类

public class DeletePlus86 {
    String num;
    public DeletePlus86(String num){
        this.num = num;
    }
    public String getnum(){
        if(num.length() > 11) {
            return num.substring(num.length()-11);
        }
        return num;
    }
}

使用

DeletePlus86 number= new DeletePlus86(num);
String phone = number.getnum();