When is while(true) true, and when is it false?

It's always true, it's never false.

Some people use while(true) loops and then use break to exit them when a certain condition is true, but it's generally quite sloppy practice and not recommended. Without the use of break, return, System.exit(), or some other such mechanism, it will keep looping forever.

I wouldn't say it's bad - but equally I would normally at least look for an alternative.

In situations where it's the first thing I write, I almost always at least try to refactor it into something clearer. Sometimes it can't be helped (or the alternative is to have a bool variable which does nothing meaningful except indicate the end of the loop, less clearly than a break statement) but it's worth at least trying.

As an example of where it's clearer to use break than a flag, consider:

while (true)
{
    doStuffNeededAtStartOfLoop();
    int input = getSomeInput();
    if (testCondition(input))
    {
        break;
    }
    actOnInput(input);
}

Now let's force it to use a flag:

boolean running = true;
while (running)
{
    doStuffNeededAtStartOfLoop();
    int input = getSomeInput();
    if (testCondition(input))
    {
        running = false;
    }
    else
    {
        actOnInput(input);
    }
}

I view the latter as more complicated to read: it's got an extra else block, the actOnInput is more indented, and if you're trying to work out what happens when testCondition returns true, you need to look carefully through the rest of the block to check that there isn't something after the else block which would occur whether running has been set to false or not.

The break statement communicates the intent more clearly, and lets the rest of the block get on with what it needs to do without worrying about earlier conditions.

Note that this is exactly the same sort of argument that people have about multiple return statements in a method. For example, if I can work out the result of a method within the first few lines (e.g. because some input is null, or empty, or zero) I find it clearer to return that answer directly than to have a variable to store the result, then a whole block of other code, and finally a return statement.

Good day to you @gyomin0315 The reason your program doesn't come out of the while loop is that you are using the equality operator (==) to compare strings in the if statement where you check if the user input is one of the valid options ("Seoul", "Busan", "Daejeon", "Sejong"). Therefore, the string comparison using == always returns false, and your program never breaks out of the while loop.

in java you should use .equal, try this and please report back.

if(addr.equals("Seoul")||addr.equals("Busan")||addr.equals("Daejeon")||addr.equals("Sejong"))
{
    break;    
}