I have a problem with this code: every time I put it:
'Button botao = (Button) findViewById(R.id.botao_id);'
It stops working, it does not run on my phone anymore. And if I take that code (and things related to it, it works again). I asked my friend to test his machine and it works perfectly. I tried to read something related to NullPointerException, but I could not solve my problem ...
Java CODE:
package com.example.android.justjava;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import java.text.NumberFormat;
public class MainActivity extends AppCompatActivity {
int quantity = 0;
Button botao = (Button) findViewById(R.id.botao_id);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
public void increment(View view) {
quantity = quantity + 1;
display(quantity);
}
public void minus(View view) {
if(quantidade>0) {
quantidade -= 1;
display();
}
if(quantidade==0){
botao.setClickable(false);
botao.setTextColor(Color.rgb(153,153,153));
}
}
public void submitOrder(View view) {
displayPrice(quantity * 5);
}
private void display(int number) {
TextView quantityTextView = (TextView) findViewById(R.id.quantity_text_view);
quantityTextView.setText("" + number);
}
private void displayPrice(int number) {
TextView priceTextView = (TextView) findViewById(R.id.price_text_view);
priceTextView.setText(NumberFormat.getCurrencyInstance().format(number));
}
}
XML CODE:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/coffee"
android:gravity="center"
android:orientation="vertical"
tools:context="com.example.android.justjava.MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="center"
android:text="Compre Seu Café"
android:textColor="#fff"
android:textSize="50sp" />
<TextView
android:id="@+id/textView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginBottom="16dp"
android:gravity="center"
android:text="Quantidade"
android:textAllCaps="true"
android:textColor="#fff"
android:textSize="30sp" />
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:orientation="horizontal">
<Button
android:id="@+id/botao_id"
android:layout_width="50dp"
android:layout_height="wrap_content"
android:layout_marginRight="8dp"
android:clickable="false"
android:onClick="decrement"
android:text="-"
android:textSize="30sp" />
<TextView
android:id="@+id/quantity_text_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="0"
android:textColor="#fff"
android:textSize="30sp" />
<Button
android:layout_width="50dp"
android:layout_height="wrap_content"
android:layout_marginLeft="8dp"
android:onClick="increment"
android:text="+"
android:textSize="30sp" />
</LinearLayout>
<TextView
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginBottom="16sp"
android:layout_marginTop="16dp"
android:gravity="center"
android:text="Preço"
android:textColor="#fff"
android:textSize="30sp" />
<TextView
android:id="@+id/price_text_view"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:text="R$0,00"
android:textColor="#fff"
android:textSize="30sp" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="16dp"
android:onClick="submitOrder"
android:text="Pedir"
android:textSize="30sp" />
Errors found on Android Monitor
07-22 15:45:46.260 18338-18338/com.example.android.justjava E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.justjava, PID: 18338
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.justjava/com.example.android.justjava.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2561)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2724)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1473)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6123)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:120)
at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:155)
at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:31)
at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:55)
at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:33)
at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:33)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:201)
at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:185)
at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519)
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
at com.example.android.justjava.MainActivity.<init>(MainActivity.java:17)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1100)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2551)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2724)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1473)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6123)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)