我正在使用Android Studio，我试图制作这个＆＃34;化学计算器＆＃34;应用程序，我收到此错误。基本上，一旦你进入应用程序，你有几个选项，你想要做什么样的计算。如果你点击＆＃34;摩尔浓度＆＃34;，你会得到2个选项。单击类名"MolarConcentration2"下的选项会停止应用，然后崩溃...与第29行有关，即calcBtnBTN.setOnClickListener(new View.OnClickListener() {。我在那里有一个onclicklistener，你将在下面看到。事实上，对于你可以选择的另一个选项而言，我的字面意思完全相同（名称改变了一点）。
这是第二个活动页面，它提供了2个选项。 MolarConcentration类工作正常，它的MolarConcentration2类给出了问题
package com.example.swapn.chemistrycalculator;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
public class iHaveForMolarConcentration extends AppCompatActivity {
Button haveMMV;
Button haveMV;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_i_have_for_molar_concentration);
haveMMV = (Button) findViewById(R.id.haveMassAndGMMandV);
haveMV = (Button) findViewById(R.id.haveMandV);
haveMMV.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
openMMV();
}
});
haveMV.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
openMC();
}
});
}
private void openMC() {
Intent goToMolarConcentration = new Intent(iHaveForMolarConcentration.this, MolarConcentration.class);
startActivity(goToMolarConcentration);
}
private void openMMV() {
Intent goToMolarConcentration2 = new Intent(iHaveForMolarConcentration.this, MolarConcentration2.class);
startActivity(goToMolarConcentration2);
}
}
这是logcat：
03-13 02:00:28.535 21718-21718/com.example.swapn.chemistrycalculator E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.swapn.chemistrycalculator, PID: 21718
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.swapn.chemistrycalculator/com.example.swapn.chemistrycalculator.MolarConcentration2}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2947)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3008)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1650)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6688)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.swapn.chemistrycalculator.MolarConcentration2.onCreate(MolarConcentration2.java:29)
at android.app.Activity.performCreate(Activity.java:6912)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1126)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2900)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3008)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1650)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6688)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)
最后，这里是类MolarConcentration2，它有问题@第29行：
package com.example.swapn.chemistrycalculator;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class MolarConcentration2 extends AppCompatActivity {
EditText massET;
EditText volumeET;
EditText gmmET;
Button calcBtnBTN;
EditText theAnswer;
@Override
protected void onCreate(Bundle savedInstanceState) {
massET = findViewById(R.id.mass);
volumeET = findViewById(R.id.volume);
gmmET = findViewById(R.id.molecularWeight);
calcBtnBTN = findViewById(R.id.calcBtn);
theAnswer = findViewById(R.id.theAnswer);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_molar_concentration2);
calcBtnBTN.setOnClickListener(new View.OnClickListener() {<--THIS LINE HERE
@Override
public void onClick(View v) {
float m = Float.parseFloat(massET.getText().toString());
float volume = Float.parseFloat(volumeET.getText().toString());
float molcWt = Float.parseFloat(gmmET.getText().toString());
float mols = m / molcWt;
float nDivV = mols / volume;
theAnswer.setText(Float.toString(nDivV));
}
});
}
}
以下是MolarConcentration2类的XML文件：
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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.example.swapn.chemistrycalculator.MolarConcentration2">
<EditText
android:id="@+id/mass"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:ems="10"
android:hint="Mass (g)"
android:inputType="numberDecimal"
android:textAlignment="center"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.503"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.24" />
<EditText
android:id="@+id/molecularWeight"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:ems="10"
android:hint="Molecular Weight (GMM)"
android:inputType="numberDecimal"
android:textAlignment="center"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.503"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.342" />
<EditText
android:id="@+id/volume"
android:layout_width="wrap_content"
android:layout_height="48dp"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:ems="10"
android:hint="Volume (L)"
android:inputType="numberDecimal"
android:textAlignment="center"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.503"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.447" />
<Button
android:id="@+id/calcBtn"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:text="Find Concentration"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.502"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.554" />
<EditText
android:id="@+id/theAnswer"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:ems="10"
android:hint="Concentration (mol/L)"
android:inputType="numberDecimal"
android:textAlignment="center"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.503"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.659" />
</android.support.constraint.ConstraintLayout>
感谢您的帮助