Instance initializer in interface java

Question

Hey I was wondering if it would be possible to give some initialization to an interface when an implementer is made. Like a blank constructor in an abstract class.

I tried something like this:

public interface State {

{
//Do something.
}

public void render();
public void tick();
}

But it does not let you have an instance initializer. Is there any way to do this? Possibly with an inner class?

So the idea is that a piece of code is automatically called when a new instance of an implementing object is created.

Praveen Kumar · Accepted Answer · 2016-04-08 07:47:59Z

4

You cannot have static or instance blocks in an interface. But as of java 8 you can have static and default methods.

public interface MyData {

default void print(String str) {
    if (!isNull(str))
        System.out.println("MyData Print::" + str);
  }

static boolean isNull(String str) {
    System.out.println("Interface Null Check");

    return str == null ? true : "".equals(str) ? true : false;
  }
}

answered Apr 8, 2016 at 7:47

Praveen Kumar

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1 Comment

Llewv Over a year ago

Yes but the idea is that a piece of code is automatically called when a new instance of an implementing object is created.

Arnaud · Accepted Answer · 2016-04-08 07:41:21Z

3

You cannot do this, an interface cannot define an initializer.

An interface is basically a list of method signatures.

edited Apr 8, 2016 at 7:41

Arnaud

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answered Apr 8, 2016 at 7:39

Guillaume Barré

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2 Comments

Sanjeev Over a year ago

Second statement is not true anymore as java8 added default implementation to methods in an interface

fabian Over a year ago

Also you can declare fields (they become static final automatically)

goncalopinto · Accepted Answer · 2016-04-08 07:52:01Z

0

An Interface doesn't work that way, it is a list of method signatures, methods that have to be implemented in a class that implements the interface. To do that you will need a class not an Interface. There is a possible solution for something like this, from Java 8 you can create static and default methods in your interface, that allows you to create methods with a body in interfaces.

answered Apr 8, 2016 at 7:52

goncalopinto

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