仅通过使用charAt（lastIndexOf，concatenate，substring，contain）进行Java String操作

Question

我想仅通过使用charAt在String中找到字符的lastIndexOf，但是我的代码仅找到第一个匹配项。 我必须改变什么？

Scanner sc = new Scanner(System.in);
char operation = sc.nextLine().charAt(0);
if (operation == 'l' ) {
        System.out.print("Please enter a string: ");
        String enteredString = sc.next();
        System.out.print("Please enter a character: ");
        char char1 = sc.next().charAt(0);
        int index = 1;
        for (int i = 0; i < enteredString.length(); i++) {
            if (enteredString.charAt(i) == char1) {
                index = i;
                break;
            }
        }
        System.out.println("The index of character " + char1 + " in string " + enteredString + " is: " + index);

    }

我成功连接了两个String：

Scanner sc = new Scanner(System.in);
char operation = sc.nextLine().charAt(0);
String c = "concatenation";
    if (operation == 'c' ) {
        System.out.println("Please enter the first string: ");
        String firstString = sc.next();
        System.out.println("Please enter the scond string: ");
        String secondString = sc.next();
        for (int i = 0; i < firstString.length(); i++) {
            char x = firstString.charAt(i);
            System.out.print(x);
        }
        for (int i = 0; i < secondString.length(); i++) {
            char y = secondString.charAt(i);
            System.out.print(y);
        }
    }

问题是，我实际上要打印此

System.out.println("The result of concatenating " + firstString + " and " + secondString + " is " + x + y);

但是我还没有找到一种打印方法，因为x和y仅在for循环中定义，如果我尝试打印它，它将被打印多次而不是一次。

当我通过charAt实现子字符串时，这也是一个问题：

if (operation == 's' ) {
        System.out.print("Please enter the string: ");
        String enteredString = sc.next();
        System.out.print("Please enter the first index: ");
        int index1 = sc.nextInt();
        System.out.print("Please enter the second index: ");
        int index2 = sc.nextInt();

        for (int i = index1; i < index2; i++) {
            char substring = enteredString.charAt(i);
            System.out.print(substring);

        }
}

我希望将其打印出来：

System.out.println("The resulting substring is: " + substring);

但是我也不知道如何实现这一目标。

Answer 1

非常简单：寻找某个字符的最后一次出现时-只需向后移动字符串即可。

从最后一个字符开始，然后在字符串内“向前”移动。 第一个匹配项是最后一个匹配项。

如果要保留从0到字符串末尾的循环：只需记住要查找的char的索引。 最初，该索引为-1，并且每次匹配时，您都将其更新为相应的索引。

Answer 2

正如@GhostCat所说
要查找字符的最后一个索引，只需替换代码

int index = 1;
    for (int i = 0; i < enteredString.length(); i++) {
        if (enteredString.charAt(i) == char1) {
            index = i;
            break;
        }
    }
    System.out.println("The index of character " + char1 + " in string " + enteredString + " is: " + index);

有了这个

        int index = -1;
        for (int i = enteredString.length() - 1; i >= 0; i--) {
            if (enteredString.charAt(i) == char1) {
                index = i;
                break;
            }
        }
        if (index == -1) {
            System.out.println("Character Not Found");
        } else {
            System.out.println("The index of character " + char1 + " in string " + enteredString + " is: " + index);
        }

对于子字符串操作，代码应如下所示

System.out.print("The resulting substring is: ");
    for (int i = index1; i < index2; i++) {
        char substring = enteredString.charAt(i);
        System.out.print(substring);
    }
    System.out.println();