无法启动活动，致命异常：main

Question

我怎样才能解决这类问题？问题在于setOnClickListener，它给了我一个NullPointerException。如果你还需要什么，请告诉我。

这是日志中的错误：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mapsprova/com.example.home.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.cardview.widget.CardView.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7656)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.cardview.widget.CardView.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
   

   
    

public class HomeActivity extends AppCompatActivity {
private CardView btn_home;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.fragment_home);

    btn_home = findViewById(androidx.appcompat.R.id.home);

    btn_home.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View view) {
            Intent intent = new Intent(HomeActivity.this, TrovamicoActivity.class);
            startActivity(intent);
        }

    });

}

Answer 1

您的问题源于您的ClickListener，问题如下：

btn_home = findViewById(androidx.appcompat.R.id.home); // This is wrong
// I believe you have to call it this way (only if its id is 'home' in your layout)
btn_home = findViewById(R.id.home);

// This call fail with NullPointerException
btn_home.setOnClickListener(new View.OnClickListener() {
    // Listener code
});

免责声明：如果没有布局，我无法确定原因，但stacktrace非常清楚，clicklistener不能在空值上调用

首先，你没有任何像在主活动中声明的那样标识为“家”的视图。所以我假设（正如你的代码所证实的，var是一个CardView）你打算使用cardview作为（某种程度上）一个按钮。所以你必须用这段代码来代替：

btn_home = findViewById(R.id.trovamico);

现在，当你点击Trovamico Cardview（来自HomeActivity）时，它应该可以工作了