Java Object Null Check for method

问题:

I need to create a null check in this formula for the books[i] and I am not entirely sure how to go about this as I am not greatly familiar with null checks and very new at programming. Any and all help is much appreciated!

public static double calculateInventoryTotal(Book[] books)
{
    double total = 0;

    for (int i = 0; i < books.length; i++)
    {
        total += books[i].getPrice();
    }

    return total;
}

回答1:

First you should check if books itself isn't null, then simply check whether books[i] != null:

if(books==null) throw new IllegalArgumentException();

for (int i = 0; i < books.length; i++){
   if(books[i] != null){
        total += books[i].getPrice();
   }
}

回答2:

You can add a guard condition to the method to ensure books is not null and then check for null when iterating the array:

public static double calculateInventoryTotal(Book[] books)
{
    if(books == null){
        throw new IllegalArgumentException("Books cannot be null");
    }

    double total = 0;

    for (int i = 0; i < books.length; i++)
    {
        if(books[i] != null){
            total += books[i].getPrice();
        }
    }

    return total;
}

回答3:

If you are using Java 7 You can use Objects.requireNotNull(object[, optionalMessage]); - to check if the parameter is null. To check if each element is not null just use

if(null != books[i]){/*do stuff*/}

Example:

public static double calculateInventoryTotal(Book[] books){
    Objects.requireNotNull(books, "Books must not be null");

    double total = 0;

    for (int i = 0; i < books.length; i++){
        if(null != book[i]){
            total += books[i].getPrice();
        }
    }

    return total;
}

回答4:

You simply compare your object to null using the == (or !=) operator. E.g.:

public static double calculateInventoryTotal(Book[] books) {
    // First null check - the entire array
    if (books == null) {
        return 0;
    }

    double total = 0;

    for (int i = 0; i < books.length; i++) {
        // second null check - each individual element
        if (books[i] != null) {
            total += books[i].getPrice();
        }
    }

    return total;
}

回答5:

If array of Books is null, return zero as it looks that method count total price of all Books provided - if no Book is provided, zero is correct value:

public static double calculateInventoryTotal(Book[] books)
{
if(books == null) return 0;
    double total = 0;
    for (int i = 0; i < books.length; i++)
    {
        total += books[i].getPrice();
    }
    return total;
}

It's upon to you to decide if it's correct that you can input null input value (shoul not be correct, but...).

回答6:

Inside your for-loop, just add the following line:

if(books[i] != null) {
     total += books[i].getPrice();
}

回答7:

This question is quite older. The Questioner might have been turned into an experienced Java Developer by this time. Yet I want to add some opinion here which would help beginners.

For JDK 7 users, Here using

Objects.requireNotNull(object[, optionalMessage]);

is not safe. This function throws NullPointerException if it finds null object and which is a RunTimeException.

That will terminate the whole program!!. So better check null using == or !=.

Also, use List instead of Array. Although access speed is same, yet using Collections over Array has some advantages like if you ever decide to change the underlying implementation later on, you can do it flexibly. For example, if you need synchronized access, you can change the implementation to a Vector without rewriting all your code.

public static double calculateInventoryTotal(List<Book> books) {
    if (books == null || books.isEmpty()) {
        return 0;
    }

    double total = 0;

    for (Book book : books) {
        if (book != null) {
            total += book.getPrice();
        }
    }
    return total;
}

Also, I would like to upvote @1ac0 answer. We should understand and consider the purpose of the method too while writing. Calling method could have further logics to implement based on the called method's returned data.

Also if you are coding with JDK 8, It has introduced a new way to handle null check and protect the code from NullPointerException. It defined a new class called Optional. Have a look at this for detail

Finally, Pardon my bad English.

回答8:

public static double calculateInventoryTotal(Book[] arrayBooks) {

    final AtomicReference<BigDecimal> total = new AtomicReference<>(BigDecimal.ZERO);
    Optional.ofNullable(arrayBooks).map(Arrays::asList).ifPresent(books -> books.forEach(book -> total.accumulateAndGet(book.getPrice(), BigDecimal::add)));
    return total.get().doubleValue();

}