在一个 activity 中保存值，然后在不同的 activity 中显示这个值

我想在一个 activity 保存我的密码，我想把密码恢复到不同的 activity 中，但是当程序开启第二个 activity 时就奔溃了，这是什么原因呢？

package com.example.test;
public class MainActivity extends Activity {
    String finall;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        String FILENAME = "hello_file.txt";
        String string = "1234";

        FileOutputStream fos;
        try
        {
            fos = openFileOutput(FILENAME, Context.MODE_PRIVATE);
            fos.write(string.getBytes());
            fos.close();
        } 
        catch (FileNotFoundException e) { e.printStackTrace(); } 
        catch (IOException e) { e.printStackTrace(); }

        try
        {
            FileInputStream in = openFileInput("hello_file.txt");
            StringBuffer fileContent = new StringBuffer("");
            byte[] buffer = new byte[4];

            while(in.read(buffer) != -1)
            {
                fileContent.append(new String(buffer));
            }
            finall = fileContent.toString();
            in.close();
        }
        catch (FileNotFoundException e) { e.printStackTrace(); } 
        catch (IOException e) { e.printStackTrace(); }

        Button button = (Button)findViewById(R.id.button);
        button.setText(finall);
        button.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v) {
                sendGo(v);

            }
        });
    }

    public void sendGo(View v)
    {
        Intent intent = new Intent(this, SecondActivity.class);
        startActivity(intent);
    }
}

第一部分正常运行，因为我在相同的activity 中能读取我保存的文件。但是当我试着在另一个activity读取时，就不能运行。

package com.example.test;
public class SecondActivity extends Activity {
    String finall="";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);
        // Show the Up button in the action bar.
        setupActionBar();
        try
        {
            FileInputStream in = openFileInput("hello_file.txt");
            StringBuffer fileContent = new StringBuffer("");
            byte[] buffer = new byte[4];
            while(in.read(buffer) != -1)
            {
                fileContent.append(new String(buffer));
            }
            finall = fileContent.toString();
            in.close();
        }
        catch (FileNotFoundException e) { e.printStackTrace(); } 
        catch (IOException e) { e.printStackTrace(); }

        TextView text = (TextView)findViewById(R.id.mehmet);
        text.setText(finall);
    }

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    private void setupActionBar() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
            getActionBar().setDisplayHomeAsUpEnabled(true);
        }
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case android.R.id.home:
            // This ID represents the Home or Up button. In the case of this
            // activity, the Up button is shown. Use NavUtils to allow users
            // to navigate up one level in the application structure. For
            // more details, see the Navigation pattern on Android Design:
            //
            // http://developer.android.com/design/patterns/navigation.html#up-vs-back
            //
            NavUtils.navigateUpFromSameTask(this);
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

}