【发布时间】：2021-01-10 16:57:39
【问题描述】：
我正在构建一个单位转换应用程序。我需要一个输入和两个微调器。微调器调节当前单位是什么，以及新单位是什么。下面是我的代码。我对如何使用 if 语句来比较两者感到困惑。我一直在尝试使用 .equals("lb")，但由于某种原因它只是返回 false。让我知道我可能哪里出错了。
主代码：
package com.stproductions.commonconverter;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Spinner;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity implements AdapterView.OnItemSelectedListener {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//Convert From Box
setContentView(R.layout.activity_main);
Spinner spinner1 = findViewById(R.id.spinner1);
ArrayAdapter<CharSequence>adapter1 = ArrayAdapter.createFromResource(this, R.array.fromchoices, android.R.layout.simple_spinner_item);
adapter1.setDropDownViewResource(android.R.layout.simple_spinner_item);
spinner1.setAdapter(adapter1);
spinner1.setOnItemSelectedListener(this);
//Convert To Box
Spinner spinner2 = findViewById(R.id.spinner2);
ArrayAdapter<CharSequence>adapter = ArrayAdapter.createFromResource(this, R.array.choices, android.R.layout.simple_spinner_item);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_item);
spinner2.setAdapter(adapter);
spinner2.setOnItemSelectedListener(this);
}
@Override
public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
String text = parent.getItemAtPosition(position).toString();
String text2 = parent.getItemAtPosition(position).toString();
// View text = parent.findViewById(R.id.spinner1);
// View text2 = parent.findViewById(R.id.spinner2);
// Spinner spinner1 = (Spinner) findViewById(R.id.spinner1);
// Spinner spinner2 = (Spinner) findViewById(R.id.spinner2);
if(text.equals("lb") && text2.equals("oz")){
Toast.makeText(this,"hello", Toast.LENGTH_SHORT).show();
}
}
@Override
public void onNothingSelected(AdapterView<?> parent) {
}
}
字符串.XML：
<resources>
<string name="app_name">Common Converter</string>
<string-array name="choices">
<item>lb</item>
<item>oz</item>
<item>g</item>
<item>kg</item>
<item>ton</item>
<item>mg</item>
</string-array>
<string-array name="fromchoices">
<item>lb</item>
<item>oz</item>
<item>g</item>
<item>kg</item>
<item>ton</item>
<item>mg</item>
</string-array>
</resources>
Activity_Main.xml：
<?xml version="1.0" encoding="utf-8"?>
<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"
android:background="#FFFFFF"
tools:context=".MainActivity">
<Spinner
android:id="@+id/spinner2"
android:layout_width="97dp"
android:layout_height="59dp"
android:background="#E6E6E6"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.831"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.595" />
<Spinner
android:id="@+id/spinner1"
android:layout_width="97dp"
android:layout_height="59dp"
android:background="#E6E6E6"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.831"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.309" />
</androidx.constraintlayout.widget.ConstraintLayout>
我现在在我的 logcat 中得到以下内容：
2021-01-06 14:48:00.985 18954-18954/com.stproductions.commonconverter E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.stproductions.commonconverter, PID: 18954
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.view.View.toString()' on a null object reference
at com.stproductions.commonconverter.MainActivity.onItemSelected(MainActivity.java:39)
at android.widget.AdapterView.fireOnSelected(AdapterView.java:957)
at android.widget.AdapterView.dispatchOnItemSelected(AdapterView.java:946)
at android.widget.AdapterView.access$300(AdapterView.java:55)
at android.widget.AdapterView$SelectionNotifier.run(AdapterView.java:910)
at android.os.Handler.handleCallback(Handler.java:938)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:223)
at android.app.ActivityThread.main(ActivityThread.java:7656)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
【问题讨论】：
标签： java android if-statement spinner equals