java - 为什么我总是收到空指针异常？

Question

您好，之前可能有人问过这个问题，但我找不到任何特别的问题。当我去运行我的程序时，我想出了一个空指针异常，程序无法运行。我对此还是有点陌生​​，不确定我的问题出在哪里。

public class MainActivity extends Activity implements OnClickListener {
public ArrayList<Short> indexP = new ArrayList<Short>();
public ArrayList<Float> linep = new ArrayList<Float>();
public Float coords = (float) 0;
public short p = 0;
public TextView info = (TextView) findViewById(R.id.info);
public int l = 0;
GLSurfaceView ourSurface;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.cad);
    ourSurface = new GLSurfaceView(this);
    FrameLayout v = (FrameLayout) findViewById(R.id.display);
    v.addView(ourSurface);
    ourSurface.setRenderer(new GLRenderer());

    Button line = (Button) findViewById(R.id.line);
    final Button enter = (Button) findViewById(R.id.enter);
    EditText cl = (EditText) findViewById(R.id.cl);
    final String value = cl.getText().toString();
    try {
        coords = Float.parseFloat(value);
    } catch (NumberFormatException e) {
    }
    ;

    line.setOnClickListener(this);
    enter.setOnClickListener(this);
    enter.setOnClickListener(this);
}

public void onClick(View v) {
    switch (v.getId()) {
    case R.id.line:
        info.setText("Input X");
        break;
    case R.id.enter:
        switch (l) {
        case (0):
            linep.add(coords);
            l++;
            break;
        case (1):
            linep.add(coords);
            indexP.add(p);
            l = 0;
            p++;

        }
    }

}

@Override
protected void onPause() {
    // TODO Auto-generated method stub
    super.onPause();
}

@Override
protected void onResume() {
    // TODO Auto-generated method stub
    super.onResume();
}

这是 logcat 信息:

threadid=1: thread exiting with uncaught exception (group=0x40a961f8)
FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.linecad/com.example.linecad.MainActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1991)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2092)
    at android.app.ActivityThread.access$600(ActivityThread.java:126)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1172)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4586)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at android.app.Activity.findViewById(Activity.java:1840)
    at com.example.linecad.MainActivity.<init>(MainActivity.java:20)
    at java.lang.Class.newInstanceImpl(Native Method)
    at java.lang.Class.newInstance(Class.java:1319)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1982)
    ... 11 more

Answer 1

在这里:

public TextView info = (TextView) findViewById(R.id.info); //<<<<

在为它设置布局之前，您正在尝试从当前 Activity 访问 View 。所以在调用 setContentView 之后将所有 View 初始化移动到一个方法中:

public TextView info;  //<<< declare here...
public int l = 0;
GLSurfaceView ourSurface;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.cad);
    info = (TextView) findViewById(R.id.info);  //<<< initialize here...
    ......