不推荐使用 Okhttp3-RequestBody.create (contentType，content)

Just had a quick look at the documentation . It reads deprecated, however the alternative is provided in the doc.
json.toRequestBody(contentType) should do the trick for you.
Below is the documentation link:
https://github.com/square/okhttp/blob/master/okhttp/src/main/java/okhttp3/RequestBody.kt

public static final MediaType JSON = MediaType.get("application/json; charset=utf-8");


OkHttpClient client = new OkHttpClient();


String post(String url, String json) throws IOException {
RequestBody body = RequestBody.Companion.create(json, JSON)
Request request = new Request.Builder()
.url(url)
.post(body)
.build();
try (Response response = client.newCall(request).execute()) {
return response.body().string();
}
}

It was deprecated since version 4.0.0 of okhttp3.

The documentation for that version says

@JvmStatic
@Deprecated(
message = "Moved to extension function. Put the 'content' argument first to fix Java",
replaceWith = ReplaceWith(
expression = "content.toRequestBody(contentType)",
imports = ["okhttp3.RequestBody.Companion.toRequestBody"]
),
level = DeprecationLevel.WARNING)
fun create(contentType: MediaType?, content: String) = content.toRequestBody(contentType)

I haven't tried it but I believe that you should be good by doing the following:

package com.example;


import java.io.IOException;
import okhttp3.MediaType;
import okhttp3.OkHttpClient;
import okhttp3.Request;
import okhttp3.RequestBody;
import okhttp3.Response;


public class Test {


public static final MediaType JSON = MediaType.get("application/json; charset=utf-8");




OkHttpClient client = new OkHttpClient();


public static void main(String[] args) {


}


String post(String url, String json) throws IOException {
//RequestBody body = RequestBody.create(JSON, json);
RequestBody body = RequestBody.Companion.create(json, JSON);


Request request = new Request.Builder()
.url(url)
.post(body)
.build();
try (Response response = client.newCall(request).execute()) {
return response.body().string();
}
}


}

Update: I tried to compile the file shown above using the following dependency version and RequestBody.Companion.create(json, JSON) doesn't seem to be deprecated.

<dependency>
<groupId>com.squareup.okhttp3</groupId>
<artifactId>okhttp</artifactId>
<version>4.0.0</version>
</dependency>

In com.squareup.okhttp3:okhttp:4.1.0

MediaType.get("application/json; charset=utf-8") no more available.

instead this we need to use "application/json; charset=utf-8".toMediaTypeOrNull().

For example how we need to create request body now since okhttp:4.1.0

import okhttp3.MediaType.Companion.toMediaTypeOrNull
import okhttp3.RequestBody.Companion.toRequestBody


val jsonObject = JSONObject()
jsonObject.put("name", "Ancd test")
jsonObject.put("city", "delhi")
jsonObject.put("age", "23")
val body = jsonObject.toString().toRequestBody("application/json; charset=utf-8".toMediaTypeOrNull())

To those wondering where the answers are coming from!

All the alternatives/solutions(as described by the answer) are documented in the corresponding deprecated code! Just manoeuvre to it (the deprecated code) using whichever means your IDE supports. For example, to see the alternative/solution to the deprecated code RequestBody.create(...,...) when using AndroidStudio or any Jetbrain's IDE, just long-press Ctrl and hover over the RequestBody.create(...,...) then click on it when it's hovered over successfully

Java Solution: Use create(String, MediaType) instead of create(MediaType, String) for example

Kotlin Solution: Use the extension function content.toRequestBody(contentType); for the File type file.asRequestBody(contentType)

Note: I'm using kotlin, but my IDE just doesn't automatically import the class or method like import okhttp3.RequestBody.Companion.toRequestBody, so I import it manually...then use it as the example given by Saeed Younus and Pratyesh below

For more: The documentation

(In Android Studio or any Jetbrain's IDE, the solution to the deprecated methods or class can be found by just holding the Ctrl and clicking on the create(...) of RequestBody.create)

You need to import these files manually may be this is a bug in android studio. It is not suggested but this is work for Okhttp 4.2.2

import okhttp3.MediaType.Companion.toMediaTypeOrNull
import okhttp3.RequestBody.Companion.asRequestBody

and use as

val file = File("path")
file.asRequestBody("image/jpeg".toMediaTypeOrNull())

ok according to okhttp 4 many thing updated as official docs

RequestBody.create() is upgraded to File.asRequestBody()

You Just Need To Flip Your Args.

@kotlin.jvm.JvmStatic @kotlin.Deprecated public final fun create(contentType: okhttp3.MediaType?, file: java.io.File): okhttp3.RequestBody { }

    @kotlin.jvm.JvmStatic @kotlin.Deprecated @kotlin.jvm.JvmOverloads public final fun create(contentType: okhttp3.MediaType?, content: kotlin.ByteArray, offset: kotlin.Int , byteCount: kotlin.Int): okhttp3.RequestBody {}


@kotlin.jvm.JvmStatic @kotlin.Deprecated public final fun create(contentType: okhttp3.MediaType?, content: kotlin.String): okhttp3.RequestBody {}


@kotlin.jvm.JvmStatic @kotlin.Deprecated public final fun create(contentType: okhttp3.MediaType?, content: okio.ByteString): okhttp3.RequestBody { }

Can u update like that

val apiRequest = RequestBody.create(MediaType.parse("text/plain;charset=utf-8"), "edit_group")
val tokenRequest = RequestBody.create(MediaType.parse("text/plain;charset=utf-8"), token)
val fileReqBody = RequestBody.create(MediaType.parse("image/*"), file)

to

val apiRequest = "edit_group".toRequestBody("text/plain;charset=utf-8".toMediaType())
val tokenRequest = token.toRequestBody("text/plain;charset=utf-8".toMediaType())
val file = File(path)
val fileReqBody = file.asRequestBody("image/*".toMediaType())

Just change ResponseBody.create(MediaType.parse("text/json"), plainBody.trim()) to ResponseBody.create(plainBody.trim(),MediaType.parse("text/json"))