Question

我遇到空指针异常的问题。请说明一些一般情况。

这是我的代码：

public class GuessActivity extends ListActivity implements OnClickListener
{
EditText GNum;
Button nxt;
int[] a = new int[4];
int[] d = new int[4];
int t = 0;
ArrayList<HashMap<String, String>> output = new ArrayList<HashMap<String, String>>();
HashMap<String, String> out = new HashMap<String, String>();
String KEY_NUM , KEY_BULLS , KEY_COWS;


protected void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.guess_activity);

    GNum = (EditText) findViewById(R.id.etGuessNum);
    nxt = (Button) findViewById(R.id.btNxt);
    a = getIntent().getExtras().getIntArray("num");

    nxt.setOnClickListener(this);
}


@Override
public void onClick(View v) 
{
    if(v.getId() == R.id.btNxt)
    {
        String gN = GNum.getText().toString();
        int l = gN.length();
        int b =0;
        int c =0;
        if(l!=4)
        {
            Toast.makeText(getApplicationContext(),"Number should be of 4 digits",
                    Toast.LENGTH_LONG).show();
        }
        else
        {
            t++;
            int x = Integer.parseInt(gN);

            for(int i=0;i<4;i++)
            {
                d[i] = x%10;
                x = x/10;
            }
            if(t>8)
                Toast.makeText(getApplicationContext(), "You Lost the Game",
                        Toast.LENGTH_SHORT).show();
            else
            {

                if(d[0] == a[0])
                    b++;
                if(d[0] == a[1] || d[0] == a[2] || d[0] == a[3])
                    c++;
                if(d[1] == a[1])
                    b++;
                if(d[1] == a[0] || d[1] == a[2] || d[1] == a[3])
                    c++;
                if(d[2] == a[2])
                    b++;
                if(d[2] == a[1] || d[2] == a[0] || d[2] == a[3])
                    c++;
                if(d[3] == a[3])
                    b++;
                if(d[3] == a[1] || d[3] == a[2] || d[3] == a[0])
                    c++;

                if(b == 4)
                    Toast.makeText(getApplicationContext(), "You Win",
                        Toast.LENGTH_SHORT).show();
            }
            out.put(KEY_NUM , gN);
            out.put(KEY_BULLS ,String.valueOf(b));
            out.put(KEY_COWS , String.valueOf(c));

            output.add(out);


            ListAdapter adapter = new SimpleAdapter(this, output,
                    R.layout.list_item,
                    new String[] { KEY_NUM , KEY_BULLS , KEY_COWS}, new int[] {
                            R.id.tvGuessNum , R.id.tvBulls ,R.id.tvCows });

            setListAdapter(adapter);
            ListView lv = getListView();
        }
    }
}
}

这是堆栈跟踪：

03-04 06:32:58.960: W/dalvikvm(844): threadid=1: thread exiting with uncaught exception (group=0xb3ab2ba8)
03-04 06:32:58.970: E/AndroidRuntime(844): FATAL EXCEPTION: main
03-04 06:32:58.970: E/AndroidRuntime(844): Process: com.example.game, PID: 844
03-04 06:32:58.970: E/AndroidRuntime(844): java.lang.NullPointerException
03-04 06:32:58.970: E/AndroidRuntime(844):  at com.example.game.GuessActivity.onClick(GuessActivity.java:66)
03-04 06:32:58.970: E/AndroidRuntime(844):  at android.view.View.performClick(View.java:4438)
03-04 06:32:58.970: E/AndroidRuntime(844):  at android.view.View$PerformClick.run(View.java:18422)
03-04 06:32:58.970: E/AndroidRuntime(844):  at android.os.Handler.handleCallback(Handler.java:733)
03-04 06:32:58.970: E/AndroidRuntime(844):  at android.os.Handler.dispatchMessage(Handler.java:95)
03-04 06:32:58.970: E/AndroidRuntime(844):  at android.os.Looper.loop(Looper.java:136)
03-04 06:32:58.970: E/AndroidRuntime(844):  at android.app.ActivityThread.main(ActivityThread.java:5017)
03-04 06:32:58.970: E/AndroidRuntime(844):  at java.lang.reflect.Method.invokeNative(Native Method)
03-04 06:32:58.970: E/AndroidRuntime(844):  at java.lang.reflect.Method.invoke(Method.java:515)
03-04 06:32:58.970: E/AndroidRuntime(844):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-04 06:32:58.970: E/AndroidRuntime(844):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
03-04 06:32:58.970: E/AndroidRuntime(844):  at dalvik.system.NativeStart.main(Native Method)

第66行采用onClick方法：d[i] = x % 10

Answer 1

看起来EditText（R.id.etGuessNum）中没有值。

您应该清理输入以避免空引用。我还建议您检查他们输入的值是否为有效数字。

我还可以通过naming conventions快速阅读Java。

Answer 2

您的3个字符串KEY_NUM, KEY_BULLS, KEY_COWS为空。
此外，您确定a不为空吗？ a = getIntent().getExtras().getIntArray("num");可能会返回null。

更改

if(l!=4)
{
    Toast.makeText(getApplicationContext(),"Number should be of 4 digits",
            Toast.LENGTH_LONG).show();
}

到

if(l!=4)
{
    Toast.makeText(getApplicationContext(),"Number should be of 4 digits",
            Toast.LENGTH_LONG).show(); 
    return;
}

Answer 3

改变这个：

String gN = GNum.getText（）。toString（）; //可以为null

为：

String gN = GNum.getText().toString(); //can be null
if(gN == null) 
return;

int l = gN.length（）; //如果编辑文本为空，则为空

整数数组中的NullPointerException

3 个答案: