把句子分成几个字母，然后通读一遍。如果匹配，则将其添加到将返回的某个列表中。

public List<Integer> findOccurences(String letter, String sentence){
    List<Integer> positions = new ArrayList<>();
    String[] letters = sentence.split("");
    for(int i = 0; i<letters.length; i++){
        if(letter.equals(letters[i])){
            positions.add(i);
        }
    }
    return positions;
}

那就跟我说吧

List<Integer> occurenceList = findOccurences(sentence2, sentence);

如果只想显示字符所在的第一个索引，可以使用 String#indexOf(int ch) . 如果要显示字母在句子中出现的所有位置，可以使用 String#indexOf(String str, int fromIndex) .
演示：

public class Main {
    public static void main(String[] args) {
        String sentence = "Hello world!";
        char ch = 'l';
        int index = sentence.indexOf(ch);
        if (index != -1) {
            System.out.println("The first occurance of '" + ch + "' is at " + index);
        } else {
            System.out.println("The letter, '" + ch + "'does not exist in the sentence");
        }

        // All positions
        System.out.println("All positions: ");
        int fromIndex = 0, count = 0;
        for (int i = 0; i < sentence.length(); i++) {
            index = sentence.indexOf(ch, fromIndex);
            if (index != -1) {
                System.out.println("'" + ch + "' was found at " + index);
                fromIndex = index + 1;
                count++;
            }
        }
        if (count == 0) {
            System.out.println("The letter, '" + ch + "'does not exist in the sentence");
        }
    }
}

输出：

The first occurance of 'l' is at 2
All positions: 
'l' was found at 2
'l' was found at 3
'l' was found at 9

或者，您可以使用 String#charAt :

public class Main {
    public static void main(String[] args) {
        String sentence = "Hello world!";
        char ch = 'l';
        int count = 0;
        for (int i = 0; i < sentence.length(); i++) {
            if (sentence.charAt(i) == ch) {
                System.out.println("'" + ch + "' was found at " + i);
                count++;
            }
        }
        if (count == 0) {
            System.out.println("The letter, '" + ch + "'does not exist in the sentence");
        }
    }
}

输出：

'l' was found at 2
'l' was found at 3
'l' was found at 9

您还可以将所有位置添加到 List<Integer> 并显示相同的。

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        String sentence = "Hello world!";
        char ch = 'l';
        List<Integer> positions = new ArrayList<>();
        for (int i = 0; i < sentence.length(); i++) {
            if (sentence.charAt(i) == ch) {
                positions.add(i);
            }
        }
        if (positions.size() == 0) {
            System.out.println("The letter, '" + ch + "'does not exist in the sentence");
        } else {
            System.out.println("The positions where '" + ch + "' was found is/are " + positions);
        }
    }
}

输出：

The positions where 'l' was found is/are [2, 3, 9]