老铁们，能具体解释一下吗？

import java.util.Arrays;
public class Demo{
    private String str="1234";
    private char[] chars={'a','b','c'};
    public static void main(String[] args){
        Demo demo=new Demo();
        System.out.println(demo.str+"  "+ Arrays.toString(demo.chars));
        change(demo.str,demo.chars);
        System.out.println(demo.str+"  "+ Arrays.toString(demo.chars));
    }
    public static void change(String str,char[] chars){
        str="5678";
        chars[2]='b';
    }
}