I am taking a course on Android, I have problems with the task.

It is necessary to make a simple traffic light, by pressing a button - the background changes.

Here is the main.class:

package com.example.opimand.oneswitchsemofor; import android.support.v4.content.ContextCompat; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.view.View; import android.widget.Button; import android.widget.RelativeLayout; import android.widget.TextView; public class MainActivity extends AppCompatActivity { private final RelativeLayout mRealtiveLayout=(RelativeLayout) findViewById(R.id.activity_main); private final TextView mTextView=(TextView) findViewById(R.id.textView); Button redButoom=(Button) findViewById(R.id.buttonRed); Button yellowButtom= (Button) findViewById(R.id.buttonRed); Button greenButtom= (Button) findViewById(R.id.buttonGreen); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); } public void onClick(View view) { switch (view.getId()) { case R.id.buttonRed: mTextView.setText(R.string.red); mRealtiveLayout.setBackgroundColor(ContextCompat.getColor(MainActivity.this,R.color.colorRed)); break; case R.id.buttonYellow: mTextView.setText(R.string.yellow); mRealtiveLayout.setBackgroundColor(ContextCompat.getColor(MainActivity.this,R.color.colorYellow)); break; case R.id.buttonGreen: mTextView.setText(R.string.green); mRealtiveLayout.setBackgroundColor(ContextCompat.getColor(MainActivity.this,R.color.colorGreen)); } } }

And here is main.xml:

 <?xml version="1.0" encoding="utf-8"?> <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Hello Kitty!" android:id="@+id/textView" /> <Button android:text="@string/green" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_below="@+id/textView" android:layout_centerHorizontal="true" android:layout_marginTop="148dp" android:id="@+id/buttonGreen" android:onClick="onClick"/> <Button android:text="@string/yellow" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_below="@+id/buttonGreen" android:layout_alignRight="@+id/buttonGreen" android:layout_alignEnd="@+id/buttonGreen" android:layout_marginTop="17dp" android:id="@+id/buttonYellow" android:onClick="onClick"/> <Button android:text="@string/red" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_marginTop="13dp" android:id="@+id/buttonRed" android:onClick="onClick" android:layout_below="@+id/buttonYellow" android:layout_alignLeft="@+id/buttonYellow" android:layout_alignStart="@+id/buttonYellow" />

    2 answers 2

    A lot of errors in the code, I do not know why

    Most of the errors are due to the fact that you call the setContentView(...) class findViewById(...) method before the setContentView(...) method is setContentView(...) .

    For example, consider this line:

     Button redButoom = (Button) findViewById(R.id.buttonRed);

    Here you declare the class field redButoom and immediately try to initialize it.

    This line is executed when creating an instance of the class, at this moment the onCreate(...) method of onCreate(...) has not started yet (respectively, the setContentView(...) method has not started, as a result, an exception occurs in the findViewById(...) method , since it is not yet clear where exactly to search for elements, as setContentView(...) has not yet worked.

    You can search for some elements in the markup using the findViewById(...) method only after you have completed the setContentView(...) method, otherwise, you will get an exception.

    How to fix it - look in the next answer .

      Initialize the view in the onCreate method

       private RelativeLayout mRealtiveLayout; private TextView mTextView; Button redButoom; Button yellowButtom; Button greenButtom; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); mRealtiveLayout=(RelativeLayout) findViewById(R.id.activity_main); mTextView=(TextView) findViewById(R.id.textView); redButoom=(Button) findViewById(R.id.buttonRed); yellowButtom= (Button) findViewById(R.id.buttonRed); greenButtom= (Button) findViewById(R.id.buttonGreen); }

      You also have no root layout in the markup.

       <?xml version="1.0" encoding="utf-8"?> <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" > <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Hello Kitty!" android:id="@+id/textView" /> <Button android:text="@string/green" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_below="@+id/textView" android:layout_centerHorizontal="true" android:layout_marginTop="148dp" android:id="@+id/buttonGreen" android:onClick="onClick"/> <Button android:text="@string/yellow" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_below="@+id/buttonGreen" android:layout_alignRight="@+id/buttonGreen" android:layout_alignEnd="@+id/buttonGreen" android:layout_marginTop="17dp" android:id="@+id/buttonYellow" android:onClick="onClick"/> <Button android:text="@string/red" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_marginTop="13dp" android:id="@+id/buttonRed" android:onClick="onClick" android:layout_below="@+id/buttonYellow" android:layout_alignLeft="@+id/buttonYellow" android:layout_alignStart="@+id/buttonYellow" /> </RelativeLayout>