Android Activity — NullPointerException (“unable to instantiate activity ComponentInfo”)?

问题

I'm trying to create a program for Android which will read in text, and then print it out, jsut to get used to using I/O for Android. However, whenever I run it on the AVD, it returns an error in LogCat saying that there is a NullPointerException in my code, and that it is therefore "unable to instantiate activity ComponentInfo".

I can't find where I'm referring to anything null.

Here is my code:

import android.app.*;
import android.os.*;
import android.view.View;
import android.view.View.*;
import android.widget.*;



 public class ButtonTestActivity extends Activity {

TextView tv = (TextView) findViewById(R.id.textView1);
EditText et = (EditText) findViewById(R.id.edittext);

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {


    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    Button b = (Button) findViewById(R.id.Enter);

    b.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View arg0) {
            tv.setText(et.getText().toString());
        }
    });

}

}

Here is my XML:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"  >
<TextView
    android:id="@+id/textView1"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/hello" />

<Button
    android:id="@+id/Enter"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    />

<EditText
    android:id="@+id/edittext"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"/>

 </LinearLayout>

Does anyone know what could be causing this? Thanks a lot.

回答1:

You need to instantiate your views inside onCreate and after the setContentView, the Activity doesn't know which views you are referring until you set the Activities layout via the setContentView method. If this doesn't fix the problem it would help if you posted the stack trace from LogCat so the exact line number is known.

@Override

public void onCreate(Bundle savedInstanceState) {


super.onCreate(savedInstanceState);
setContentView(R.layout.main);


TextView tv = (TextView) findViewById(R.id.textView1);
EditText et = (EditText) findViewById(R.id.edittext);

Button b = (Button) findViewById(R.id.Enter);

b.setOnClickListener(new OnClickListener() {
    @Override
    public void onClick(View arg0) {
        tv.setText(et.getText().toString());
    }
});

}

回答2:

I assume that the problem is caused by the two fields tv and et which you initialize directly when the class instance is created.

You should move the initialization of both fields in the onCreate method as you have done it with "Button b".

回答3:

Before setting the content you are trying to instantiate the textview and EditText that's why you got null pointer exception
Thanks

回答4:

Null pointer exception means that you are referencing something that isn't yet created.
setContentView creates the fields neccesary by the variables tv and et.
So, you need to have tv and et after the setContentView.

回答5:

yeah, additional of above answers, i give that error because define a variable before onCreate method:

boolean in = true;

@Override
protected void onCreate .......

来源：https://stackoverflow.com/questions/9537150/android-activity-nullpointerexception-unable-to-instantiate-activity-compon

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