problems with Java's Scanner

[shadylookin]

I'm having a problem with the Scanner class in java(at least I think so). Whenever I ask for user input from the keyboard none of my if and else if statements are properly recognizing the String from the scanner.

for instance when I have command = userInput.next();
even if the String perfectly matches the word "quit"(without quotations) that is supposed to set the boolean running equal to false the if statement fails to run.

Also when I type "insert"(w/o quotations) the else if statement that checks to see if command == "insert" fails to execute. I've checked the debugger and the variable command is correct.

So what am I doing that's causing this problem?

thanks for the help

here's the code I'm having difficulty with

import java.util.Scanner;
import java.io.File;


public class Hash {

private Scanner userInput = new Scanner(System.in);
private LinkedList[] hashTable = new LinkedList[17];

public Hash(){
// initialize the hashTable
for(int i = 0; i<17; i++){
hashTable[i] = new LinkedList();
}
}

public static void main(String args[]){
Hash hash = new Hash();
hash.welcomeMessage();

}// end main

/**
* welcomeMessage prints a message that gives a description and a list of commands
* the user can input. then it gives a prompt asking for a command and then
* gives that command to userCommand.
* Valid commands are insert delete search save quit and load.
*
*/
public void welcomeMessage(){
boolean running = true;
String command;
System.out.println("This program creates and loads a hash table that uses linking" +
" that allows you to insert delete search save quit and load");

while(running == true){
System.out.print("enter command > ");
command = userInput.next();
if(command == "quit"){
running = false;//if command is quit then quit the loop
}
else{
userCommand(command);
}
}//end while loop
}// end welcome message

public void userCommand(String command){
String in;
int inInt;
if(command == "save"){
System.out.print("enter the name to save under >");
in = userInput.next();
save(in);
}
else if(command == "load"){
System.out.print("enter the file to load >");
in = userInput.next();
load(in);
}
else if(command == "insert"){
System.out.print("enter the integer to insert >");
inInt = userInput.nextInt();
insert(inInt);
}
else if(command == "delete"){
System.out.print("enter the integer to delete >");
inInt = userInput.nextInt();
delete(inInt);
}
else if(command == "search"){
System.out.print("enter the integer to search for >");
inInt = userInput.nextInt();
search(inInt);
}
else{
System.out.println("invalid command");
}
}// end userCommand

public void insert(int valueToInsert){
int key = valueToInsert % 17;
System.out.println(hashTable[key].add(valueToInsert));
}// end insert

public void search(int valueToSearchFor){

}// end search

public void delete(int valueToDelete){

}// end delete

public void save(String valueToSaveAs){

}// end save

public void load(String valueToLoad){

}// end load

}// end class

[Phristen]

You can't compare strings with == in Java...
Use String.equals() method.

For example, "quit".equals("quit") will be true, but "quit" == "quit" won't.

[shadylookin]

Eureka it works. Thanks for the help.

[kavon89]

To compare a string and ignore case, use:


if(input.equalsIgnoreCase("quit")) {
//stuff
}

I would recommend using that since it looks like your program would be taking lots of input from the user.

[ceclauson]

This is true, you should be using the equals() method to compare strings for equality of content, not ==.

Two other thoughts. First, the scanner class has a number of methods for parsing the next element of input. In the context that you seem to be using it, you probably want to use nextLine() instead of next(). I think next() only gets the next whitespace delimited token. I'm not sure that that would be significant in this context, but it might be.

Second, whenever I do what you're doing, I usually use the String class's trim() method before doing the comparison. This eliminates leading and trailing whitespace. Again, I don't know if this would be significant in this context, but it might be.

So here are the lines of code that you emphasized as I would write them:


command = userInput.nextLine().trim();
if(command.equals("quit")){
running = false;//if command is quit then quit the loop
}


I would try that, and if that doesn't work, post again :-)