在Java中使用if与逻辑OR进行选择

Question

This might seems bit foolish, but it has really caught me.

I want to throw an error if a charString doesn't contain a word "Hi".

So I wrote

if(!charString.contains("Hi"))
   /// throw error on screen

Now, the requirement changed. I want to also throw an error if a charString does not contain "Hello".

So, the requirement is I should not throw an error if the charString contains either Hi or Hello

So I wrote:

if(!charString.contains("Hi") || !charString.contains("Hello"))
   /// throw error on screen

To my horror, I realized I am terribly wrong in writing above code. How can I write it in a single if statement then?

The requirement is:

"If charString contains either Hi or Hello, do not throw error"

Answer 1

A very clear way (at the expense of a superfluous block that requires a comment) is to write

if (charString.contains("Hi") || charString.contains("Hello")){
    // do nothing
} else {
    // throw an error
}

This is equivalent to the less eyebrow-raising

if (!(charString.contains("Hi") || charString.contains("Hello"))){
    // throw error
}

or (due to De Morgan's Law https://en.wikipedia.org/wiki/De_Morgan%27s_laws )

if (!charString.contains("Hi") && !charString.contains("Hello"))){
    // throw error
}

Answer 2

As I understand, you want to throw error when it doesn't contain Hi AND doesn't contain Hello .

if(!charString.contains("Hi") && !charString.contains("Hello"))

Your previous code was:

if the String doesn't contain "Hi" or the String doesn't contain "Hello" throw error. It means that the luck of at least one of those Strings resulted in throwing the error.

Let's describe your flow in plain English:

IF charString contains "Hi" or "Hello" (so is a way of saying "Welcome")
    proceed
ELSE
    throw the error.

We can also use the negation:

 IF charString doesn't contain one of "Hi" or "Hello" (so is a way of saying "Welcome")
    throw the error
ELSE
    proceed.

In Java they would be:

if (charString.contains("Hi") || charString.contains("Hello")) {
    //it's good
} else {
    //it's bad
}

or the second case:

    NOT(charString contains Hi or charString contains Hello)
if (!(charString.contains("Hi") || charString.contains("Hello"))) {
    //it's bad
} else {
    //it's good
}