Java APIs: Add a Builder interface implemented by all Builders

[GrahamLea]

In Java, each Builder class should implement a generic Builder interface.

Use Case

We can use extension methods in Kotlin to make it less verbose to build props, like this:

        fun stackProps(): StackProps =
            StackProps.Builder().props {
                env(Environment.Builder().props {
                    account(accountToUse)
                    region(regionToUse)
                })
            }

which works by using this extension method:

        fun <B : Any, T> B.props(init: B.() -> Unit): T {
            init(this)
            val buildFunction = this::class.memberFunctions.single { it.name == "build" }
            return buildFunction.call(this) as T
        }

Unfortunately, the extension method has to use unsafe reflection and casting due to there being no relationship in the type system between Builders and the types they build.

A simple Builder interface would help make helper methods like these simpler and type-safe, like this:

        fun <B: Builder<T>, T> B.props(init: B.() -> Unit): T {
            init(this)
            return this.build()
        }

Proposed Solution

The Java API should include this interface:

package software.amazon.awscdk.core;

@FunctionalInterface
public interface Builder<T> {
    T build();
}

and each Builder class should implement it:

public interface StackProps extends software.amazon.jsii.JsiiSerializable {
    ....
    public static final class Builder implements software.amazon.awscdk.core.Builder<StackProps> {
        ....

Other

This should be backwards-compatible for almost all cases. In theory, it simply adds an interface for functions that are already present. In the wild, if some code relies on the fact that Builder classes don't implement an interface, that code may break.

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

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This is a 🚀 Feature Request