java - 尝试调用虚拟方法时的 NPE

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

findViewById returns null (4 个回答)

已关闭 5 年前。

我明白了

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.ViewGroup.addView(android.view.View)' on a null object reference

这是我的代码:

 protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    b1=(Button)findViewById(R.id.btn);
    f1=(FrameLayout)findViewById(R.id.frame_layout);
    f1.addView(new myview(getApplicationContext()));
    animation= 
AnimationUtils.loadAnimation(getApplicationContext(),R.anim.translation);
    b1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            f1.startAnimation(animation);
        }
    });

错误显示在 f1.addView(new myview(getApplicationContext())); 行中。我创建了一个动画程序，在其中我想将其他类调用到代码中提到的框架布局中，因此当我运行此程序时，会显示运行时错误，如下所示。

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2706)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2767)
    at android.app.ActivityThread.-wrap12(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1514)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:163)
    at android.app.ActivityThread.main(ActivityThread.java:6221)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794)  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.ViewGroup.addView(android.view.View)' on a null object reference
    at com.cenet.animationclass.MainActivity.onCreate(MainActivity.java:25)
    at android.app.Activity.performCreate(Activity.java:6875)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
    at  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2659

Answer 1

您需要首先向您的 Activity 添加布局:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    b1=(Button)findViewById(R.id.btn);
    f1=(FrameLayout)findViewById(R.id.frame_layout);
}