Question

here is the logcat below, i cannot understand what a null pointer exeption means, or any of the stuff under it.

08-18 14:59:21.080: E/AndroidRuntime(13333): FATAL EXCEPTION: main
08-18 14:59:21.080: E/AndroidRuntime(13333): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{k.t.m/k.t.m.MainActivity}: java.lang.NullPointerException
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1894)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1995)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.app.ActivityThread.access$600(ActivityThread.java:128)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1161)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.os.Handler.dispatchMessage(Handler.java:99)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.os.Looper.loop(Looper.java:137)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.app.ActivityThread.main(ActivityThread.java:4514)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at java.lang.reflect.Method.invokeNative(Native Method)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at java.lang.reflect.Method.invoke(Method.java:511)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:980)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:747)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at dalvik.system.NativeStart.main(Native Method)
08-18 14:59:21.080: E/AndroidRuntime(13333): Caused by: java.lang.NullPointerException
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.app.Activity.findViewById(Activity.java:1794)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at k.t.m.MainActivity.<init>(MainActivity.java:14)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at java.lang.Class.newInstanceImpl(Native Method)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at java.lang.Class.newInstance(Class.java:1319)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.app.Instrumentation.newActivity(Instrumentation.java:1027)
08-18 14:59:21.080: E/AndroidRuntime(13333):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1885)
08-18 14:59:21.080: E/AndroidRuntime(13333):    ... 11 more

here is the main java that i have so far… Thanks to everyone that helps me out with this problem.

package k.t.m;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.support.v4.app.NavUtils;
import android.widget.*;

public class MainActivity extends Activity {

    TextView main = (TextView) findViewById(R.id.main);
    Button mainButton1 = (Button) findViewById(R.id.mainButton1);
    Button mainButton2 = (Button) findViewById(R.id.mainButton2);

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mainButton1.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                milesToKilometers();
            }

        });

        mainButton2.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                kilometersToMiles();
            }

        });

    }

    protected void kilometersToMiles() {

        main.setVisibility(View.INVISIBLE);
        mainButton1.setVisibility(View.INVISIBLE);
        mainButton2.setVisibility(View.INVISIBLE);

    }

    protected void milesToKilometers() {

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }


}

Answer 1

You are calling findViewById() before you have called setContentView(), which will not work well.