Logs:

2019-04-01 19:18:07.458 5768-5768/com.example.car E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.car, PID: 5768 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.car/com.example.car.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) at android.app.ActivityThread.-wrap12(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6077) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at com.example.car.MainActivity.onCreate(MainActivity.java:57) at android.app.Activity.performCreate(Activity.java:6662) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)

Code:

  MainActivity: public class MainActivity extends AppCompatActivity { private Button Act_Change, btn; private EditText pass; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); Toolbar toolbar = findViewById(R.id.toolbar); setSupportActionBar(toolbar); FloatingActionButton fab = findViewById(R.id.fab); fab.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View view) { Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG) .setAction("Action", null).show(); } public void AddListenerOnButton () { Act_Change = (Button)findViewById(R.id.Act_Change); btn = (Button)findViewById(R.id.btn); btn.setOnClickListener( new View.OnClickListener() { @Override public void onClick(View v) { btn.setText("DONE"); btn.setBackgroundTintList(ColorStateList.valueOf(Color.RED)); Toast.makeText( MainActivity.this, pass.getText(), Toast.LENGTH_SHORT ).show(); } } ); } }); { Act_Change.setOnClickListener( new View.OnClickListener() { @Override public void onClick(View v) { Intent intent = new Intent("MainActivity2"); startActivity(intent); } }); } } @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_main, 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); } }

    1 answer 1

    In the onCreate () method, you did not initialize your buttons, hence the NullPointerException. You initialize them in the wrong place, in an anonymous class, create a method to initialize them and do not call it. Move the code to the onCreate method:

     Act_change = findViewById( id вашей кнопки ); btn = findViewById(id кнопки);

    Or take this method from an anonymous class into your Activity:

      public void AddListenerOnButton () { Act_Change = (Button)findViewById(R.id.Act_Change); btn = (Button)findViewById(R.id.btn); btn.setOnClickListener( new View.OnClickListener() { @Override public void onClick(View v) { btn.setText("DONE"); btn.setBackgroundTintList(ColorStateList.valueOf(Color.RED)); Toast.makeText( MainActivity.this, pass.getText(), Toast.LENGTH_SHORT ).show(); } } ); }

    And onCreate call it.