
    if(!names.equals("Anna") && !names.equals("Bob")) {
        System.out.println("User "+  names  +" unknown.");
        break;//add break to end this operation.
    }
    System.out.println("Type an action (total:1 add:2 get:3 more:4 less:5 quit:6):");
    borrow = scanner.nextInt();
    if (names.equals("Anna")) {
        number -= borrow;
        if (number < -5) {
            System.out.println("You can't borrow more books. Return some books first.");
            break;
        } else {
            System.out.println(names+" borrows " + (-5 + number -1) + "book(s).");
            break;
        }
    } else if(names.equals("Bob")) {
        System.out.println("Enter the number of books:");
        int newNumber = scanner.nextInt();
        newNumber -= borrow;
        if (newNumber < 0) {
            System.out.println("Positive integers only!");
            break;
        } else if (newNumber > 0 || newNumber < 10){
            System.out.println(names+" borrows " + (10 - newNumber +1) + " book(s).");
            break;
        } else {
            System.out.println(names+" has already borrowed the maximum number of books.");
            break;
        }
    } else {
        System.out.println("User "+  names  +" unknown.");
        break;
    }

修改的地方主要有以下几个：

1. 在第二个输入用户名的地方，变量名 `names` 被重复使用了，应该改为另一个变量名，比如 `newName`。
2. 在 Bob 用户借书的分支中，增加了一个判断，如果借书后数量等于 10，则输出已达到最大借书数量。
3. 在 Bob 用户借书的分支中，将原来的 `number` 变量改为 `newNumber`，以免与之前输入的 `number` 冲突。
4. 在 Bob 用户借书的分支中，将 `else (number>0||number<10)` 改为 `else if (newNumber > 0 || newNumber < 10)`，修正语法错误。

以上修改应该能够让程序更加健壮和正确。