MainActivity.java

package com.example.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    // 1
    EditText etHeight;
    EditText etWeight;
    Button btnCalculate;
    TextView tvBmi;

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

        // 2
        etHeight = findViewById(R.id.etHeight);
        etWeight=findViewById(R.id.etWeight);
        btnCalculate = findViewById(R.id.btnCalculate);
        tvBmi = findViewById(R.id.tvBmi);
    }

    public void calaculate(View view) {
        String stw = etWeight.getText().toString();
        if (stw.length()>0) {
            double weight = Double.parseDouble(stw);

        double height = Double.parseDouble(etHeight.getText().toString());

        double bmi = weight / Math.pow(height,2);
        tvBmi.setText(bmi+"");

        if (bmi>=20 && bmi<=24) {
            Toast.makeText(this,"Very good",Toast.LENGTH_LONG).show();
        }
        }
    }

}