Java 在第二个活动中,通过intent通过Parcelable从检索到的ArrayList获取ArrayList的元素
我正在尝试构建一个简单的安卓系统,它可以模拟使用卡片学习词汇。为了实现最终的应用程序,我将通过学习新事物和添加新事物,逐步增加功能和复杂性 我将告诉你我的困境,但首先,我的应用程序应该做些什么,到目前为止(我离我想让它去的地方还很远,但你不必介意) 此时,应用程序应执行以下操作:Java 在第二个活动中,通过intent通过Parcelable从检索到的ArrayList获取ArrayList的元素,java,android,arraylist,parcelable,Java,Android,Arraylist,Parcelable,我正在尝试构建一个简单的安卓系统,它可以模拟使用卡片学习词汇。为了实现最终的应用程序,我将通过学习新事物和添加新事物,逐步增加功能和复杂性 我将告诉你我的困境,但首先,我的应用程序应该做些什么,到目前为止(我离我想让它去的地方还很远,但你不必介意) 此时,应用程序应执行以下操作: 1) 在主要活动中: a) 创建的实现的3个实例的数组 可包裹接口(类VOCARD实现可包裹),VOCARD[] voc1,在这种情况下。由于类VOCARD实现了Parcelable,因此 将获得用于构建3个实例的地
- 1) 在主要活动中:
- a) 创建的实现的3个实例的数组 可包裹接口(类VOCARD实现可包裹),VOCARD[] voc1,在这种情况下。由于类VOCARD实现了Parcelable,因此 将获得用于构建3个实例的地块
- b) 创建一个名为vocCardList的VocCard类型的ArrayList并添加 voc1的所有3个元素都被添加到voc1卡列表中
- c) 创建“开始”按钮的实例,该按钮为 单击时启动名为PracticeActivity的第二个活动
- d) 将带有Parcelable的ArrayList和CardList添加到 意图
- a) 获取MainActivity创建的意图
- b) 从intent检索ArrayList和CardList
- c) 获取vocCardsList的任何元素,并指定该类型的变量 这是我的名片
- d) 通过调用其 方法
- e) 通过将TextView设置为值的字符串来显示该值 价值观
- f) 创建一个按钮nextButton,用于创建启动意图 第二个活动再次练习活动,作为某种递归
- g) 将ArrayList和CardList中的parcelable添加到intent
- h) 重复2)a)-g)直到应用程序通过关闭图标关闭
java.lang.ClassCastException:java.lang.String不能强制转换为com.undiclosed.smartcards.VocCard
使用vocCardList1.get(2):java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“java.lang.String com.unclosed.smartcards.VocCard.returnVocfeign()”
问题:
为什么我不能按预期的方式访问ArrayList的元素?当我在网上搜索的时候,我可能找错了东西
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);
}
});
}
}
package com.unclosed123.smartcards;
导入android.content.Intent;
导入android.os.packet;
导入android.os.Parcelable;
导入android.support.v7.app.AppActivity;
导入android.os.Bundle;
导入android.view.view;
导入android.widget.Button;
导入java.util.ArrayList;
导入java.util.List;
公共类MainActivity扩展了AppCompatActivity{
私人名单和信用卡名单;
私有字符串[]voc_f={“面包”、“苹果”、“水”};
私有字符串[]voc_n={“pain”、“pomme”、“eau”};
@凌驾
创建时受保护的void(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//创建卡片并添加到列表中
vocCardList=newarraylist();
VOCARD[]voc1=新的VOCARD[3];
Parcel in=Parcel.get();
对于(int i=0;i<3;i++){
voc1[i]=新的voc1卡(in);
voc1[i].setVocForeign(voc_f[i]);
voc1[i].setvocinate(voc_n[i]);
新增(voc1[i]);
}
//创建意图并分配包裹列表,以便在btn单击时发送到第二个活动
按钮startBtn=(按钮)findViewById(R.id.Button);
startBtn.setOnClickListener(新视图.OnClickListener(){
@凌驾
@抑制警告(“未选中”)
公共void onClick(视图v){
意向意向=新意向(MainActivity.this、PracticeActivity.class);
intent.putParcelableArrayListExtra(“voc1”,(ArrayList)VOCARDLIST);
getApplicationContext().startActivity(intent);
}
});
}
}
以及下面的PracticeActivity.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);
}
});
}
}
(对于那些被注释掉的大部分,我感到很抱歉,我认为这有助于传达我对该课程的进一步意图)
package com.unclosed123.smartcards;
导入android.content.Intent;
导入android.support.v7.app.AppActivity;
导入android.os.Bundle;
导入android.view.view;
导入android.widget.Button;
导入android.widget.TextView;
导入java.util.ArrayList;
公共课堂实践活动扩展了课堂实践活动{
@凌驾
创建时受保护的void(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_practice);
//获取启动此活动的意图并提取字符串
Intent=getIntent();
final ArrayList vocCardList1=intent.getParcelableArrayListExtra(“voc1”);//
//从语音卡中获取数据
//VOCARD card4count=VOCARDLIST1.get(2);
//card4count.increaseCount();
//int count=card4count.getCount();
/*如果(计数>=vocCardList1.size()){
//待办事项
//为EndPracticeActivity创建新的意图
//临时声明
计数--;
}*/
VOCARD card0=VOCARDLIST1.get(2);
//VOCARD card1=VOCARDLIST1.get(1);
字符串test1=card0.returnVocForeign();
//card0.increaseCount();
//字符串test1=“test1”;
//使文本视图显示传输的字符串
Te
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];
}
};
}
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;
}
...
...
}
@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);
}
});
}
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
...
...
final ArrayList<VocCard> vocCardList = getIntent().getParcelableArrayListExtra("voc1");
final VocCard card = vocCardList.get(2);
String test = card.getVocForeign();
...
...
}