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[英]NullPointerException when sending nested ArrayList via Parcelable
[英]Get elements of ArrayList from retrieved ArrayList via intent via Parcelable in 2nd activity
我正在尝试构建一个简单的机器人,该机器人应该模拟卡片的学习词汇。 为了进入最终的应用程序,我将通过学习新事物并添加新事物来逐步添加功能和复杂性。
我要告诉你我被卡住的地方,但是首先,这是我的应用程序应该做的事情,到目前为止(我距离我想要去的地方还很远,但是你没有请注意。):
此时,应用应执行以下操作:
a)在这种情况下,创建一个由Parcelable接口实现的3个实例组成的数组(类VocCard实现Parcelable),即VocCard [] voc1。 由于VocCard类实现了Parcelable,因此获得了用于构建3个实例的Parcel。
我目前停留在2)c)的范围内 ,因为该应用程序只能按上述索引0的方式工作。仅VocCard card0 = vocCardList1.get(0); 作品,vocCardList1.get(1)或vocCardList1.get(2); 不会,尽管1和2在ArrayList边界之内。
奇怪的是, 使用索引1和索引2的Runtime Exeption消息并不相同:
使用vocCardList1.get(1): java.lang.ClassCastException: java.lang.String cannot be cast to com.undiclosed.smartcards.VocCard
与vocCardList1.get(2): java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.undisclosed.smartcards.VocCard.returnVocForeign()' on a null object reference
为什么我不能按我期望的方式访问ArrayList的元素? 当我在网上搜索时,我可能正在寻找错误的内容。
MainActivity.java:
package com.undisclosed123.smartcards;
import android.content.Intent;
import android.os.Parcel;
import android.os.Parcelable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import java.util.ArrayList;
import java.util.List;
public class MainActivity extends AppCompatActivity {
private List<VocCard> vocCardList;
private String[] voc_f = {"bread","apple","water"};
private String[] voc_n = {"pain","pomme","eau"};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//Create VocCards and add to List
vocCardList = new ArrayList<VocCard>();
VocCard[] voc1 = new VocCard[3];
Parcel in = Parcel.obtain();
for(int i = 0; i < 3; i++){
voc1[i] = new VocCard(in);
voc1[i].setVocForeign(voc_f[i]);
voc1[i].setVocNative(voc_n[i]);
vocCardList.add(voc1[i]);
}
// Create Intent and assign the parcelable List for sending to second activity on btn click
Button startBtn = (Button) findViewById(R.id.button);
startBtn.setOnClickListener(new View.OnClickListener() {
@Override
@SuppressWarnings("unchecked")
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, PracticeActivity.class);
intent.putParcelableArrayListExtra("voc1",(ArrayList)vocCardList);
getApplicationContext().startActivity(intent);
}
});
}
}
下面是PracticeActivity.java:
(很抱歉,大部分内容已被注释掉,我认为这可以帮助传达我对该课程的进一步打算)
package com.undisclosed123.smartcards;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import java.util.ArrayList;
public class PracticeActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_practice);
// Get the Intent that started this activity and extract the string
Intent intent = getIntent();
final ArrayList<VocCard> vocCardList1 = intent.getParcelableArrayListExtra("voc1"); //
//Get the Data from the VocCards
//VocCard card4count = vocCardList1.get(2);
// card4count.increaseCount();
//int count = card4count.getCount();
/* if(count >= vocCardList1.size()){
// TODO
//Create new intent for EndPracticeActivity
//makeshift statement
count--;
}*/
VocCard card0 = vocCardList1.get(2);
// VocCard card1 = vocCardList1.get(1);
String test1 = card0.returnVocForeign();
// card0.increaseCount();
// String test1 = "test1";
//Make a TextView display the transfered String
TextView textView = findViewById(R.id.textView);
textView.setText(test1);
//Create another intent that recalls same activity recursively
Button nextBtn = (Button) findViewById(R.id.button2);
nextBtn.setOnClickListener(new View.OnClickListener() {
@Override
@SuppressWarnings("unchecked")
public void onClick(View v) {
Intent intent = new Intent(PracticeActivity.this, PracticeActivity.class);
intent.putParcelableArrayListExtra("voc1",(ArrayList)vocCardList1);
getApplicationContext().startActivity(intent);
}
}); /**/
}
}
最后,VocCard.java:
package com.undisclosed123.smartcards;
import android.os.Parcel;
import android.os.Parcelable;
public class VocCard implements Parcelable {
private String voc_foreign;
private String voc_native;
private boolean learned;
private int error_level;
private static int counter;
public String returnVocForeign(){
return voc_foreign;
}
public void setVocForeign(String voc_f){
voc_foreign = voc_f;
}
public String returnVocNative(){
return voc_native;
}
public void setVocNative(String voc_n){
voc_native = voc_n;
}
public boolean checkLearned(){
return learned;
}
public int getErrorLevel(){
return error_level;
}
public void makeLearned(){
learned = true;
}
public void increaseErrorLevel(){
error_level++;
}
public int getCount(){
return counter;
}
public void increaseCount(){
counter++;
}
public VocCard(Parcel in) {
voc_foreign = in.readString();
voc_native = in.readString();
}
@Override
public void writeToParcel(Parcel dest, int flags) {
dest.writeString(voc_foreign);
dest.writeString(voc_native);
dest.writeInt((Boolean) learned ? 1 : 0);
dest.writeInt(error_level);
}
@Override
public int describeContents() {
return 0;
}
public static final Creator<VocCard> CREATOR = new Creator<VocCard>() {
@Override
public VocCard createFromParcel(Parcel in) {
return new VocCard(in);
}
@Override
public VocCard[] newArray(int size) {
return new VocCard[size];
}
};
}
问题在于将数据写入Parcel
并从中读取数据。
public class VocCard implements Parcelable {
...
...
@Override
public void writeToParcel(Parcel dest, int flags) {
dest.writeString(voc_foreign);
dest.writeString(voc_native);
dest.writeInt(learned ? 1 : 0);
dest.writeInt(error_level);
}
/**
* This constructor is invoked by the method
* createFromParcel(Parcel source) of the object CREATOR.
*
* The order and number of writing and reading data to and from
* Parcel should be same
**/
private VocCard(Parcel in) {
voc_foreign = in.readString();
voc_native = in.readString();
learned = in.readInt() == 1;
error_level = in.readInt();
}
/**
* A constructor that initializes the VocCard object.
**/
VocCard(String voc_foreign, String voc_native) {
this.voc_foreign = voc_foreign;
this.voc_native = voc_native;
}
...
...
}
onCreate
内MainActivity
一些更改
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
...
...
//Create VocCards and add to List
mVocCardList = new ArrayList<>(3);
for (int i = 0; i < 3; i++) {
mVocCardList.add(new VocCard(voc_f[i], voc_n[i]));
}
Button startBtn = (Button) findViewById(R.id.button);
startBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent intent = new Intent(MainActivity.this, PracticeActivity.class);
intent.putParcelableArrayListExtra("voc1", (ArrayList<? extends Parcelable>) mVocCardList);
startActivity(intent);
}
});
}
现在在PracticeActivity
获取VocCard
列表
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
...
...
final ArrayList<VocCard> vocCardList = getIntent().getParcelableArrayListExtra("voc1");
final VocCard card = vocCardList.get(2);
String test = card.getVocForeign();
...
...
}
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