同时替换多个匹配字符串

public static void main(String[] args) {
        String str="ab cdef";
        char c;
        String str1="";
        StringBuffer buffer=new StringBuffer();
        for(int i=0;i<str.length();i++) {
            c=str.charAt(i);
            switch(c) {
            case ‘a‘:
                c=‘1‘;
                break;
            case ‘b‘:
                c=‘2‘;
                break;
            case ‘c‘:
                c=‘3‘;
                break;
            case ‘ ‘:
                c=‘D‘;
                break;
            }
            str1=buffer.append(c).toString();
            
        }
        System.out.println(str1);