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 constructor be made protected?
Protecting a constructor prevents the users from creating the instance of the class, outside the package. During overriding, when a variable or method is protected, it can be overridden to other subclass using either a public or protected modifier only. Outer class and interface cannot be protected.
Can a constructor be private or protected?
No, Constructors can be public , private , protected or default (no access modifier at all). Making something private doesn’t mean nobody can access it. It just means that nobody outside the class can access it. So private constructor is useful too.
Is a constructor always public?
A constructor is a special member function of a class which initializes objects of a class. In C++, constructor is automatically called when object of a class is created. By default, constructors are defined in public section of class.
What happens when a constructor is defined as protected?
A protected constructor means that only derived members can construct instances of the class (and derived instances) using that constructor. This sounds a bit chicken-and-egg, but is sometimes useful when implementing class factories.
Can constructor be static or final?
Java constructor can not be static
One of the important property of java constructor is that it can not be static. We know static keyword belongs to a class rather than the object of a class. A constructor is called when an object of a class is created, so no use of the static constructor.
Can we declare constructor as 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 declare constructor as final?
No, a constructor can’t be made final. A final method cannot be overridden by any subclasses.
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.
Why return type is not allowed for constructor?
So the reason the constructor doesn’t return a value is because it’s not called directly by your code, it’s called by the memory allocation and object initialization code in the runtime. Its return value (if it actually has one when compiled down to machine code) is opaque to the user – therefore, you can’t specify it.
Can constructor have return type in Java?
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.
Can constructor be default in Java?
Java compiler automatically creates a default constructor (Constructor with no arguments) in case no constructor is present in the java class.
Can abstract class have protected constructor?
Constructors on abstract types can be called only by derived types. Because public constructors create instances of a type and you cannot create instances of an abstract type, an abstract type that has a public constructor is incorrectly designed.
Can you use this () and super () both in a constructor?
“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 override static method?
No, we cannot override static methods because method overriding is based on dynamic binding at runtime and the static methods are bonded using static binding at compile time. So, we cannot override static methods. The calling of method depends upon the type of object that calls the static method.
Can a constructor be synchronized?
Note that constructors cannot be synchronized — using the synchronized keyword with a constructor is a syntax error. Synchronizing constructors doesn’t make sense, because only the thread that creates an object should have access to it while it is being constructed.
Why constructor is not overridden?
Constructor Overriding is never possible in Java. This is because, Constructor looks like a method but name should be as class name and no return value. Overriding means what we have declared in Super class, that exactly we have to declare in Sub class it is called Overriding.
Can we overload private constructor in Java?
1) NO! A constructor belongs to the class in which it is declared. A sub class is a different class and must have its own constructor. So, constructors simply can’t be overridden.
Can destructor be private?
Destructors with the access modifier as private are known as Private Destructors. Whenever we want to prevent the destruction of an object, we can make the destructor private.
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 have static class in Java?
Java supports Static Instance Variables, Static Methods, Static Block, and Static Classes. The class in which the nested class is defined is known as the Outer Class. Unlike top-level classes, Inner classes can be Static. Non-static nested classes are also known as Inner classes.
Can we call constructor without creating object?
NO. You can’t invoke a constructor without creating an object.
Can constructor be virtual?
Constructor can not be virtual, because when constructor of a class is executed there is no vtable in the memory, means no virtual pointer defined yet.
How many constructors can a class have?
You can have 65535 constructors in a class(According to Oracle docs).
Can constructor throw exception?
Yes, constructors are allowed to throw an exception in Java. A Constructor is a special type of a method that is used to initialize the object and it is used to create an object of a class using the new keyword, where an object is also known as an Instance of a class.
Is Super called automatically?
With super(parameter list) , the superclass constructor with a matching parameter list is called. Note: If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass.
Can constructor be overloaded in derived class?
Since the constructors can’t be defined in derived class, it can’t be overloaded too, in derived class.
Is overriding possible in Java?
In Java, method overriding occurs when a subclass (child class) has the same method as the parent class. In other words, method overriding occurs when a subclass provides a particular implementation of a method declared by one of its parent classes.
