Java中如何调用构造函数

Question

I have following code in java:

// SingleMaltView class
class abc {
    public abc(display p) {
        // display is another class.
    }
}

class sample {
    public static void main(String[] args) {
        abc ob = new abc(); // error
    }
}

How to call constructor of abc class?

Answer 1

Your constructor has an argument / parameter, use it:

public abc(display p)
//         ↑  here!!!!

This means you must provide a display object to create the abc object, so in the main method you must make something like that:

public static void main(String[] args) {
    display d = new display(); // assuming display has no-argument constructor
    abc ob = new abc(d);   // NO error! :)
}

NOTES:

• java objects by convention starts in UPPERCASE , so names of files and classes should be Display and Abc (or ABC ).
• if you don't put any code into constrctor, dont create it
• java provides a default empty constructor to instantiate your objects when you don't create any.

Answer 2

If you dont provide constructor in a class, compiler inserts by default no arguments constructor but if you have provided then compiler wont insert default constructor. Here you are creating an object of a class in which parameterized constructor is present and you are creating object of class ABC with no parameters so compiler is unable to locate no argument constructor and hence gives error below code can solve this riddle.

    public abc()    // another constructor
    {

    }

or change creation type

     public static void main(String[] args)
     {
              Display d =new Display();
              abc ob = new abc(d);   // error
     }

Answer 3

Here you are calling default constructor which is not there in your abc class . rather you have a single parameterized constructor.

public static void main(String[] args) {
    abc ob = new abc();   // error
} 

And here you are creating an object which has no argument.

So you should pass here something like this,

display obj = new display();
abc ob = new abc(obj);

Answer 4

We can define a constructor in 2 ways: (i) Default constructor (ii) Parametrised constructor

(i)Default constructor syntax: ConstructorName() {Statements.. }

(ii)Parametrised constructor syntax: ConstructorName(Parameter/Arguments) {Statements.. }

*Syntax to call a constructor ClassName reference variable = new TypeOfConstructor()

*To call the variable and method of constructor which defined under the scope, call it by using reference variables.

Note: Constructor doesn't have any return type and access specifier