Constructor a parameter in a method call

Question

What is this type of passing constructor to a method called in Java (the Class_name is an internal class)?

method_name(new Class_name(arguments));

How can we pass constructor without having an instance of the object?

Answer 1

Well, actually new Class_name(arguments) will create a new instance of the class. A construction might be useful - it is more concise than

Class_name instance = new Class_name(arguments);
method_name(instance);

but you would not be able to access instance later on.

Answer 2

As has been said, you are passing a new object.

If you want to pass just the information about this class, then your method would need to take a Class an an argument, for example...

public void method_name(Class clz) { ... do something... )

You would call it like this...

method_name( Class_name.class);

The other thing doing literally what you wanted would be to give the Constructor, which you can get, for example, by...

Constructor const = Class_name.class.getConstructor( ArgumentType.class, AnotherArgumentType.class, ...);

for example...

Constructor const = Class_name.class.getConstructor( String.class);

...if your constructor took one String argument.

And then call your method...

public void method_name(Constructor constructor) { ... do something... )

... with...

method_name( const );

The last way is very specific and I doubt that it's actually what you NEED, no matter what you think you might need.

Answer 3

new creates a object and this is passed to the function. You are not passing class you are passing "new" of that class which will create a object

Answer 4

You are not passing a constructor, an instance :

When you do :

MyClass myClass = new Myclass();
method_name(myClass);

You have an instance of MyClass into myClass . then you call method_name with myClass as an argument

So this can be shortened as :

method_name(new Myclass());

Answer 5

These types of use cases are very common for Decorator design pattern , where you do not require the intermediate objects.

Example:

FileReader fr = new FileReader(filename);
LineNumberReader lnr = new LineNumberReader(fr);

or

LineNumberReader lnr = new LineNumberReader(new FileReader(filename));

Take a look at I/O classes from nio framework for more understanding on these types of use cases.