翻转单词顺序列

import java.util.Stack;

public class Reverse {
    
    public static void reverse(String str){
        
        Stack<String> st=new Stack<String>();
        StringBuffer sb;
        int i=0;
        while(i<str.length()){
            
            if(str.charAt(i)==' '||i==str.length()-1){
                int j=0;
                if(str.charAt(i)==' '){j=i-1;}
                if(i==str.length()-1){j=i;}
                
                while(str.charAt(j)!=' '&&j!=0)
                {
                    j--;
                }
                 sb=new StringBuffer();
                    for(;j<i;j++){
                        sb.append(str.charAt(j));
                    }
                st.push(sb.toString());
            }        
            i++;
            
        }
        
        System.out.println(st.pop());
        System.out.println(st.pop());
        System.out.println(st.pop());
        System.out.println(st.pop());
    }
    
    
    

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        
        String str="I am a student.";
        reverse(str);

    }

}