多个 if 语句的意图
Intent with multiple if statements
我正在学习 Intents 的概念，当我尝试从不是“Arabica”意图的任何其他意图出发时，我正在处理的应用程序崩溃了，意图对象在微调器上的 onItemSelectedListener 上触发。我想这样做，以便当用户在微调器上选择其他选项时，它将执行该意图并转到另一个 activity，它显示我的咖啡应用程序的文本代码。
代码如下：
public class MainActivity extends AppCompatActivity {
private Spinner spinner;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
spinner = findViewById(R.id.List);
List<String> categories = new ArrayList<>();
categories.add(0, "Choose Category");
categories.add("Arabica");
categories.add("Liberica");
categories.add("Robusta");
categories.add("Excelsa");
ArrayAdapter<String> dataAdapter;
dataAdapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_item, categories);
dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(dataAdapter);
spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
if (parent.getItemAtPosition(position).equals("Choose Category"))
{
}
else
{
String item = parent.getItemAtPosition(position).toString();
Toast.makeText(parent.getContext(), "Selected: " +item, Toast.LENGTH_SHORT).show();
if (parent.getItemAtPosition(position).equals("Arabica")) {
Intent intent = new Intent(MainActivity.this, Arabica.class);
startActivity(intent);
} else if (parent.getItemAtPosition(position).equals("Liberica")) {
Intent intent = new Intent(MainActivity.this, Liberica.class);
startActivity(intent);
} else if (parent.getItemAtPosition(position).equals("Robusta")) {
Intent intent = new Intent(MainActivity.this, Robusta.class);
startActivity(intent);
} else if (parent.getItemAtPosition(position).equals("Excelsa")) {
Intent intent = new Intent(MainActivity.this, Excelsa.class);
startActivity(intent);
}
}
}
@Override
public void onNothingSelected(AdapterView<?> parent) {
}
});
}
}
这里是 xml:
<TextView
android:id="@+id/MainTitle"
android:layout_width="199dp"
android:layout_height="143dp"
android:layout_marginStart="16dp"
android:text="Selecciona un nombre cientifico para ver su descripcion"
android:textAppearance="@style/TextAppearance.AppCompat.Body1"
android:textSize="24sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintHorizontal_bias="0.047"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.022" />
<Spinner
android:id="@+id/List"
android:layout_width="268dp"
android:layout_height="48dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.559"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.499" />
<Button
android:id="@+id/Button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="60dp"
android:text="Caracteristicas del Cafe"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/List" />
这也是我的清单：
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/Theme.CoffeeAssessor">
<activity
android:name=".Excelsa"
android:exported="false" />
<activity
android:name=".Liberica"
android:exported="false" />
<activity
android:name=".Robusta"
android:exported="false" />
<activity
android:name=".Arabica"
android:exported="false" />
<activity
android:name=".MainActivity"
android:exported="true">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
最后是错误日志：
2021-11-01 12:52:48.840 10365-10365/com.revolution.coffeeassessor E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.revolution.coffeeassessor, PID: 10365
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.revolution.coffeeassessor/com.revolution.coffeeassessor.Liberica}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.revolution.coffeeassessor.Liberica.onCreate(Liberica.java:21)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
我没有为按钮实现 setOnCLickListener，但我不认为这是我的 onCreate 的问题，尽管我使用了所有 Liberica 代码：
public class Liberica extends AppCompatActivity {
Button back2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
back2 = findViewById(R.id.back);
back2.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(Liberica.this, MainActivity.class);
startActivity(intent);
}
});
}
}
和我的 xml:
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.revolution.coffeeassessor.Liberica">
<TextView
android:id="@+id/LibericaTitle"
android:layout_width="219dp"
android:layout_height="74dp"
android:layout_marginStart="16dp"
android:layout_marginTop="16dp"
android:text="Libérica (Coffea Libérica)"
android:textAppearance="@style/TextAppearance.AppCompat.Body1"
android:textSize="24sp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/LibericaDescription"
android:layout_width="321dp"
android:layout_height="533dp"
android:text=" Esta variedad es originaria de Monrovia (Liberia), de donde toma su nombre. El aroma y sabor de este grano de café es muy peculiar, por lo que su consumo está muy poco extendido, se centra principalmente en los países Escandinavos. "
android:textAppearance="@style/TextAppearance.AppCompat.Body1"
android:textSize="24sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<Button
android:id="@+id/back2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginBottom="16dp"
android:text="Back"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintStart_toStartOf="parent" />
原因是打错了。在 Liberica Class 中更改此行：
back2 = findViewById(R.id.back);
为此：
back2 = findViewById(R.id.back2);
因为按钮的实际 id 是 back2 而不是 back 如您所见：
<Button
android:id="@+id/back2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginBottom="16dp"
android:text="Back"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintStart_toStartOf="parent" />
还要确保 setContentView(R.layout.activity_main); 在 Liberica 中 Class 指向 Liberica XML 布局文件，例如如果它被调用 activity_liberica.xml 将其设置为：
setContentView(R.layout.activity_liberica);
