Android studioでswitch文を使うとエラーが出る

エラー: 定数式が必要です
            case R.id.btn1:
                     ^

package com.example.calculate;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.util.Log;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private Button btn1;
    private Button btn2;

    (略

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        btn1 = findViewById(R.id.btn1);
        btn1.setOnClickListener(this);
        btn2 = findViewById(R.id.btn2);
        btn2.setOnClickListener(this);

        (略

        calctext = findViewById(R.id.calctext);

    }

    @Override
    public void onClick(View view) {
        Log.d("MainActivity", "Button ID: " + view.getId());
        switch(view.getId()) {

            case R.id.btn1: //問題の箇所
                calctext.setText("1");
                break;

        }

    }
}