JAVA方法调用查询

Question

class Temp
{
    private Temp(int data)
    {
        System.out.printf(" Constructor called ");
    }
    protected static Temp create(int data)
    {Temp obj = new Temp(data);
        return obj;
    }
    public void myMethod()
    {
        System.out.printf(" Method called ");
    }
}

public class Test
{
    public static void main(String[] args)
    {
        Temp obj = Temp.create(20); //How this is a method call?
        obj.myMethod();
    }
}

1. In the above program, the commented line is not understandable for me? Can any one have good explaination about how static method is called using object creation

Answer 1

A static method means that it can be called without creating an Object of that class, in your example Temp item.

 static Temp create(int data)

The static word in your method is what is allowing you to do that. The method is then callable in a static way, which means using className.methodName , in your example, Temp.create()

And since your method is returning a Temp object, you are putting that into a Temp object

Temp obj = Temp.create(20);

In the Temp obj you are putting the result of the object created in your method, in the line

{Temp obj = new Temp(data);
    return obj; //this is your object
}

Answer 2

class Temp
{
    private Temp(int data)
    {
        System.out.printf(" Constructor called ");
    }
    protected static Temp create(int data)
    {Temp obj = new Temp(data);
        return obj;
    }
    public void myMethod()
    {
        System.out.printf(" Method called ");
    }
}

public class Test
{
    public static void main(String[] args)
    {
        Temp obj = Temp.create(20); //How this is a method call?
        obj.myMethod();
    }
}

A constructor of a class always follows a same signature: [access modifier -> protected, public, private, default] [name of the class] (parameters)

So, in your code,

private Temp(int data) {
  System.out.printf(" Constructor called ");
}

is your constructor.

In order to use your Temp class, at least the non-static members of it, you'll need an instance of the class to be able to use it, but, since your constructor is declared private, an instance can only be created within the very same class.

That is what you do here:

protected static Temp create(int data)
        {Temp obj = new Temp(data);
            return obj;
        }

This method is a static method, meaning it 'exists' as soon as the class is loaded in the memory, even without the class being instantiated. Since it is declared protected, not private, it can be used by subclasses of Temp , and by classes that are in the same package as Temp , like your Test class.

Your Test class calls this method, which calls the constructor, and returns the created instance to the Test class. This way of working is used in certain scenario's, for instance if you want to limit the number of instances created per VM, like with the Singleton pattern.