为什么当我将从EditText转换为整数的值时出现错误

Question

为什么当我从String转换为int时出现错误，但我认为转换是正确的。但我知道错误是什么 这是代码：

bhstudentgrouplist.count() = 0

错误消息：

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    EditText weight;
    EditText age;
    Button result;
    TextView txtresult;

    String w;
    String a;
    double  r1;
    int w1;
    int a1;

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

        weight=findViewById(R.id.w);
        age=findViewById(R.id.a);
        result=findViewById(R.id.btnresult);
        txtresult=findViewById(R.id.txtresult);

        w= weight.getText().toString();
        w1=Integer.parseInt(w);


        a= age.getText().toString();
        a1=Integer.parseInt(a);

        r1=w1/2.2*a1/28.3/8;
        result.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                txtresult.setText("number of cups :"+r1);

            }
        });

    }
}