Method Chaining (enhancing Java syntax)

[Tomáš Bleša]

/* I sent the following to discuss@ mailing list yesterday. (wrong list for the topic) I hope this will be more appropriate mailing list */

Hi all,

this is a request for feedback on a topic that has been on my mind for a few weeks. I have written a short document in JEP format and would like to ask you to comment if you find the described proposal useful.

Thanks,
Tomas Blesa
__________________________________

Summary
-------
Enhance the Java language syntax to better support the method chaining (named parameter idiom) programming pattern.

Goals
-----
The primary goal is to remove unnecessary boilerplate code in class methods designed for type-safe chained calls, especially when combined with inheritance.

Motivation
----------
[Method chaining](https://en.wikipedia.org/wiki/Method_chaining) is a widely used and popular programming pattern, particularly in creating libraries (APIs) or configuration objects. Programmers can easily create a method that returns `this` with a method signature that specifies the returning type of the containing class.

```java
class Shape {
    public Shape scale(double ratio) {
        // recalculate all points
        return this;
    }
}
```

The problem arises when we combine this pattern with inheritance. We can lose type information when calling the method on a subclass. For example, let's create two subclasses of the `Shape` superclass:

```java
class Rectangle extends Shape {
    public Rectangle roundCorners(double pixels) {
        // ...
        return this;
    }
}

class Circle extends Shape {
}
```

Now, imagine the following piece of code using the mini-library above:

```java
var myRect = new Rectangle().scale(1.2).roundCorners(10);
```

The code won't compile because `scale()` returns the type `Shape`, which doesn't have the `roundCorners` method. There is also a problem even without the final `roundCorners()` call:

```java
var myRect = new Rectangle().scale(1.2);
```

The inferred type of `myRect` is `Shape` and not `Rectangle`, so the following line will also be invalid:

```java
myRect.roundCorners(10);
```

Straightforward solutions to the problem could be:

1) Override the `scale()` method in all subclasses and change the return type:

```java
class Rectangle extends Shape {
    // ...
    @Override
    public Rectangle scale(double ratio) {
        super.scale(ratio);
        return this;
    }       
}
```

2) Split object construction and method calls:

```java
var myRect = new Rectangle();
myRect.scale(1.2);
myRect.roundCorners(10);
```

3) Partial solution - reorder chained calls (if possible):

```java
var myRect = new Rectangle();
myRect.roundCorners(10).scale(1.2); // roundCorners called first
```

All of these solutions add unnecessary lines of code, and as the library of shapes grows, keeping the desired return type will introduce more and more boilerplate code.

Description
-----------
The proposed solution to the problem described in the previous section is to extend the Java syntax for the returned type in method signatures:

```java
class Shape {
    public this scale(double ratio) { // <=== returns this
        // recalculate all points
        return this;
    }
}
```

Methods declared or defined as returning `this` can only return the instance on which they are called. The following code will be type-safe and perfectly valid:

```java
var myRect =                     // inferred Rectangle type
    new Rectangle()              // returns Rectangle instance
    .scale(1.2)                  // returns Rectangle instance
    .roundCorners(10);           // returns Rectangle instance
```

The constructed type `Rectangle` is preserved throughout the entire call chain.

It is possible to override methods returning `this`, but the subclass' implementation must also be declared with the `this` keyword instead of a concrete returning type.

It is even possible to remove the explicit return statement altogether:

```java
class Shape {
    public this scale(double ratio) {
        // recalculate all points
    }
}
```

Or simply remove the value `this` from the return statement:

```java
class Shape {
    public this scale(double ratio) {
        // recalculate all points
        if (condition) return;         // <== automatically returns this
        // do something else
    }
}
```

In fact, methods returning `this` can be compiled to the same bytecode as methods returning `void`. This is because the instance reference (and the returned value) is already known to the caller, eliminating the need to pass that value back through the call stack. As a result, both CPU cycles and memory are saved.

In the Java world, it is common to create getters and setters according to the Java Beans specification in the form of `getProperty`/`setProperty` pairs or `isProperty`/`setProperty`. Setters are defined as returning `void`. These setters can be more useful if defined as returning `this`:

```java
class Customer {
    public this setFirstname() { ... }
    public this setSurname() { ... }
    public this setEmail() { ... }
}
```

This allows for more concise usage when constructing and configuring an instance without adding more code:

```java
customers.add(
    new Customer()
        .setFirstname(resultSet.getString(1))
        .setSurname(resultSet.getString(2))
        .setEmail(resultSet.getString(3))
);
```

It is also possible to declare an interface with methods returning `this`:

```java
interface Shape {
    this scale(double ratio);
}
```

In this case, all implementing classes must define the method as returning `this`.

The proposed syntax is a bit less useful for enums or records, as neither of them allows for inheritance. But enums and records can also implement interfaces and for this reason and for overall consistency, "return this" syntax should be allowed for enums and records.

To accommodate the syntax with the Java Reflection API, it will probably be required to create a special final placeholder class `This` (with an uppercase "T"), similar to `java.lang.Void`.

Alternatives
------------
It is probably possible to help auto-generate overriding methods in subclasses using annotation processing, but this option wasn't fully explored. However, such an approach would add extra unnecessary code to compiled subclasses and go against the primary goal of reducing boilerplate.

Risks and Assumptions
---------------------
The proposed syntax is likely to break the compatibility of library-dependent code whose author decides to switch to the "return this" syntax between versions.

Older code that looks like this:

```java
class MyUglyShape extends Shape {
    @Override
    public MyUglyShape scale(double ratio) {
        return this;
    }
}
```

will have to be rewritten as:

```java
class MyUglyShape extends Shape {
    @Override
    public this scale(double ratio) {    // signature change
        // optional removal of the return this statement
    }
}
```

or 

```java
class MyUglyShape extends Shape {
	// override removed
}
```

This problem can be mitigated with the help of smart IDEs automatically suggesting such refactoring.

Another possible risk is breaking old code that relies on the Reflection API for scanning the returning types of methods.
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