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Why super constructor should be first statement in subclass constructor in java?
Because, the parent class also needs to be initialized when you create an object in the inheritance hierarchy.
What happens when no constructor function is declared in a class - in Java?
When any constructor is deffined in your class, the java compiler create a default no argument constructor for you. This constructor only have an invocation to the super class constructor (" super( ) ").
How do explicitly invoke superclass constructor from a subclass?
Using the super keyword. If you call super() from within your constructor, it will explicitly invoke the superclass version of the constructor.
When invoking a constructor from subclass its super class's no-arg constructor is always invoked?
Implicitly: (i.e., you do not code for it, but works as if you did)calling the no-argument constructor of the subclass, and there is no explicitly "redirect" codes.Explicitly:a constructor with base() / super() in the implementation, even that invoked constructor required some arguments.C# example: public SubClass(string whatever) : base() {//...}
How is a constructor in the super class called?
By using the reference super(); When you invoke super(); the JVM knows that you are trying to invoke the constructor from the parent class and calls the super class constructor automatically. In fact, the invocation to super(); is usually the first line of any class constructor. This is done to ensure that all the parent class objects are initialized before the current child class is created.
How does constructor work?
First line in any constructor has to be either super() or this() not both. If any constructor does not contain either of super() and this(), compiler adds super(). When any constructor is called before excuting the code of the constructor, if it founds this(), it will call another constructor else it will call super() which is the call for the constructor of super class, now again from the super class constructor it will call the super class constructor if available. This is continued until it reaches the top of the class hierarchy. ---- Basically, a constructor is a block of code that gets executed each time a particular instance of a class is created. So, say you've designed a class for working with a database of some sort. When you create an instance of that class, copies of all the variables and functions of that class get attached to the instance-object, and if one of the functions is a constructor function, it will be run as soon as the instance-object is created. This lets you automatically set up conditions for the instance (i.e. establishing connections to different databases or reading data from different tables, or etc.). Depending on the language you're using, classes may or may not automatically call the constructor function of a parent or super class (if such exists, and if you do not provide a constructor for the class in question).
Why we can't use super and this keywords in the same constructor?
because, the super refers to the constructor of the parent class while this refers to the constructor of the current class. Either statements must be the first line in a constructor. So, since we cannot have two first lines in a method we cannot have both keywords in the same constructor.public RandomTest() {super();this();}The above code will never compile. Even if we flip the positions of super and this, we will get the same compilation error.
What are the rules on how to construct java constructor?
1. The constructor has to have the same name as the classthat it is in.2. It does not have a return type. If it has a return type, then it is a method (even though it is legal, it's not ideal to have name a method the same name as the class).3. It can use any access modifier (this includes private).4. The default constructor does not take arguments.5. The first statement in a constructor has to have a super() type or this() type. If this is not written, by default, it's super(). It's illegal to have it in any other line other than the first line.6. Constructors can only access static variables.7. Only constructors have access to another constructor.Remember that interfaces do not have a constructor.
A subclass can call constructor method defined by its super class?
True
Why am I getting Java unreported exception while compiling my class?
It may be because you are using inheritance and your super class constructor has a throws clause in its declaration. If your super class has a throws clause in its constructor declaration, you must do the same in your child class constructor in order to eliminate this compiler error.
How constructor called?
You cannot invoke a constructor explicitly. It will get invoked implicitly when you call the new keyword on the class to create an object of the class. Ex: private ClassExample obj = new ClassExample(); here this new keyword usage on the ClassExample class will invoke the constructor of this class and create an object of that class.
Using default constructor in inheritance and in what reason it throws error?
The compiler places a default no-arg constructor in any java class that does not have an explicit constructor coded into it. for ex: public class Car { ... ... //lots of code but no constructor } In the above case, the compiler will place the below constructor into the code: public Car() { super(); } But, if you have a constructor in your class that takes arguments then the compiler will not put the default constructor. Ex: public class Car { public Car(String name){ ... } ... //lots of code } Above, we have a Car constructor that takes a string name as argument. so, the compiler wont put the default constructor in the code. now, if you try to do: Car obj = new Car(); you will get an error because this constructor is not defined.
