Java constructor returning null

Can a constructor return null?

Thanks In Advance !! We can not return any value from constructor. A constructor does not return anything; the «new» operator returns an object that has been initialized using a constructor. If you really want a constructor that may return null, perhaps you should check out the factory method pattern.

Can new Java return null?

Can constructors have a return?

No, constructor does not have any return type in Java. Constructor looks like method but it is not. It does not have a return type and its name is same as the class name. Mostly it is used to instantiate the instance variables of a class.

Does a constructor have a return type of void?

A constructor does not have a return type, so as you wrote void in front of A(), you’ve defined a method, not a constructor.

Can a constructor return false?

Can a constructor return a NULL value — C++

Can constructor be static?

No, we cannot define a static constructor in Java, If we are trying to define a constructor with the static keyword a compile-time error will occur. In general, static means class level. A constructor will be used to assign initial values for the instance variables.

Can we make constructor final?

No, a constructor can’t be made final. A final method cannot be overridden by any subclasses. As mentioned previously, the final modifier prevents a method from being modified in a subclass. The main intention of making a method final would be that the content of the method should not be changed by any outsider.

What happens if constructor has a return type?

Explanation: The constructor cannot have a return type. It should create and return new objects. Hence it would give a compilation error.

Can a constructor be private?

Yes, we can declare a constructor as private. If we declare a constructor as private we are not able to create an object of a class. We can use this private constructor in the Singleton Design Pattern.

Can a constructor be overloaded?

Constructors can be overloaded in a similar way as function overloading. Overloaded constructors have the same name (name of the class) but the different number of arguments. Depending upon the number and type of arguments passed, the corresponding constructor is called.

Can constructor return a value in C++?

You do not specify a return type for a constructor. A return statement in the body of a constructor cannot have a return value.

Is null constructor?

Can we return null in catch block?

Is returning null in catch block is best practice.. Best is a relative word. If you want to swallow / disregard the exception then you would return null. i.e. you cannot do anything about the exception but still want to continue processing then this might be «best».

Can you print null?

It’s just the string and the character array parameters that cause ambiguity as character arrays and objects can happily coexist. The char array null cannot be printed by the PrintStream since it causes a NullPointerException .

Can constructors be virtual?

In C++, the constructor cannot be virtual, because when a constructor of a class is executed there is no virtual table in the memory, means no virtual pointer defined yet. So, the constructor should always be non-virtual. But virtual destructor is possible.

Can a constructor be overridden?

Constructor looks like method but it is not. It does not have a return type and its name is same as the class name. But, a constructor cannot be overridden. If you try to write a super class’s constructor in the sub class compiler treats it as a method and expects a return type and generates a compile time error.

Can we create immutable class in Java?

Immutable class in java means that once an object is created, we cannot change its content. In Java, all the wrapper classes (like Integer, Boolean, Byte, Short) and String class is immutable. We can create our own immutable class as well.

Can abstract methods have constructor?

Like any other classes in Java, abstract classes can have constructors even when they are only called from their concrete subclasses.

Can you use this () and super () both in a constructor?

If we include “this()” or “super()” inside the constructor, it must be the first statement inside it. “this()” and “super()” cannot be used inside the same constructor, as both cannot be executed at once (both cannot be the first statement). “this” can be passed as an argument in the method and constructor calls.

Can we create object of abstract class?

We cannot create objects of an abstract class. To implement features of an abstract class, we inherit subclasses from it and create objects of the subclass. A subclass must override all abstract methods of an abstract class.

Can static method be overridden?

Static methods are bonded at compile time using static binding. Therefore, we cannot override static methods in Java.

Can a main () method be declared final?

Yes, we can declare the main () method as final in Java. The compiler does not throw any error. If we declare any method as final by placing the final keyword then that method becomes the final method.

Can we overload main method?

Yes, we can overload the main method in Java, but When we execute the class JVM starts execution with public static void main(String[] args) method.

Can we inherit constructor in Java?

Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.

Can you declare an abstract class as final?

If you declare a class abstract, to use it, you must extend it and if you declare a class final you cannot extend it, since both contradict with each other you cannot declare a class both abstract and final if you do so a compile time error will be generated.

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Java object constructor returning «null»

Here is an example: Solution 2: You have your constructor assignments the wrong way: Solution 3: Java constructor objects should be created like so: To create a new User, use this code: Solution 1: Yes, you can throw an exception, which will terminate object creation.

Java object constructor returning «null»

New to Java! I have created a class that has constructors for various fields. However, when I try and print the fields, I recieve «null» as the output. Please help me understand why. Here is the class:`

 public class User < //All relevant information to identify each user. private String FirstName; private String LastName; private String Email; private long PhoneNum; private long CardNum; //Build Constructors for User info. public User(String Fname, String Lname, String Mail, long num, long card) < Fname=FirstName; Lname=LastName; Mail=Email; num=PhoneNum; card=CardNum; >//Method to set FirstName. public void setFirstName (String Fname) < FirstName=Fname; >//Method to get FirstName. public String getFirstName() < return FirstName; >//Method to set LastName. public void setLastName (String Lname) < LastName=Lname; >//Method to get Lastname. public String getLastname() < return LastName; >//Method to set email. public void setEmail (String Mail) < Email=Mail; >//Method to get email. public String getEmail() < return Email; >//Method to set phonenumber. public void setPhoneNum(long num) < PhoneNum=num; >//Method to get phonenumber. public long getPhoneNum() < return PhoneNum; >//Method to set credit card number. public void setCardNum(long card) `enter code here` < CardNum=card; >// Method to get credit card number. public long getCardNum()

Now when I run this code, I receive «null»:

What am I doing wrong? Thank you in advance!