我正在学习 Intents 的概念，当我尝试从不是“Arabica”意图的任何其他意图出发时，我正在处理的应用程序崩溃了，意图对象在微调器上的 onItemSelectedListener 上触发。我想这样做，以便当用户在微调器上选择其他选项时，它将执行该意图并转到另一个 activity，它显示我的咖啡应用程序的文本代码。
代码如下：
public class MainActivity extends AppCompatActivity {
private Spinner spinner;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
spinner = findViewById(R.id.List);
List<String> categories = new ArrayList<>();
categories.add(0, "Choose Category");
categories.add("Arabica");
categories.add("Liberica");
categories.add("Robusta");
categories.add("Excelsa");
ArrayAdapter<String> dataAdapter;
dataAdapter = new ArrayAdapter<>(this, android.R.layout.simple_spinner_item, categories);
dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(dataAdapter);
spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
if (parent.getItemAtPosition(position).equals("Choose Category"))
{
}
else
{
String item = parent.getItemAtPosition(position).toString();
Toast.makeText(parent.getContext(), "Selected: " +item, Toast.LENGTH_SHORT).show();
if (parent.getItemAtPosition(position).equals("Arabica")) {
Intent intent = new Intent(MainActivity.this, Arabica.class);
startActivity(intent);
} else if (parent.getItemAtPosition(position).equals("Liberica")) {
Intent intent = new Intent(MainActivity.this, Liberica.class);
startActivity(intent);
} else if (parent.getItemAtPosition(position).equals("Robusta")) {
Intent intent = new Intent(MainActivity.this, Robusta.class);
startActivity(intent);
} else if (parent.getItemAtPosition(position).equals("Excelsa")) {
Intent intent = new Intent(MainActivity.this, Excelsa.class);
startActivity(intent);
}
}
}
@Override
public void onNothingSelected(AdapterView<?> parent) {
}
});
}
}
这里是 xml:
<TextView
android:id="@+id/MainTitle"
android:layout_width="199dp"
android:layout_height="143dp"
android:layout_marginStart="16dp"
android:text="Selecciona un nombre cientifico para ver su descripcion"
android:textAppearance="@style/TextAppearance.AppCompat.Body1"
android:textSize="24sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintHorizontal_bias="0.047"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.022" />
<Spinner
android:id="@+id/List"
android:layout_width="268dp"
android:layout_height="48dp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.559"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.499" />
<Button
android:id="@+id/Button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="60dp"
android:text="Caracteristicas del Cafe"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@+id/List" />
这也是我的清单：
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/Theme.CoffeeAssessor">
<activity
android:name=".Excelsa"
android:exported="false" />
<activity
android:name=".Liberica"
android:exported="false" />
<activity
android:name=".Robusta"
android:exported="false" />
<activity
android:name=".Arabica"
android:exported="false" />
<activity
android:name=".MainActivity"
android:exported="true">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
最后是错误日志：
2021-11-01 12:52:48.840 10365-10365/com.revolution.coffeeassessor E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.revolution.coffeeassessor, PID: 10365
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.revolution.coffeeassessor/com.revolution.coffeeassessor.Liberica}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.revolution.coffeeassessor.Liberica.onCreate(Liberica.java:21)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
我没有为按钮实现 setOnCLickListener，但我不认为这是我的 onCreate 的问题，尽管我使用了所有 Liberica 代码：
public class Liberica extends AppCompatActivity {
Button back2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
back2 = findViewById(R.id.back);
back2.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(Liberica.this, MainActivity.class);
startActivity(intent);
}
});
}
}
和我的 xml:
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.revolution.coffeeassessor.Liberica">
<TextView
android:id="@+id/LibericaTitle"
android:layout_width="219dp"
android:layout_height="74dp"
android:layout_marginStart="16dp"
android:layout_marginTop="16dp"
android:text="Libérica (Coffea Libérica)"
android:textAppearance="@style/TextAppearance.AppCompat.Body1"
android:textSize="24sp"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:id="@+id/LibericaDescription"
android:layout_width="321dp"
android:layout_height="533dp"
android:text=" Esta variedad es originaria de Monrovia (Liberia), de donde toma su nombre. El aroma y sabor de este grano de café es muy peculiar, por lo que su consumo está muy poco extendido, se centra principalmente en los países Escandinavos. "
android:textAppearance="@style/TextAppearance.AppCompat.Body1"
android:textSize="24sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<Button
android:id="@+id/back2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginBottom="16dp"
android:text="Back"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintStart_toStartOf="parent" />
原因是打错了。在 Liberica Class 中更改此行：
back2 = findViewById(R.id.back);
为此：
back2 = findViewById(R.id.back2);
因为按钮的实际 id 是 back2 而不是 back 如您所见：
<Button
android:id="@+id/back2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="16dp"
android:layout_marginBottom="16dp"
android:text="Back"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintStart_toStartOf="parent" />
还要确保 setContentView(R.layout.activity_main); 在 Liberica 中 Class 指向 Liberica XML 布局文件，例如如果它被调用 activity_liberica.xml 将其设置为：
setContentView(R.layout.activity_liberica);