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Converting EditText to TextView type in android studio [duplicate]

This question already has an answer here:

I'm trying to convert my EditText to String type and then evaluate a mathematical expression(given by user) to display it in a textView type but my app crashes every time a try to run it.

package com.example.abhishek.edittexttrial;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
//import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import net.objecthunter.exp4j.Expression;
import net.objecthunter.exp4j.ExpressionBuilder;


public class MainActivity extends AppCompatActivity {
private TextView txtScreen = (TextView)findViewById(R.id.txtScreen);
private EditText etxt = (EditText)findViewById(R.id.editText);
private double ans;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
findViewById(R.id.btnCalc).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
display();
}
});

}
public void display(){
try{
String str = etxt.getText().toString().trim();
String result = str.substring(0, str.length() - 1);
Expression exp = new ExpressionBuilder(result).build();
this.ans = exp.evaluate();
txtScreen.setText(Double.toString(ans));


}catch(NumberFormatException exception){
this.ans = 0;
txtScreen.setText(Double.toString(ans));
}

}

}


This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

findViewById()

onCreate()

setContentView()

private TextView txtScreen;

onCreate()

txtScreen = (TextView)findViewById(R.id.txtScreen);