null

问题 I wrote a custom static method to create my fragment. Fragment is a subclass of android.support.v4.app.Fragment class. Method to create my fragment is bellow. public static AddItemFragment newInstance(UUID listId, UUID itemId){ AddItemFragment fragment=new AddItemFragment(); Bundle bundle=new Bundle(); bundle.putSerializable(EXTRA_DATA_LIST_ID,listId); bundle.putSerializable(EXTRA_DATA_ITEM_ID, itemId); fragment.setArguments(bundle); return fragment; } In my onCreate method, I am attempting

问题 I wrote a custom static method to create my fragment. Fragment is a subclass of android.support.v4.app.Fragment class. Method to create my fragment is bellow. public static AddItemFragment newInstance(UUID listId, UUID itemId){ AddItemFragment fragment=new AddItemFragment(); Bundle bundle=new Bundle(); bundle.putSerializable(EXTRA_DATA_LIST_ID,listId); bundle.putSerializable(EXTRA_DATA_ITEM_ID, itemId); fragment.setArguments(bundle); return fragment; } In my onCreate method, I am attempting

问题 I use postgresql and yii2 framework. Well I got a very interesting error message: SQLSTATE[23502]: Not null violation: 7 ERROR: null value in column "id" violates not-null constraint DETAIL: Failing row contains (null, 1, null, null, null, null, 1, Demo, , , , 1998-01-01, , , , 345345435453453, , , , , 1, , , f, f, f, f, 10, f, 1, f, f, f, null, null, null, 1470477479, 1470477479, null). But I checked my Insert command, and there is not "id" column there! INSERT INTO "advertiser" ("languages"

问题 I use postgresql and yii2 framework. Well I got a very interesting error message: SQLSTATE[23502]: Not null violation: 7 ERROR: null value in column "id" violates not-null constraint DETAIL: Failing row contains (null, 1, null, null, null, null, 1, Demo, , , , 1998-01-01, , , , 345345435453453, , , , , 1, , , f, f, f, f, 10, f, 1, f, f, f, null, null, null, 1470477479, 1470477479, null). But I checked my Insert command, and there is not "id" column there! INSERT INTO "advertiser" ("languages"

问题 I am learning java concurrent programming recently. I know that the final keyword can guarantee a safe publication. However, when I read the LinkedBlockingQueue source code, I found that the head and last field did not use the final keyword. I found that the enqueue method is called in the put method, and the enqueue method directly assigns the value to last.next . At this time, last may be a null because last is not declared with final . Is my understanding correct? Although lock can

问题 Let's assume we define this Book class where we ensure that isbn can never be assigned a null value: public class Book { private String isbn; public Book(String isbn) { setIsbn(isbn); } public void setIsbn(String isbn) { if (isbn == null) { throw new NullPointerException(); } this.isbn = isbn; } public String getIsbn() { return this.isbn; } } This prevents creation of a Book object where isbn is null but after a book object with a non-null isbn is created, we can modify the value via

问题 Let's assume we define this Book class where we ensure that isbn can never be assigned a null value: public class Book { private String isbn; public Book(String isbn) { setIsbn(isbn); } public void setIsbn(String isbn) { if (isbn == null) { throw new NullPointerException(); } this.isbn = isbn; } public String getIsbn() { return this.isbn; } } This prevents creation of a Book object where isbn is null but after a book object with a non-null isbn is created, we can modify the value via

问题 I have a function which returns a string under certain circumstances, namely when the program runs in Linux or MacOS, otherwise the return value should be nil in order to omit some OS-specific checks further in code. func test() (response string) { if runtime.GOOS != "linux" { return nil } else { /* blablabla*/ } } however when I try to compile this code I get an error: test.go:10:3: cannot use nil as type string in return argument. If I return just an empty string like return "" , I cannot

问题 If I have a struct containing a nil pointer of type A , using reflect.DeepEqual to check if that property is nil will result in false , which strikes me as odd behaviour. type Container struct { O *Obj } type Obj struct { Message string } var c Container eq := reflect.DeepEqual(c.O, nil) fmt.Printf("O value: %v, is nil: %t", c.O, eq) // Prints: "O value: <nil>, is nil: false" Specifically, I am marshaling a JSON object into a struct, where I would like to test that a specific property is nil

问题 In my DataFrame, there are columns including values of null and NaN respectively, such as: df = spark.createDataFrame([(1, float('nan')), (None, 1.0)], ("a", "b")) df.show() +----+---+ | a| b| +----+---+ | 1|NaN| |null|1.0| +----+---+ Are there any difference between those? How can they be dealt with? 回答1: null values represents "no value" or "nothing", it's not even an empty string or zero. It can be used to represent that nothing useful exists. NaN stands for "Not a Number", it's usually