I am making a simple 3x3 grid type application that when I click a button, the last button clicked is " " and the button I just clicked is "!". However, I keep getting the error,
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View
android.view.Window.findViewById(int)' on a null object reference
Here is the code: package com.example.ryanfolz.gridgame;
import android.content.Context;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
public class GameActivity extends ActionBarActivity {
private Context ctx;
public int j = 1;
public int i = 1;
public final int[][] buttons = new int[][] {
{R.id.top_left_button, R.id.top_center_button, R.id.top_right_button},
{R.id.left_button, R.id.center_button, R.id.right_button},
{R.id.bottom_left_button, R.id.bottom_center_button, R.id.bottom_right_button}};
private Button lastButton;
public void setPlayer(Button button){
lastButton = (Button)findViewById(R.id.center_button);
button.setText("!");
lastButton.setText(" ");
lastButton = button;
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_game);
ctx = this;
final GameEngine game = new GameEngine();
lastButton = (Button)findViewById(R.id.center_button);
for (i = 0; i < buttons.length; i++) {
for (j = 0; j < buttons[i].length; j++) {
final Button button = (Button)findViewById(buttons[i][j]);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Button b = (Button)v;
game.updatePlayer(b);
}
});
}
}
Button backButton = (Button)findViewById(R.id.back_button);
backButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent gotoMenu = new Intent(ctx, MainActivity.class);
startActivity(gotoMenu);
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_game, 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();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
The other method is this
public class GameEngine {
GameActivity game = new GameActivity();
public void updatePlayer(Button buttonID){
game.setPlayer(buttonID);
}
Log cat is this:
01-07 15:52:38.892 1929-1929/com.example.ryanfolz.gridgame E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.ryanfolz.gridgame, PID: 1929
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
at android.app.Activity.findViewById(Activity.java:2071)
at com.example.ryanfolz.gridgame.GameActivity.setPlayer(GameActivity.java:25)
at com.example.ryanfolz.gridgame.GameEngine.updatePlayer(GameEngine.java:21)
at com.example.ryanfolz.gridgame.GameActivity$1.onClick(GameActivity.java:54)
at android.view.View.performClick(View.java:4756)
at android.view.View$PerformClick.run(View.java:19749)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
01-07 15:52:40.775 2285-2300/com.example.ryanfolz.gridgame D/OpenGLRenderer﹕ Render dirty regions requested: true
01-07 15:52:40.778 2285-2285/com.example.ryanfolz.gridgame D/﹕ HostConnection::get() New Host Connection established 0xa6c4a590, tid 2285
01-07 15:52:40.873 2285-2285/com.example.ryanfolz.gridgame D/Atlas﹕ Validating map...
Thanks for any help
} `
I believe the problem is here:
GameActivity game = new GameActivity();
public void updatePlayer(Button buttonID){
game.setPlayer(buttonID);
}
You're creating the GameActivity
by calling the constructor directly, rather than it being constructed by the Android system itself. I suspect that means you'll find onCreate
hasn't been called - so you've never inflated your layout. With no layout, there's nothing to find the view in.
It's not clear why you're constructing an activity yourself, but I would suggest you should try to find an alternative design which allows you to get the relevant activity for your context instead - an activity which the rest of Android "knows" about in the normal way.
See more on this question at Stackoverflow