java - Android:尝试实例化一个类

Question

总的来说，我对 Java 编程还是个新手，我现在正在尝试解决实例化问题，并遇到了一些问题。我已尝试使用尽可能简单的方法来对此进行测试，但我的程序将强制关闭。

*我的类(class)

public class InterfaceScreen extends Activity implements testInterface1 {
TextView tv1, tv2;
String String1 = "I ate a pizza!";
String String2 = "sleeping......";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_interface_screen);
    tv1 = (TextView)findViewById(R.id.textView1);
    tv2 = (TextView)findViewById(R.id.textView3);   

    //Attempint to create a new InterfaceScreen (which is an animal by the way....)
    InterfaceScreen myAnimal = new InterfaceScreen();

    //now telling our animal to eat and sleep
    myAnimal.eat(String1);
    myAnimal.sleep(String2);

}
public void eat(String text1) {
    tv1.setText(text1);
}

public void sleep(String text2) {
    tv2.setText(text2);
}

*我的界面屏幕，再次使其尽可能简单。

public interface testInterface1 {

    void eat(String text1);
    void sleep(String text2);
}

我的 Activity 的 XML

  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:paddingBottom="@dimen/activity_vertical_margin"
     android:paddingLeft="@dimen/activity_horizontal_margin"
     android:paddingRight="@dimen/activity_horizontal_margin"
     android:paddingTop="@dimen/activity_vertical_margin"
     tools:context=".InterfaceScreen" >

<TextView
    android:id="@+id/textView2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/textView2"
    android:layout_below="@+id/textView2"
    android:layout_marginTop="21dp"
    android:text="TextView" />

<TextView
    android:id="@+id/textView3"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/textView1"
    android:layout_below="@+id/textView1"
    android:layout_marginTop="30dp"
    android:text="TextView" />

 </RelativeLayout>

记录 Cat 输出

02-28 16:03:36.621: E/AndroidRuntime(2557): FATAL EXCEPTION: main
02-28 16:03:36.621: E/AndroidRuntime(2557): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.generaljavalearning/com.example.generaljavalearning.InterfaceScreen}: java.lang.NullPointerException
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.os.Handler.dispatchMessage(Handler.java:99)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.os.Looper.loop(Looper.java:137)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.app.ActivityThread.main(ActivityThread.java:5103)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at java.lang.reflect.Method.invokeNative(Native Method)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at java.lang.reflect.Method.invoke(Method.java:525)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at dalvik.system.NativeStart.main(Native Method)
02-28 16:03:36.621: E/AndroidRuntime(2557): Caused by: java.lang.NullPointerException
02-28 16:03:36.621: E/AndroidRuntime(2557):     at com.example.generaljavalearning.InterfaceScreen.eat(InterfaceScreen.java:33)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at com.example.generaljavalearning.InterfaceScreen.onCreate(InterfaceScreen.java:28)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.app.Activity.performCreate(Activity.java:5133)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
02-28 16:03:36.621: E/AndroidRuntime(2557):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
02-28 16:03:36.621: E/AndroidRuntime(2557):     ... 11 more

Answer 1

在 onCreate 中，您已经在 InterfaceScreen 的实例中进行操作。它叫做 this。您无需创建新实例。

代替

myAnimal.eat(String1);
myAnimal.sleep(String2);

随便说说

this.eat(String1);
this.sleep(String2);

甚至更好

eat(String1);
sleep(String2);

此外，变量通常以小写字母开头:string1 和 string2。类以大写字母开头。

NullPointerException 发生是因为您的新实例没有获得正确的 tv1 和 tv2 引用。在 Android 中，我们不会创建 Activity 的实例。