nameof equivalent in Java

Question

C# 6.0 introduced the nameof() operator, that returns a string representing the name of any class / function / method / local-variable / property identifier put inside it.

If I have a class like this:

class MyClass
{
    public SomeOtherClass MyProperty { get; set; }

    public void MyMethod()
    {
        var aLocalVariable = 12;
    }
}

I can use the operator like this:

// with class name:
var s = nameof(MyClass); // s == "MyClass"

// with properties:
var s = nameof(MyClass.OneProperty); // s == "OneProperty"

// with methods:
var s = nameof(MyClass.MyMethod); // s == "MyMethod"

// with local variables:
var s = nameof(aLocalVariable); // s == "aLocalVariable".

This is useful since the correct string is checked at compile time. If I misspell the name of some property/method/variable, the compiler returns an error. Also, if I refactor, all the strings are automatically updated. See for example this documentation for real use cases.

Is there any equivalent of that operator in Java? Otherwise, how can I achieve the same result (or similar)?

Answer 1

It can be done using runtime byte code instrumentation, for instance using Byte Buddy library.

See this library: https://github.com/strangeway-org/nameof

The approach is described here: http://in.relation.to/2016/04/14/emulating-property-literals-with-java-8-method-references/

Usage example:

public class NameOfTest {
    @Test
    public void direct() {
        assertEquals("name", $$(Person.class, Person::getName));
    }

    @Test
    public void properties() {
        assertEquals("summary", Person.$(Person::getSummary));
    }
}

Answer 2

Sadly, there is nothing like this. I had been looking for this functionality a while back and the answer seemed to be that generally speaking, this stuff does not exist.

See Get name of a field

You could, of course, annotate your field with a "Named" annotation to essentially accomplish this goal for your own classes. There's a large variety of frameworks that depend upon similar concepts, actually. Even so, this isn't automatic.

Answer 3

You can't.

You can get a Method or Field using reflection, but you'd have to hardcode the method name as a String, which eliminates the whole purpose.

The concept of properties is not built into java like it is in C#. Getters and setters are just regular methods. You cannot even reference a method as easily as you do in your question. You could try around with reflection to get a handle to a getter method and then cut off the get to get the name of the "property" it resembles, but that's ugly and not the same.

As for local variables, it's not possible at all.

Answer 4

You can't.

If you compile with debug symbols then the .class file will contain a table of variable names (which is how debuggers map variables back to your source code), but there's no guarantee this will be there and it's not exposed in the runtime.

Answer 5

Lombok has an experimental feature @FieldNameConstants

After adding annotation you get inner type Fields with field names.

@FieldNameConstants
class MyClass {
 String myProperty;
}
...

String s = MyClass.Fields.myProperty; // s == "myProperty"

Answer 6

I was also annoyed that there is nothing comparable in Java, so I implemented it myself: https://github.com/mobiuscode-de/nameof

You can simply use it like this:

Name.of(MyClass.class, MyClass::getProperty)

which would just return the String

"property"

It's also on , so you can add it to your project like this:

<dependency>
    <groupId>de.mobiuscode.nameof</groupId>
    <artifactId>nameof</artifactId>
    <version>1.0</version>
</dependency>

or for Gradle:

implementation 'de.mobiuscode.nameof:nameof:1.0'

I realize that it is quite similar to the library from strangeway, but I thought it might be better not to introduce the strange $/$$ notation and enhanced byte code engineering. My library just uses a proxy class on which the getter is called on to determine the name of the passed method. This allows to simply extract the property name.

I also created a blog post about the library with more details.