如何在构建Android应用程序时初始化TextView以显示动态内容？

Question

Here is the problem that I am trying to solve, I ask the user to perform some computation in the current activity which I call as activity1 and I ask the user to press a button to perform the computation after the user presses the button I move him to another activity where I need to display the results of the computation performed by the user, but I am unable to set the TextView for the activity, I read several stackoverflow posts and then finally landed on one that I found somewhat relevant - NullPointer Exception when assigning value to TextView . I keep on getting these NullPointerExceptions and I don't know now what to do. Here is the code for activity2 file wherein I want to display the contents of the computation to the user.

package com.example.mathcalculator;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class CalculatorActivity3 extends ActionBarActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Bundle bundle = new Bundle();
        bundle.putString("key", getIntent().getStringExtra(CalculatorActivity2.EXTRA_MESSAGE));

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
        }

        setContentView(R.layout.activity_calculator_activity3);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.calculator_activity3, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_calculator_activity3, container, false);
            TextView textView = (TextView) rootView.findViewById(R.id.text_view);
            String strtext = getArguments().getString("key");
            textView.setText("The result of the computation is :" + strtext);
            return rootView;
        }
    }

}

This is the activity through which I am passing the content:

package com.example.mathcalculator;

import java.math.BigDecimal;
import android.content.Intent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;

public class CalculatorActivity2 extends ActionBarActivity {

    // this is the operation requested by the user and we store it in a global variable
    public static String operationRequested = "";

    // this is the message that is passed along with the result of the computation
    public static String EXTRA_MESSAGE = "com.example.mathcalculator.CalculatorActivity2.Message";

    // this function is used to perform the requisite operation as requested by the user on the previous screen of the application
    public void computeOperation(View view){

        // get the numbers from the 2 fields
        EditText et1 = (EditText)findViewById(R.id.number_1);
        EditText et2 = (EditText)findViewById(R.id.number_2);
        String et1String = et1.getText().toString();
        String et2String = et2.getText().toString();

        // these are the 2 decimal numbers input by the user in the 2 edit text fields
        BigDecimal num1 = new BigDecimal(et1String);
        BigDecimal num2 = new BigDecimal(et2String);

        // make an intent to pass the result to the next activity
        Intent intent = new Intent(this,CalculatorActivity3.class);

        if(operationRequested.equals("sum")){

            // this is the result of the operation
            BigDecimal result = num1.add(num2);
            Log.e("The operation requested was sum and the result of the operation is : " , result.toString());
            // put the key-value pair in the intent
            intent.putExtra(EXTRA_MESSAGE, result.toString());
        }
        else if(operationRequested.equals("difference")){

            // this is the result of the operation
            BigDecimal result = num1.subtract(num2);
            Log.e("The operation requested was difference and the result of the operation is : " , result.toString());
            // put the key-value pair in the intent
            intent.putExtra(EXTRA_MESSAGE, result.toString());
        }
        else if(operationRequested.equals("product")){

            // this is the result of the operation
            BigDecimal result = num1.multiply(num2);
            Log.e("The operation requested was product and the result of the operation is : " , result.toString());
            // put the key-value pair in the intent
            intent.putExtra(EXTRA_MESSAGE, result.toString());
        }
        else if(operationRequested.equals("division")){

            // this is the result of the operation
            BigDecimal result = num1.divide(num2);
            Log.e("The operation requested was division and the result of the operation is : " , result.toString());
            // put the key-value pair in the intent
            intent.putExtra(EXTRA_MESSAGE, result.toString());
        }
        else;

        // start the new activity
        startActivity(intent);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_calculator_activity2);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction().add(R.id.container, new PlaceholderFragment()).commit();
        }

        // this is the intent received from the previous activity
        Intent intent = getIntent();
        operationRequested = intent.getStringExtra(CalculatorActivity1.EXTRA_MESSAGE);

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.calculator_activity2, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(
                    R.layout.fragment_calculator_activity2, container, false);
            return rootView;
        }
    }

}

and the fragment file for the same activity above

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="@drawable/calculato_activity_1">

    <TextView android:id="@+id/text_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

</LinearLayout>

Answer 1

The problem is that you are trying to add a fragment to the transaction before you set the layout for you acitivity.

problem:

if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderF()).commit();ragment
    }

    setContentView(R.layout.activity_calculator_activity3);

As you can see you set the layout after to do a transaction thus it will catch a exception to the .add(R.id.container, new PlaceholderF()).commit(); R.id.container is not yet added so it will return null.

solution:

add the fragment after you set the layout for your activity

     setContentView(R.layout.activity_calculator_activity3);

 if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderF()).commit();ragment
        }