如何檢查列表 object 有 null 值或空字符串？

Question

我有一本書 class 然后我制作了一個List<Book> 。 我在檢查所有屬性時遇到問題。 以下是class Book的程序代碼示例：

class Book {  
    int id;  
    String name,author;
    public Book(int id, String name, String author) {  
        this.id = id;  
        this.name = name;  
        this.author = author;  
    }   
}

這是List<Book>上的代碼片段：

 Book b = new Book(1, "", "Ok");
 Book c = new Book(2, "Z", "");
 Book d = new Book(0, "C", "Ok");
 List<Book> x = new ArrayList<>();
 x.add(b);
 x.add(c);
 x.add(d);

如何檢查List<Book>中的值是否為空字符串或null然后返回 Boolean 並返回消息示例id 2 has empty value ？

Answer 1

您可以使用Apache Commons中的StringUtils.isEmpty(...)方法，或者如果您不想要依賴項，您可以像這樣編寫它：

public static boolean isEmpty(String s) {
  return s == null || s.isEmpty();
}

要檢查List中的所有authors ，請使用 Streams：

public boolean anyAuthorEmpty(List<Book> books) {
  return books.stream().anyMatch(b -> isEmpty(b.getAuthor());
}

要找到確切的Book ，您可以像這樣使用.findFirst() ：

public Book findBookWithEmptyAuthor(List<Book> books) {
  return books.stream().filter(b -> isEmpty(b.getAuthor())
                .findFirst().orElse(null);
}

如果沒有找到，這將返回Book或null 。

如果您需要沒有作者的所有內容，您可以使用Collectors.toList() ：

public List<Book> findBooksWithNoAuthor(List<Book> books) {
  return books.stream().filter(b -> isEmpty(b.getAuthor())
                .collect(Collectors.toList());

Answer 2

您可以使用反射，這樣您就不需要單獨手動測試每個文件：

static void testAllFieldsForNull(List<Book> bookList) throws IllegalAccessException {
    for (int i = 0; i < bookList.size(); i++){
        Book book = bookList.get(i);
        Field[] fields = book.getClass().getDeclaredFields();
        for(Field field: fields){
            Class<?> fieldType = field.getType();
            if(!fieldType.isPrimitive()){
                if (field.get(book) == null){
                    System.out.println("Field [" + field.getName() + "] has null value for book at position " + i);
                    continue;
                }
                if(fieldType.isAssignableFrom(String.class) && ((String)field.get(book)).isEmpty()){
                    System.out.println("Field [" + field.getName() + "] is empty String for book at position " + i);
                }
            }
        }
    }
}

測試：

public static void main(String[] args) throws IllegalAccessException {
    Book b = new Book(1, "", "Ok");
    Book c = new Book(2, "Z", "");
    Book d = new Book(0, "C", null);
    List<Book> x = new ArrayList<>();
    x.add(b);
    x.add(c);
    x.add(d);
    testAllFieldsForNull(x);
}

Output：

Field [name] is empty String for book at position 0
Field [author] is empty String for book at position 1
Field [author] has null value for book at position 2

或者，如果您只需要收集“好”書籍（實際上是任何類型的物品），您可以使用：

public static boolean testObject(Object obj){
    Field[] fields = obj.getClass().getDeclaredFields();
    boolean okay = true;
    for(Field field: fields){
        Class<?> fieldType = field.getType();
        try{
            if(!fieldType.isPrimitive()){
                if (field.get(obj) == null){
                    okay = false;
                    continue;
                }
                if(fieldType.isAssignableFrom(String.class) && ((String)field.get(obj)).isEmpty()){
                    okay = false;
                }
            }
        }catch (IllegalAccessException e){
            e.printStackTrace();
            return false;
        }
    }
    return okay;
}

然后使用它進行過濾：

public static void main(String[] args) throws IllegalAccessException {
    Book b = new Book(1, "", "Ok");
    Book c = new Book(2, "Z", "");
    Book d = new Book(0, "C", null);
    Book a = new Book(3, "C", "D");
    List<Book> x = new ArrayList<>();
    x.add(b);
    x.add(c);
    x.add(d);
    x.add(a);
    System.out.println(
            x
            .stream()
            .filter(FieldTest::testObject)
            .collect(Collectors.toList()).get(0).id
    );
}

Answer 3

要檢查 class Book的多個屬性，可以提供以下解決方案（假設有輔助方法isNullOrEmpty的實現）：

class SONullEmpty {
    static boolean isNullOrEmpty(String str) {
        return str == null || str.isEmpty();
    }

    static List<Book> booksWithNullOrEmpty(List<Book> books) {
        return books
            .stream()
            .filter(book -> Stream.of(
                    book.getName(), book.getAuthor()
                ).anyMatch(SONullEmpty::isNullOrEmpty)
            )
            .collect(Collectors.toList());
    }
}

---

類似地，可以實現一個接受Book屬性的多個 getter 的方法，然后調用：

// in the same SONullEmpty class
static List<Book> withNullOrEmptyAttribs(List<Book> books, Function<Book, String> ... getters) {
    return books
        .stream()
        .filter(book -> Arrays.stream(getters).anyMatch(g -> isNullOrEmpty(g.apply(book))))
        .collect(Collectors.toList());
}

測試：

Book b = new Book(1, "", "Ok");
Book c = new Book(2, "Z", "");
Book d = new Book(3, null, "Ok");
List<Book> x = Arrays.asList(b, c, d);

withNullOrEmptyAttribs(x, Book::getName)
    .forEach(book -> System.out.printf("Book with id=%d has null or empty name%n", book.getId()));

withNullOrEmptyAttribs(x, Book::getAuthor)
    .forEach(book -> System.out.printf("Book with id=%d has null or empty author%n", book.getId()));

Output：

Book with id=1 has null or empty name
Book with id=3 has null or empty name
Book with id=2 has null or empty author

Answer 4

或者你可以使用 Java Stream 也許

public class Test1 {
private Test1()
{
    Book b = new Book(1, "", "Ok");
    Book c = new Book(2, "Z", "");
    Book d = new Book(3, "C", "Ok");
    List<Book> x = new ArrayList<>();
    x.add(b);
    x.add(c);
    x.add(d);
    
    boolean empty = x.stream()
            .filter(book -> book.name == null || book.name.isEmpty() || book.author == null || book.author.isEmpty())
            .count() > 0;
}

class Book
{
    int id;
    String name, author;

    public Book(int id, String name, String author)
    {
        this.id = id;
        this.name = name;
        this.author = author;
    }
}

public static void main(String[] args)
{
    new Test1();
}
}

Answer 5

所以如果你需要檢查字符串是 null 還是空的，你可以 go

//代碼

String s = "somestring" if(s == "" || s == null){ //true }