+++
title = "Define the HitCounter API"
weight = 100
+++

## Create a new file for our hit counter construct

Create two new files under `src/main/java/com/myorg` called `HitCounterProps.java` with the following content:

```java
package com.myorg;

import software.amazon.awscdk.services.lambda.IFunction;

public interface HitCounterProps {
    // Public constructor for the props builder
    public static Builder builder() {
        return new Builder();
    }

    // The function for which we want to count url hits
    IFunction getDownstream();

    // The builder for the props interface
    public static class Builder {
        private IFunction downstream;

        public Builder downstream(final IFunction function) {
            this.downstream = function;
            return this;
        }

        public HitCounterProps build() {
            if(this.downstream == null) {
                throw new NullPointerException("The downstream property is required!");
            }

            return new HitCounterProps() {
                @Override
                public IFunction getDownstream() {
                    return downstream;
                }
            };
        }
    }
}
```

AND `HitCounter.java` with this content:

``` java
package com.myorg;

import software.constructs.Construct;

public class HitCounter extends Construct {
    public HitCounter(final Construct scope, final String id, final HitCounterProps props) {
        super(scope, id);
        // TODO
    }
}
```

Save the files.

## What's going on here?

* We declared a new construct class called `HitCounter`.
* As usual, constructor arguments are `scope`, `id` and `props`, and we
  propagate them to the `Construct` base class.
* The `props` argument is of type `HitCounterProps` which includes a single
  property `getDownStream` of type `IFunction` and a `Builder` to create the
  props. This is where we are going to "plug in" the Lambda function we created
  in the previous chapter so it can be hit-counted.

----

Next, we are going to write the handler code of our hit counter.