What is the difference between val and const val in kotlin?

If you’re new to Kotlin, you might be wondering what is need of const val when we have val? 🤔

Both are the same, Right? 🤔

Then this article is for you.

• The main difference between val and const val is that val can be assigned a value at runtime, whereas const val must be assigned a value at compile time and cannot be changed afterward(their values are hardcoded).

/*
const val eg.
*/
const val baseUrl  = "https://rickandmortyapi.com/api"
/*
val eg.
*/
val db = FirebaseDatabase.getInstance()
/*
const val eg.
*/
const val baseUrl  = "https://rickandmortyapi.com/api"

/*
val eg.
*/
val db = FirebaseDatabase.getInstance()
/*
const val eg.
*/
const val baseUrl  = "https://rickandmortyapi.com/api"

/*
val eg.
*/
val db = FirebaseDatabase.getInstance()

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• const val can only be declared at the top level of a file or inside an object declaration, whereas val can be declared anywhere within a function, class or object.

Example of const val deceleration at the top level of file

package com.example.admindashboard
import android.content.Intent
import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import com.example.admindashboard.databinding.ActivityMainBinding
import com.example.admindashboard.ui.AddBooks
import com.example.admindashboard.ui.ControlPanelActivity
import com.example.admindashboard.ui.NotifficationActivity
const val message = "Hello "                // correct
class MainActivity : AppCompatActivity() {
    const val message2 = "hello again"      // incorrect
// other code
    var binding: ActivityMainBinding? = null
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding!!.root)
    }
}
package com.example.admindashboard

import android.content.Intent
import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import com.example.admindashboard.databinding.ActivityMainBinding
import com.example.admindashboard.ui.AddBooks
import com.example.admindashboard.ui.ControlPanelActivity
import com.example.admindashboard.ui.NotifficationActivity

const val message = "Hello "                // correct

class MainActivity : AppCompatActivity() {

    const val message2 = "hello again"      // incorrect

// other code
    var binding: ActivityMainBinding? = null
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding!!.root)

    }

}

package com.example.admindashboard

import android.content.Intent
import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import com.example.admindashboard.databinding.ActivityMainBinding
import com.example.admindashboard.ui.AddBooks
import com.example.admindashboard.ui.ControlPanelActivity
import com.example.admindashboard.ui.NotifficationActivity

const val message = "Hello "                // correct

class MainActivity : AppCompatActivity() {

    const val message2 = "hello again"      // incorrect

// other code
    var binding: ActivityMainBinding? = null
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding!!.root)

    }

}

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Example of const val declaration inside object

package com.example.admindashboard.utils
import com.google.firebase.database.FirebaseDatabase
import com.google.firebase.storage.FirebaseStorage
object FirebaseUtils {
    const val bookNodeName = "books"  // const val declaration
    val firebaseDatabase = FirebaseDatabase.getInstance()
    var pyqRef = firebaseDatabase.getReference("pyq")
}
package com.example.admindashboard.utils

import com.google.firebase.database.FirebaseDatabase
import com.google.firebase.storage.FirebaseStorage

object FirebaseUtils {

    const val bookNodeName = "books"  // const val declaration

    val firebaseDatabase = FirebaseDatabase.getInstance()

    var pyqRef = firebaseDatabase.getReference("pyq")


}
package com.example.admindashboard.utils

import com.google.firebase.database.FirebaseDatabase
import com.google.firebase.storage.FirebaseStorage

object FirebaseUtils {

    const val bookNodeName = "books"  // const val declaration

    val firebaseDatabase = FirebaseDatabase.getInstance()

    var pyqRef = firebaseDatabase.getReference("pyq")


}

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原文链接：What is the difference between val and const val in kotlin?