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Week 110 — What is the null literal and how can it be used in Java applications?

Question of the Week #110
What is the null literal and how can it be used in Java applications?
7 Replies
Mercy
Mercy•4d ago
The literal null refers to a special value that can be used for any reference type in a Java program. It refers to the absence of a value i.e. if a variable is null, it means there is no value of the type set. 
Object someVariable = null;
LocalDateTime someLocalDateTimeVariable = null;
// etc
Object someVariable = null;
LocalDateTime someLocalDateTimeVariable = null;
// etc
It is possible to check for an object being null using the == operator: 
if (someVariable == null) {
  System.out.println("the variable is null");
} else {
  System.out.println("the variable has a value");
}
if (someVariable == null) {
  System.out.println("the variable is null");
} else {
  System.out.println("the variable has a value");
}
Attempting to access any field or call a method of a variable that's set to null causes a NullPointerException to be thrown 
Object someNullObject = null;
someNullObject.toString();//throws java.lang.NullPointerException: Cannot invoke "Object.toString()" because "someNullObject" is null
Object someNullObject = null;
someNullObject.toString();//throws java.lang.NullPointerException: Cannot invoke "Object.toString()" because "someNullObject" is null
class ClassWithVariable{
  int i;
}
ClassWithVariable nullInstance = null;
int j = nullInstance.i;//throws java.lang.NullPointerException: Cannot read field "i" because "nullInstance" is null
class ClassWithVariable{
  int i;
}
ClassWithVariable nullInstance = null;
int j = nullInstance.i;//throws java.lang.NullPointerException: Cannot read field "i" because "nullInstance" is null
The Objects class provides some utility methods for dealing with null. For example, the method Objects.requireNonNullElse returns the variable passed as the first argument if it is non-null and the second argument otherwise. The Objects.requireNonNull method can be used as a simple null assertion and throws a NullPointerException if the argument is null. 
LocalDateTime someLocalDateTimeVariableOrNow = Objects.requireNonNullElse(someLocalDateTimeVariable, LocalDateTime.now());

Objects.requireNonNull(someVariable, "exception message in case someVariable is null");
LocalDateTime someLocalDateTimeVariableOrNow = Objects.requireNonNullElse(someLocalDateTimeVariable, LocalDateTime.now());

Objects.requireNonNull(someVariable, "exception message in case someVariable is null");
📖 Sample answer from dan1st
Mercy
Mercy•4d ago
null is a key word that means a non-reference. It is used to state that a Reference-Type variable/field is not pointing to any valid reference. Also, it is known as the 1 bilion dollar error. List<Integer> myList = List.of(1, 2, 3); // Now, myList is pointing to a list returned by List.Of() method myList = null; // From this point, myList is not pointing to that List anymore.
Submission from retro.alvar
Mercy
Mercy•4d ago
A null literal is a constant value that can be assigned to ReferenceType variables to point to a null reference.
Submission from oneplusiota
Mercy
Mercy•4d ago
0x01 ACONST_NULL pushes a null reference onto the stack. Its used to represent the absence of an object. 
// Pushes a null ref onto the stack
// [0x01]
ACONST_NULL

// Sets the value of MyClass.someField to NULL
// [0xB3, 0xFieldRef, 0xFieldDescriptor]
PUTSTATIC com/example/MyClass/someField Ljava/lang/Object;
// Pushes a null ref onto the stack
// [0x01]
ACONST_NULL

// Sets the value of MyClass.someField to NULL
// [0xB3, 0xFieldRef, 0xFieldDescriptor]
PUTSTATIC com/example/MyClass/someField Ljava/lang/Object;
Myclass.someField = null;
Myclass.someField = null;
Submission from tonicbox
Mercy
Mercy•4d ago
In Java, null indicates the absence of an object. If null is assigned to a non-primitive variable or field, then that variable or field points to nothing. Furthermore, any non-primitive field that is not explicitly assigned a value will be null. Some ways that null can be used: * As the initial value of a variable or field to indicate that it hasn't been assigned a real value yet * As a method call argument to indicate "no value" for that parameter * Returned from a method to indicate that no sensible value could be returned
Mercy
Mercy•4d ago
Care should be taken with how nulls are used and handled. If the code is not expecting and guarding against them, NullPointerExceptions will result. Furthermore, because potential null values complicate the code or open it up to potential bugs, other mechanisms (like Optional<T>) should be used whenever possible.
⭐ Submission from dangerously_casual
Mercy
Mercy•4d ago
The null literal in Java represents the absence of a value or a reference that doesn't point to any object. According to my research, the null keyword in java is a literal. It is neither a data type nor an object. Null represents the absence of a value. If you assign a null to an object of string Class, it does not refer to any value in the memory. The null cannot be assigned to primitive data types. The reserved word null is case sensitive and cannot be written as Null or NULL as the compiler will not recognize them and will certainly give an error. null is used as a special value to signify: 1. It can also be used to initialise variables String str = null; Uninitialized state Termination condition Non-existing object An unknown value The null is used to express a particular value within a program. They are constant values that directly appear in a program and can be assigned now to a variable. 2. Checking for Null Values NullPointer exception is thrown when you try to use a variable with a null value, so to avoid it, you need to check the NullPointerException as I did below 
if (str != null) {
    System.out.println(str.length());
} else {
    System.out.println("String is null");
}
if (str != null) {
    System.out.println(str.length());
} else {
    System.out.println("String is null");
}
Explicitly Dereferencing You can use null to dereference an object and release its memory, then it will be eligible for garbage collection. 
MyClass obj = new MyClass();
obj = null;
MyClass obj = new MyClass();
obj = null;
Did you know that, you can explicitly add null values in ArrayLists. 
List<String> list = new ArrayList<>();
list.add(null); 
List<String> list = new ArrayList<>();
list.add(null);
⭐ Submission from mercy_17083

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