如何正确使用此布尔值？

Question

I'm making an IRC bot. If someone types "!tell username: message" I want to be able to see if the user is online or not. Here's the first part of my code, which looks through the users in the channel and checks if the user is already online:

if (messageIC.startsWith("!tell ")) {
    boolean userIsOnChannel;
    String messagey = message.substring(6);
    String[] messager = messagey.split(":");
    String username = messager[0];
    User[] users = getUsers(channel);
    for (final User user : getUsers(channel)) {
        if (user.getNick().equalsIgnoreCase(username)) {
            userIsOnChannel = true;
            sendMessage(channel, username + " is online now!");
            break;
        }
        else {
            userIsOnChannel = false;
        }
    }

}

So when I have this in NetBeans, it's telling me that the variable userIsOnChannel is never used. I thought I used it in the if/else statements. This creates problems because I want to be able to store the user's message and send something along the lines of "I'll pass that along" to the channel if the boolean returns false. When I try to use the boolean, I get an error that it has never been initialized. What did I do wrong?

Answer 1

That variable goes out of scope as soon as your code block finishes. You "use" it by setting it but you never use it as a boolean (you never check the value.

You probably need to change

if (messageIC.startsWith("!tell ")) {
boolean userIsOnChannel;

to

boolean userIsOnChannel = false;
if (messageIC.startsWith("!tell ")) {

so that userIsOnChannel is available after that code block.

You get the "may not have been initialized" because it does not get a value just by writing

boolean userIsOnChannel;

you need to assign it a value (probably false in this case)..

Answer 2

If the default boolean value is "false" then initialize it to false and remove the else block.

if (messageIC.startsWith("!tell ")) {
    boolean userIsOnChannel = false;
    String messagey = message.substring(6);
    String[] messager = messagey.split(":");
    String username = messager[0];
    User[] users = getUsers(channel);
    for (final User user : getUsers(channel)) {
        if (user.getNick().equalsIgnoreCase(username)) {
            userIsOnChannel = true;
            sendMessage(channel, username + " is online now!");
            break;
        }

    }

}

Answer 3

You need to have something use or return the field.

If you add a getter method for the field the warning should go away.

getUserIsOnChannel(){
return userIsOnChannel;
}

Answer 4

Here I will try to explain it in simple words through comments.

boolean userIsOnChannel;
// at this place userIsChannel is undefinded and definitely you will 
// try to get its value.
for(....) {
    if(condition) {
        userIsOnChannel = ...
    } else {
        userIsOnChannel = ...
    }
    // Here you will not get any error if you use userIsOnChannel
    // because it is obvious that if if statement is not satisfied
    // then else will be and userIsOnChannel will definitely be assigned.
}
// But here you will again get error because it compiler again here cannot
// confirm that the userIsOnChannel has a value (may be for statement not run even 1 time

So you will have to initialize your variable before you use it and compiler could confirm that its value will be assigned.