更改java.util.List中的对象引用String java.util.List.set(int index, String element)

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

public static void main(String[] args) { 
    List<String> list = new ArrayList<String>(); 
    list.add("a"); 
    list.add("b"); 
    list.add("c"); 
    for (int i = 0; i < list.size(); i++) { 
        String s = list.get(i); 
        if (s.equals("b")) { 
            s = "bb"; 
            String oldValue = list.set(i, s); 
            System.out.println("change " + oldValue + " to " + s); 
        } 
    } 
    for (String s : list) { 
        System.out.println(s); 
    } 
}