Java中的每个对象都有自己的构造函数吗？

Question

This question is because of this line i read A new instance of an object is created by calling a constructor method. i agrre but constructor method of what..? an OObject or a Class itself..?.. Sorry if its an amateur question but i'm still learning java and i'm finding it hard to understand.

and this reminded me of which is first chicken or egg..?

EDIT:

May be my question was not clear, i know how objects are created and every class has a constructor but what i want to know is, every Objects in the heap have their own copy of instance variable. in the same way will they also have a Constructor with them or its just something that only classes have.

Answer 1

In Object-oriented design, a constructor is what creates an object out of your class definition...

2 key concepts here

• Class - a class is the blueprint for what an instantiated object should contain, both behaviour (methods) and information (properties). Usually contains a Constructor.

• Object - The thing that is Created by the constructor, a instatiated version of the Class in practical use.

Example of a Constructor in use

public class MyClass {

    public int intProperty;

    // This is the Constructor, Notice it shares a name with the Class
    public MyClass(int value) {
        intProperty = value;
    }
}

Now to Use the class

//                            |----------This is calling the constructor
//                            |          and placing a new MyClass object
//                            v          in myClassObejct
MyClass myClassObject = new MyClass(3);

myClassObject.intProperty; // 3

This creates a new MyClass Object

Java does not work without Classes and Constructors, it is core to the design pattern of the language...

Only Classes have constructors, Objects are the product of Constructors , an Object itself does not contain a constructor.

Answer 2

Yes, you call the constructor of the class:

MyClass instance = new MyClass();

but note that some objects have special constructors allowed by the language, for example String:

String x = "foobar";

Is very similar to (but not exactly the same as):

String x = new String("foobar");

Note that if no constructors are defined for the class, a default (no-args) constructor is implied .

Answer 3

Yes every class has a constructor.

If you do not explicitly define one, Java will create a default empty one for you.

Answer 4

In Java every class has one or more constructors, and when you create an object using the new keyword, a constructor of that class is called.

example:

Integer i = new Integer("1");
//i is the object, and the Integer class constructor gets called

Answer 5

Every class must have its own constructor. Either you provide one or more ctor's or the compiler generates a default no-argument ctor for you. If you have your own ctor, than the compiler doesn't generate anything.

Answer 6

Every object in Java must be created through some Class's constructor, with the exception of a few primitive classes like String, which has special allocation rules. Even at the most basic level, you can always call

Object o = new Object();

and since all objects in Java inherit from the Object superclass, most objects will inherit the default constructor.

The exception to this is when a class only has a private constructor--one that cannot be called by any outside classes. In that case, because the default constructor is no longer necessary, it will not be accessible either.

class MyClass
{
    private MyClass()
    {
        //cannot be called by outside classes
    }
}

In general, this is used either for purely static classes (which don't need instances) or Singleton objects (that want to restrict instantiation). Therefore all objects are created from some constructor, but not all classes necessarily have a usable constructor.

Answer 7

The statement "A new instance of an object is created by calling a constructor method" is an incorrect statement and is what's causing the confusion. There's no such thing as an instance of an object. An object is an instance of a class. The following 2 statements are valid:

An object is created by calling a constructor method.

An instance of a class (ie an object) is created by calling a constructor method.