How do you call a class (that's not an activity) in an activity class in Android Studio?

Question

I'm learning Android Studio and I decided to create a Java class and then call it in MainActivity. However, the app crashes on startup - see below. I just don't understand what the error means. Any thoughts?

MainActivity.java

   package com.example.daniel.hamblaster;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;


public class MainActivity extends AppCompatActivity {

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

        generateText obj = new generateText();
        obj.generate();

    }
}

Java class:

package com.example.daniel.hamblaster;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class generateText extends AppCompatActivity {

    Button myButton = (Button) findViewById(R.id.myButton);

    public void generate() {
        myButton.setOnClickListener(
            new Button.OnClickListener() {
                public void onClick(View v) {
                    TextView myText = (TextView) findViewById(R.id.myText);
                    myText.setText("blablaba");
                }
            }
        );
    }
}

Error:

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.daniel.hamblaster, PID: 5560 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.daniel.hamblaster/com.example.daniel.hamblaster.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6119) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) at com.android.internal.os.Zygot eInit.main(ZygoteInit.java:776) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference at android.support.v7.app.AppCompatDelegateImplBase.(AppCompatDelegateImplBase.java:120) at android.support.v7.app.AppCompatDelegateImplV9.(AppCompatDelegateImplV9.java:151) at android.support.v7.app.AppCompatDelegateImplV11.(AppCompatDelegateImplV11.java:31) at android.support.v7.app.AppCompatDelegateImplV14.(AppCompatDelegateImplV14.java:55) at android.support.v7.app.AppCompatDelegateImplV23.(AppCompatDelegateImplV23.java:33) at android.support.v7.app.AppCompatDelegateImplN.(AppCompatDelegateImplN.java:33) at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:201) at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185) at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:525) at android.support.v7.app.AppCompatActivity. findViewById(AppCompatActivity.java:193) at com.example.daniel.hamblaster.generateText.(generateText.java:9) at com.example.daniel.hamblaster.MainActivity.onCreate(MainActivity.java:14) at android.app.Activity.performCreate(Activity.java:6679) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6119) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) Application terminated.

Answer 1

You are trying to make impossible stuff.

Activities are not to be created as an ordinary class. I can see that you are starting to get a grip of what Java is. Take your time and learn Java basics before running into Android.

For short: Activities are not to be instantiated with new Activity(); If you are trying to, please, use Intents.

Intent a = new Intent(this, ActivityB.class);
this.startActivity(a);

This is the way to open an activity from another.

And if you REALLY want to just instantiate a class, remove that extension from generateText class and just handle it just like a normal and ordinary class.

You should, as well, check some Java code standards :) Do never create a Class with lowercase first letter.

Best of luck.

Answer 2

1) If you are working with UI, do it in the activity you are currently in.

2) If you want to start another activity, use:

Intent intent = new Intent(ActivityA.this, ActivityB.class); startActivity(intent);

3) If you want to execute a method of another class, let it be

public static <return-type> method() {...} in that class. This way you even do not need to initialize you class (make it static too, btw).