空指針異常Java

Question

我剛剛開始學習Java，並遇到了一個問題。 嘗試運行Android應用程序時，調用方法makeResponse時會出現NullPointerException。

代碼提取（本帖末尾附有完整代碼）：

private String makeResponse(String input){
    //This doesn't work yet.  I keep getting null pointer exceptions on the line beginning "int id" line, but can't tell why.
    String response = "You picked "+input;
    if (sw==null){
        response+=" and sw is null"; //This doesn't activate
    }
    if (input==null){
        response+=" and input is null"; //This doesn't activate
    }
    int id = sw.getIdFromName(input); //If this line (and the following one) are commented out, the method runs with no problem, but neither of the if clauses above trigger.
    response+=", that has id "+String.valueOf(id);
    return response;
}

（sw是父類的一個字段，在另一個方法中設置.sw是一個自制類的實例 - 最后的完整代碼）

從“int id =”開始的行拋出異常

我最初搜索NullPointerException告訴我，當應用程序在需要對象的情況下嘗試使用null時，它會“拋出”。 - 因此我上面的代碼中的兩個“if”子句，試圖找到哪個對象意外為null。 由於這兩者都不是，我得出結論，sw.getIdFromName必須返回一個Integer類型的null（就像在這個類似的問題中： Java：取消裝箱整數時的空指針異常？ ）。 但是，我沒有看到如何在sw.getIdFromName中實現這一點，如下所示（nameLookup是一個String數組，一個sw字段）：

public int getIdFromName(String name){
    for (int i=0;i<267;i++){
        if (nameLookup[i].equals(name)){
            return i;
        }
    }
    return -1;
}

（順便說一下，如果有一種更好的方法來搜索字符串數組中的搜索詞，我會很高興有人能告訴我 - binarySearch似乎沒有在字符串數組中定義）。

根據上面鏈接的問題中的頂級評論者的建議，我嘗試在makeResponse中用“Integer id”替換“int id”，但沒有效果 - 在同一個地方拋出相同的異常。

任何建議將不勝感激。

從上面鏈接的stackoverflow問題中的注釋來看，提供堆棧跟蹤不會提供任何新信息，但如果被問到，我會很樂意這樣做。

Ps這是我在這里的第一個問題，如果我犯了禮儀或者錯誤的話，請道歉。

完整代碼清單：

ConversationActivity.java：

package com.example.Conversation;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.AutoCompleteTextView;
import android.widget.TextView;

public class ConversationActivity extends Activity {
/** Called when the activity is first created. */
StationsWrapper sw;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    /**Setting the adapter for the AutoComplete*/
    final AutoCompleteTextView textView = (AutoCompleteTextView) findViewById(R.id.ACTextView1);
    String[] stationsArray = getResources().getStringArray(R.array.stations);
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, R.layout.list_item, stationsArray);
    textView.setAdapter(adapter);

    /**Code below grabs the data from stations.xml and puts it in a readable object */
    this.sw = new StationsWrapper();

    /** Code below is to set a click function for the AutoComplete*/
    OnItemClickListener ACListener = new OnItemClickListener(){

        public void onItemClick(AdapterView<?> parent, View v, int position,
                long id) {
            TextView reply = (TextView) findViewById(R.id.reply);
            reply.setText("working...");
            String ChosenStation = (String) parent.getItemAtPosition(position);
            reply.setText(makeResponse(ChosenStation));
            //Toast.makeText(ConversationActivity.this, "You clicked "+parent.getItemAtPosition(position), Toast.LENGTH_SHORT).show();
            textView.setText("");

        }
    };
    textView.setOnItemClickListener(ACListener);

}

private String makeResponse(String input){
    //This doesn't work yet.  I keep getting null pointer exceptions on the line beginning "int id" line, but can't tell why.
    String response = "You picked "+input;
    if (sw==null){
        response+=" and sw is null"; //This doesn't activate
    }
    if (input==null){
        response+=" and input is null"; //This doesn't activate
    }
    int id = sw.getIdFromName(input); //If this line (and the following one) are commented out, the method runs with no problem, but neither of the if clauses above trigger.
    response+=", that has id "+String.valueOf(id);
    return response;
}

}

StationsWrapper.java：

package com.example.Conversation;


import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

public class StationsWrapper {

    private int[][] stats;
    private String[] nameLookup;

    public StationsWrapper(){
        //Constructor.  Grabs data from XML, and whacks it into relevant arrays.
        //stats is an integer array, indexed first by station id (1-267), and then by datatype (0 for line, 1 for zone) 
        final int[][] stats = new int[267][2];
        final String[] nameLookup = new String[267];

        try {

            SAXParserFactory factory = SAXParserFactory.newInstance();
            SAXParser saxParser = factory.newSAXParser();

            DefaultHandler handler = new DefaultHandler() {

                boolean bline = false;
                boolean bzone= false;
                String curStation;
                int curID;
                String curLine;
                String curZone;

                public void startElement(String uri, String localName,String qName, 
                        Attributes attributes) throws SAXException {

                    if (qName.equalsIgnoreCase("STATION")){
                        curStation=attributes.getValue(0);
                        curID=Integer.parseInt(attributes.getValue(1));
                    }

                    if (qName.equalsIgnoreCase("LINE")) {
                        bline = true;
                    }

                    if (qName.equalsIgnoreCase("ZONE")) {
                        bzone = true;
                    } 
                }

                public void endElement(String uri, String localName,
                        String qName) throws SAXException {
                    if (qName.equalsIgnoreCase("Station")){
                        nameLookup[curID-1]=curStation;
                        int intLine=(convLineToInt(curLine));
                        stats[curID-1][0]=intLine;
                        int intZone=(convZoneToInt(curZone));
                        stats[curID-1][1]=intZone;
                    }
                }

                public void characters(char ch[], int start, int length) throws SAXException {

                    if (bline) {
                        //System.out.println("Line : " + new String(ch, start, length));
                        curLine=new String(ch, start, length);
                        bline = false;
                    }

                    if (bzone) {
                        //System.out.println("Zone : " + new String(ch, start, length));
                        curZone=new String(ch, start, length);
                        bzone = false;
                    } 
                }

            };

            saxParser.parse("c:\\Users\\Jack Jackson\\Coding\\Java\\stations.xml", handler);

        } catch (Exception e) {
            e.printStackTrace();
        }

        this.stats=stats;
        this.nameLookup=nameLookup;

    }

    public static void main(String[] args){
        //Nothing to see here, move it along folks.
    }

    public String[] getNameLookup(){
        return nameLookup;
    }

    public int getIdFromName(String name){
        for (int i=0;i<nameLookup.length;i++){
            if (nameLookup[i].equals(name)){
                return i;
            }
        }
        return -1;
    }

    public int returnData(int id, int datapoint){
        return stats[id][datapoint];
    }

    public void displayStats(){
        for (int i=0;i<267;i++){
            for (int j=0;j<2;j++){
                System.out.print(stats[i][j]);
                System.out.print(" ");
            }
            System.out.println("");
        }
    }
   }

Answer 1

在不運行代碼的情況下， nameLookup數組之一似乎很可能為null ，因此嘗試調用nameLookup[i].equals()會拋出NullPointerException 。

如果nameLookup元素可以合法地為null ，則處理此問題的一種方法是在getIdFromName()反轉比較的順序：

if (name.equals(nameLookup[i])) {

無論如何，我建議您確保nameLookup本身及其元素都已完全初始化。