Is the other way around, instead of:

public User(String Fname, String Lname, String Mail, long num, long card)

public User(String Fname, String Lname, String Mail, long num, long card)

To avoid this kind of thing you can use the this keyword. Here is an example:

String firstName; public Constructor(String firstName)

You have your constructor assignments the wrong way:

public User(String Fname, String Lname, String Mail, long num, long card) < // this refers to the current object being constructed this.FirstName = Fname; this.LastName = Lname; this.Email = Mail; this.PhoneNum = num; this.CardNum = card; >

Java constructor objects should be created like so:

public User(String Fname, String Lname) < this.firstName = Fname; //keyword 'this' refers to the new object, so the new object's Fname would equal the firstName place below this.lastName = Lname; //same as above >

To create a new User, use this code:

User Matt = new("Matt", "Burns"); //Creates a new User Matt, with firstName "Matt" and lastName "Burns" System.out.println(Matt.getfirstName()); //prints User Matt's firstName to the console. 

When does a java object become non-null during, If another thread were to check the someObject variable «during» construction, I believe it may (due to quirks in the memory model) see a partially initialized object. The new (as of Java 5) memory model means that any final fields should be set to their values before the object becomes visible to other threads (so long as the reference to the newly created object doesn’t escape from the Code samplepublic static Foo f = null;public static void main(String[] args) Feedback

Can Java object arrays initialize elements as non-null values?

I’m pretty new at Java and I’m having a tough time figuring out how to fix this null pointer exception that has been troubling me.

I know where the problem occurs and I know what a null pointer exception is, but I have no idea how I’m going to make my program work.

Here’s the code snippet where the problem is occuring:

public static void main(String[] args)

Like I said, I know that it happens because the objects in atm[] are null, but I’m not sure how to fix the problem.

I’m sure it’s some silly mistake because those are the kinds of mistakes I make on a regular basis, but any help that you guys can give would make my day.

Your entire array is null ! remember , arrays are never automatically initialized in java, unless they are arrays of ints,floats,doubles, or booleans.

Scanner input = new Scanner//System.in.Scanner; Account[] atm = new Account[10]; for (int i = 0; i

When you’re declaring arrays that hold objects, read it as, «I’m creating an array that will hold ‘x’ objects.» (correct), and then proceed to instantiate those objects

«I’m creating an array with ‘x’ objects in it.» (incorrect) since there aren’t any objects in there yet because they haven’t been created.

Java constructor style: check parameters aren’t null, For less verbosity use Validate.notNull (obj, message) from commons-lang. Thus your constructor will look like: public SomeClass (Object one, Object two) < …

Can constructor return a null object?

While looking through some old code I came across this gem:

MyObject o = new MyObject("parameter"); if (o == null) o = new MyObject("fallback parameter"); 

The second line is marked in Eclipse as dead code, and I understand why. No exception seems to be explicitly thrown, and it isn’t possible for the MyObject constructor to throw any kind of exception (such as NullPointerException s).

My question is why there is a null check? Was it previously possible in an old version of Java for a constructor to return null? Or is this simply useless and dead code?

The code is dead in any version of Java. It’s not possible for a constructor to return null , and even if an exception would be thrown from the constructor, the next line won’t be called.

No, it has never been possible. Maybe a previous version of the code used some factory method which could return null:

MyObject o = createMyObject("parameter"); if (o == null) o = createMyObject("fallback parameter"); 

From section 15.9.4 of the JLS:

The value of a class instance creation expression is a reference to the newly created object of the specified class. Every time the expression is evaluated, a fresh object is created.

So no, it can never return null.

My guess is that it was written by a C programmer who is used to testing the return value of malloc() for NULL , malloc() can return NULL if your system runs out of memory.

The code doesn’t make sense in Java since Java will throw an OutOfMemoryError` if it runs out of memory.

Java object constructor returning «null», 0. Java constructor objects should be created like so: public User (String Fname, String Lname) < this.firstName = Fname; //keyword 'this' refers to …

Is it possible to stop an object from being created during construction?

For example if an error occurs inside the constructor (e.g. the parameters pass in were invalid) and I wanted to stop an object being created would it be possible to return null rather than a reference to a new object. (I know the term return is technically correct in this case but you know what I mean).Basically, is it possible to cancel object creation?

Please guide me to get out of this issue.

Yes, you can throw an exception, which will terminate object creation. You cannot return null or any other value from a constructor.

If the parameters passed in are invalid, you should throw an IllegalArgumentException with a descriptive message. NullPointerException should be thrown in case of illegal null values.

Here’s an example of a BigCar class that requires its engine to be > 4000 CC.

public class BigCar < private final Engine engine; public BigCar(Engine engine) < if (engine == null) throw new NullPointerException("Must provide an engine to our car"); if (engine.getCC() public static void main(String. args) < // Size of engine is 2000CC Engine engine = new Engine(2000); BigCar myBigCar = new BigCar(engine); // Exception; Engine should be // bigger than 4000 CC >> 

Use getters and setters method to set the values for parameters and throw exception if the value is invalid or show a messagebox that object can not be created.

UPDATE

public class A < private String Name; public void SetName(String name) < if (name.equals(null) || name.equals("")) throw new IllegalArgumentException("name can not be null or empty"); >public A(String name) < SetName(name); >> 

Java — How to check if object is null or not except == null, The easiest way to check is entity == null. There is no shorter way to do that. Note that there is a method for this in the standard lib: Objects.isNull …

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