java - How to write a simple input validation for this code -

i need on writing user input validation programm. have been given homework write java program, using array or arraylist, store 10 messages in , ask user choose between 0 , 10, display message , generate random message.

i able complete program , submit it, program crashes if user puts other character integer, or if puts integer,v not within range of 0 10.

i want resubmit homework input validation better grade decided ask pros, once again started learning programming 5 weeks ago.

below homework submitted:

public static void main(string[] args) {       system.out.println("please read through folowing 10 messages , follow instruction under them.");      list<string> list = new arraylist <string>(10); // making object in arraylist         //create arraylist object       //add elements arraylist      list.add(0, "i try good.");       list.add(1, "nobody perfect.");       list.add(2, "life good");       list.add(3, "this me.");      list.add(4, "system out");       list.add(5, "it's summer time");       list.add(6, "i green pepper,");       list.add(7, "he funny");      list.add(8, "there challenges");       list.add(9, "what name");       shoutoutcannedmessage(list);       system.out.println("retrieving stored messages arraylist");     shoutoutrandommessage();       }         //this method retrieves values arraylist using method       public static void shoutoutcannedmessage(list<string> message) {     int size = message.size();      for(int i=0;i<size;i++)      { system.out.println(message.get(i));       }     // retrieve user's choice         int userchoice;      scanner scanner = new scanner(system.in);      // inform user select 1 of messages poped above      system.out.println("please enter number of choice 0 10");      userchoice = scanner.nextint();      system.out.println(message.get(userchoice));     } /**  *  */ public static void shoutoutrandommessage() {      //holds words generated.    string[] subject= {" he", " me", " she", "we"};   string[] verb= {" do", " say", " get", " make", " know"};   string[] adjective= {" good", " new", " first", " last", " long"};   string[] object= {" cup", " map", " house", " computer"};   string[] adverb= {" up. ", " so. ", " out. ", " now. ", " just"};      random r = new random(); //intialize random    int selectedelement = r.nextint(subject.length);    //randomly create sentence.     {   string randomsentence=subject[selectedelement]    + verb[selectedelement]          + adjective[selectedelement]    + object[selectedelement]   + adverb[selectedelement];   system.out.println("shoutout: " + randomsentence );    }   } } 

the number user stored in variable userchoice. need check if number valid. can adding if.

replace code:

 system.out.println("please enter number of choice 0 10");  userchoice = scanner.nextint();  system.out.println(message.get(userchoice)); 

with this:

 system.out.println("please enter number of choice 0 10");  userchoice = scanner.nextint();  if(userchoice >=0 && userchoice <=10)      system.out.println(message.get(userchoice));  else      system.out.println("the number entered not valid."); 

you can go step further , check if user entered valid number. can done by:

 system.out.println("please enter number of choice 0 10");  string userchoicestr = scanner.next();  try{      userchoice = integer.parseint(userchoicestr);      if(userchoice >=0 && userchoice <=10)          system.out.println(message.get(userchoice));      else          system.out.println("the number entered not valid.");  } catch (numberformatexception e) {     system.out.println("the number entered not valid.");  } 

to let user try again if entered wrong input: want process repeat if enters wrong input. repeating processes means loops. there many constructs in java write loops. in case best choice use do-while loop want user try @ least once.

boolean isinputvalid = false; do{     system.out.println("please enter number of choice 0 10");     string userchoicestr = scanner.next();     try{         userchoice = integer.parseint(userchoicestr);         if(userchoice >=0 && userchoice <=10) {             system.out.println(message.get(userchoice));             isinputvalid = true;         }         else             system.out.println("the number entered not valid.");     } catch (numberformatexception e) {        system.out.println("the number entered not valid.");     } } while (!isinputvalid);