Can constructor have non access modifiers?
Like methods, constructors can have any of the access modifiers: public, protected, private, or none (often called package or friendly). Unlike methods, constructors can take only access modifiers. Therefore, constructors cannot be abstract , final , native , static , or synchronized .
How do I create a private constructor?
We can declare a constructor private by using the private access specifier. Note that if a constructor is declared private, we are not able to create an object of the class. Instead, we can use this private constructor in Singleton Design Pattern.
Can static methods be private?
Yes, we can have private methods or private static methods in an interface in Java 9.
Why we use parameterized constructor in Java?
A parameterized constructor accepts parameters with which you can initialize the instance variables. Using parameterized constructor, you can initialize the class variables dynamically at the time of instantiating the class with distinct values.
What is the return type of a constructor?
A constructor doesn’t have any return type. The data type of the value retuned by a method may vary, return type of a method indicates this value. A constructor doesn’t return any values explicitly, it returns the instance of the class to which it belongs.
Can abstract method be final?
therefore, a final abstract combination is illegal for classes. Hence, a final class cannot contain abstract methods whereas an abstract class can contain a final method.
Is it OK to call abstract method from constructor in Java?
It’s a very bad practice to call an abstract method from a constructor. Methods called from constructors should always be private or final, to prevent overriding.
Why main method is static in Java?
When java runtime starts, there is no object of the class present. That’s why the main method has to be static so that JVM can load the class into memory and call the main method. If the main method won’t be static, JVM would not be able to call it because there is no object of the class is present.
Can we call one constructor from another constructor?
The invocation of one constructor from another constructor within the same class or different class is known as constructor chaining in Java. If we have to call a constructor within the same class, we use ‘this’ keyword and if we want to call it from another class we use the ‘super’ keyword.
Why multiple inheritance is not allowed in Java?
The reason behind this is to prevent ambiguity. Consider a case where class B extends class A and Class C and both class A and C have the same method display(). Now java compiler cannot decide, which display method it should inherit. To prevent such situation, multiple inheritances is not allowed in java.
Can we overload final method?
Can We Override a Final Method? No, the Methods that are declared as final cannot be Overridden or hidden. For this very reason, a method must be declared as final only when we’re sure that it is complete.
Can return type change in overriding?
From Java 5 onwards, we can override a method by changing its return type only by abiding the condition that return type is a subclass of that of overridden method return type.
Can we declare interface as abstract?
The interface body can contain abstract methods, default methods, and static methods. An abstract method within an interface is followed by a semicolon, but no braces (an abstract method does not contain an implementation).
Can we write non abstract methods in interface?
Interface methods are by definition public and abstract, so you cannot have non-abstract methods in your interface.
Why constructor have no return type?
So the reason the constructor doesn’t return a value is because it’s not called directly by your code, it’s called by the memory allocation and object initialization code in the runtime. Its return value (if it actually has one when compiled down to machine code) is opaque to the user – therefore, you can’t specify it.
Can we declare constructor as 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 constructor return a value?
No, constructor does not return any value.
Can we declare constructor as final?
No, a constructor can’t be made final. A final method cannot be overridden by any subclasses.
Can we override private methods?
You cannot override a private or static method in Java. If you create a similar method with same return type and same method arguments in child class then it will hide the super class method; this is known as method hiding. Similarly, you cannot override a private method in sub class because it’s not accessible there.
Can a constructor be protected?
Protecting a constructor prevents the users from creating the instance of the class, outside the package. During overriding, when a variable or method is protected, it can be overridden to other subclass using either a public or protected modifier only.
Can singleton class have multiple constructors?
EDIT: IF you have a singleton-class you won´t need both constructor. All your initializations can be done by the default one, where you set all important members of that instance whereas the static one can be omitted.
Should destructor be private for Singleton?
If the singleton is implemented as a variable at global scope, it must have a public destructor. Only public members are accessible at global scope. If it’s declared as a static member or static local within its own class, then the destructor may be private.
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 we call default constructor explicitly?
Yes, it is possible to call special member functions explicitly by the programmer.