Both this() and super() are constructor calls. Constructor call must always be the first statement. So we can not have two statements as first statement, hence either we can call super() or we can call this() from the constructor, but not both.
this and super are two special keywords in Java, which is used to represent current instance of a class and it’s super class. As I said in the first line, the main difference between this and super in Java is that this represents current instance of a class, while super represent current instance of the parent class.
Subsequently, question is, why does this () and super () have to be the first statement in a constructor? The Eclipse compiler says “Constructor call must be the first statement in a constructor“. So, it is not stopping you from executing logic before the call to super. It is just stopping you from executing logic that you can’t fit into a single expression.
how are this () and super () used with constructors?
Constructors use this to refer to another constructor in the same class with a different parameter list. Constructors use super to invoke the superclass’s constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.
Can we use super in method?
super is a keyword. It is used inside a sub-class method definition to call a method defined in the superclass. Only public and protected methods can be called by the super keyword. It is also used by class constructors to invoke constructors of its parent class.
What is super () in Java?
super is a keyword. It is used inside a sub-class method definition to call a method defined in the super class. Private methods of the super-class cannot be called. It is also used by class constructors to invoke constructors of its parent class. Super keyword are not used in static Method.
What is final method?
You use the final keyword in a method declaration to indicate that the method cannot be overridden by subclasses. The Object class does this—a number of its methods are final . A class that is declared final cannot be subclassed.
Can we override static method?
Static methods cannot be overridden because they are not dispatched on the object instance at runtime. The compiler decides which method gets called. Static methods can be overloaded (meaning that you can have the same method name for several methods as long as they have different parameter types).
What is final and super keyword difference between them?
super vs final Java are keywords of Java but doing very different jobs. super keyword is used to access super class variables and methods by subclass object when they are overridden by subclass. final is used in three places in Java with different jobs.
Why super is first line in Java?
super() must be first because conceptually, the subclass instance “is-a” instance of the superclass, and must be a properly set-up superclass object. The superclass shouldn’t need to know about the existence of subclasses, so it should be allowed to assume that its constructor is the first thing that runs.
What is this () in Java?
this is a keyword in Java which is used inside method or constructor of class. It is used to reference the current object whose method or constructor is being invoked. this keyword can be used to refer any member of current object from within an instance method or a constructor.
What is this and super keyword?
super keyword is used to access methods of the parent class while this is used to access methods of the current class. this is used to refer current-class’s instance as well as static members.
What is constructor in OOP?
A constructor is a special method of a class or structure in object-oriented programming that initializes an object of that type. A constructor is an instance method that usually has the same name as the class, and can be used to set the values of the members of an object, either to default or to user-defined values.
Can you make a constructor final?
No, a constructor can’t be made final. A final method cannot be overridden by any subclasses. In other words, constructors cannot be inherited in Java therefore, there is no need to write final before constructors. Therefore, java does not allow final keyword before a constructor.
What is this () Calling statement why we use it?
Use of this keyword It clarify that you are talking about a field, when there is another variable of the same name. Some use this as a means to differentiate instance variables from local variables or method parameters. Invoke other constructor of the current class in your parameterized constructor.
What is method overloading and overriding?
Overloading occurs when two or more methods in one class have the same method name but different parameters. Overriding means having two methods with the same method name and parameters (i.e., method signature). One of the methods is in the parent class and the other is in the child class.
What is Polymorphism in Java?
Polymorphism in Java is a concept by which we can perform a single action in different ways. We can perform polymorphism in java by method overloading and method overriding. If you overload a static method in Java, it is the example of compile time polymorphism. Here, we will focus on runtime polymorphism in java.
What is true constructor?
What is true about constructor? Explanation: Constructor returns a new object with variables defined as in the class. Instance variables are newly created and only one copy of static variables are created. Explanation: No instance can be created of abstract class.
What is static keyword in Java?
The static keyword in Java is used mainly for memory management. It is used with variables, methods, blocks and nested classes. It is a keyword that is used to share the same variable or method of a given class. This is used for a constant variable or a method that is the same for every instance of a class.