Question

我有一个问题，我有一个可序列化的类，可以设置和获取多个列表。我在其中添加了一个列表，但是当我想从getter方法获取该列表时，它返回NullPointerException。这就是为什么我在这里提到错误和Setter和Getter方法来解释问题以及我如何检索它。我不知道为什么并且不知道它将如何被删除？任何人都可以帮我解决这个问题吗？

错误堆栈：

09-24 13:04:18.923: ERROR/AndroidRuntime(529): FATAL EXCEPTION: main
09-24 13:04:18.923: ERROR/AndroidRuntime(529): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.equinix.android.common/com.equinix.android.showmyorders.ShowMyOrders}: java.lang.NullPointerException
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at android.app.ActivityThread.access$2300(ActivityThread.java:125)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at android.os.Looper.loop(Looper.java:123)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at android.app.ActivityThread.main(ActivityThread.java:4627)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at java.lang.reflect.Method.invokeNative(Native Method)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at java.lang.reflect.Method.invoke(Method.java:521)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at dalvik.system.NativeStart.main(Native Method)
09-24 13:04:18.923: ERROR/AndroidRuntime(529): Caused by: java.lang.NullPointerException
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at com.equinix.android.showmyorders.ShowMyOrders.onCreate(ShowMyOrders.java:87)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
09-24 13:04:18.923: ERROR/AndroidRuntime(529):     ... 11 more

设置和获取列表的类：

package com.equinix.android.model;

import java.io.Serializable;
import java.util.List;

public class BaseIBXValues implements Serializable{

    private static final long serialVersionUID = 7526472295622776147L;
    private List<String> IBXId;
    private List<String> IBXName;
    private List<String> IBXNaturalKey;

    public List<String> getIBXId()
    {
        return IBXId;
    }
    public void setIBXId(List<String> IBXId)
    {
        this.IBXId = IBXId;
    }

    public List<String> getIBXName()
    {
        return IBXName;
    }
    public void setIBXName(List<String> IBXName)
    {
        this.IBXName = IBXName;
    }

    public List<String> getIBXNaturalKey()
    {
        return IBXNaturalKey;
    }
    public void setIBXNaturalKey(List<String> IBXNaturalKey)
    {
        this.IBXNaturalKey = IBXNaturalKey;
    }
}


**Code for Setting the value of arraylist:**
public class Parse_Json {

    String json_String;
    List<String> IBXId;
    BaseIBXValues ibx;------> It is the class where setter and getter mentioned

    public Parse_Json(String json_String)
    {
        this.json_String = json_String;
    }

    public void parse()
    {
        ibx = new BaseIBXValues();
        try{
        JSONObject ja = new JSONObject(json_String); 

        JSONArray  jo = ja.getJSONArray("ibx");
        for(int i=0;i<jo.length();i++)
        {
            JSONObject j_data = jo.getJSONObject(i);
            System.out.println("The id is:"+j_data.getString("id"));
            IBXId.add(j_data.getString("id"));
            LoginScreen.id.add(j_data.getString("id"));
            System.out.println("The Name is:"+j_data.getString("name"));
            LoginScreen.name.add(j_data.getString("name"));
            System.out.println("The naturalKey is:"+j_data.getString("naturalKey"));
            LoginScreen.naturalKey.add(j_data.getString("naturalKey"));

        }
        }catch(Exception e)
        {
            e.printStackTrace();
        }
        ibx.setIBXId(IBXId);------------> set the list
    }

从Getter获取列表：

ibx = new BaseIBXValues();
        ibId = ibx.getIBXId();

        for(int i=0;i<6;i++)
        {
            System.out.println("The IBX ID:"+ibId.get(i));
        }

Answer 1

很少有事情需要注意：

在Parse_Json课程中：

List<String> IBXId;

IBXID永远不会被提升，因此它是null。

稍后当它尝试将值添加到IBXId时：

IBXId.add(j_data.getString("id"));

这将抛出NullPointerException。

但如果您的JSONArray jo = ja.getJSONArray("ibx");返回jo大小0，则该行

ibx.setIBXId(IBXId);

会将IBXId设为null

在以下代码中：

    ibId = ibx.getIBXId();    

    for(int i=0;i<6;i++)
    {
        System.out.println("The IBX ID:"+ibId.get(i));
    }

ibId为null，因为在BaseIBXValues类中，该行

 private List<String> IBXId;

没有初始化IBXId，所以它的值是null所以调用ibId.get(i)会导致NullPointerException

修改

请参阅此处修改的代码：

public class Parse_Json { String json_String; List<String> IBXId = new ArrayList<String>(); BaseIBXValues ibx;------> It is the class where setter and getter mentioned

但是当你打电话给吸气剂时

ibId = ibx.getIBXId(); for(int i=0;i<6;i++) { System.out.println("The IBX ID:"+ibId.get(i)); }

，您可以确保您具有非空值

ibId = ibx.getIBXId(); If(ibId != null) { for(String index : ibId) { System.out.println("The IBX ID:"+index); } }

<强>被修改

List<String> ibId=Collections.emptyList(); @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.showmyorders); ibx = new BaseIBXValues(); ibId = ibx.getIBXId(); if(ibId != null) { for(String index : ibId) { System.out.println("The IBX ID:"+index); } }

修改这应该是：

public class BaseIBXValues implements Serializable{ private static final long serialVersionUID = 7526472295622776147L; private List<String> IBXId = new ArrayList<String>(); // initialize to empty array of string private List<String> IBXName;

创建新的BaseIBXValues对象时，

ibx = new BaseIBXValues(); ibId = ibx.getIBXId();

ibId应该是一个空字符串数组

从类中获取List时出错

1 个答案: