启动后我添加了 addListenerOnButtonClick();在 mainactivity.java 然后应用程序崩溃

Question

mainactivity.java

包 com.example.newsplash;

进口androidx.appcompat.app.AppCompatActivity;

进口android.os.Bundle；导入 android.view.View；导入 android.widget.Button；导入 android.widget.CheckBox；导入 android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    CheckBox Chapathi, Dosai, Biriyani, Coffee;
    Button order;`enter code here`
    @Override

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        addListenerOnButtonClick();

    }



    public void addListenerOnButtonClick() {
        Chapathi = (CheckBox) findViewById(R.id.Chapathi);
        Biriyani = (CheckBox) findViewById(R.id.Biriyani);
        Dosai = (CheckBox) findViewById(R.id.Dosai);
        Coffee = (CheckBox) findViewById(R.id.Coffee);


        order.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick (View view)
            {
                int totalamount = 0;
                StringBuilder result = new StringBuilder();
                result.append("Selected Items:");
                if (Chapathi.isChecked()) {
                    result.append("\nChapathi 10RS");
                    totalamount += 10;

                }
                if (Biriyani.isChecked()) {
                    result.append("\nBiriyani is 100RS");
                    totalamount += 100;
                }
                if (Dosai.isChecked()) {
                    result.append("\nDosai is 10RS");
                    totalamount += 10;

                }
                if (Coffee.isChecked()) {
                    result.append("\n Coffee is 10RS");
                    totalamount += 10;
                }
                result.append(("\nTotal"+totalamount + "RS"));
                Toast.makeText(getApplicationContext(), result.toString(), Toast.LENGTH_LONG).show();
            }


        });

    }

}

logcat 2020-11-05 00:33:48.523 27490-27966/com.example.newsplash D/mmscene: getHint applicationScene= com.example.newsplash,idx=0 none 2020-11-05 00:33:48.577 27490-27490/com.example.newsplash I/Choreographer: 跳过7帧！应用程序可能在其主线程上做了太多工作。 2020-11-05 00:33:51.378 27490-27490/com.example.newsplash W/ActivityThread：handleWindowVisibility：没有 activity 令牌 android.os.BinderProxy@3268ab0 2020-11-05 00:33:51.928 27490-27490/com.example.newsplash D/AndroidRuntime: 关闭 VM 2020-11-05 00:33:51.938 27490-27490/com.example.newsplash E/AndroidRuntime ：致命例外：主要进程：com.example.newsplash，PID：27490 java.lang.RuntimeException: 无法启动 activity ComponentInfo{com.example.newsplash/com.example.newsplash.MainActivity}: java.lang.NullPointerException: 尝试调用虚拟方法 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' 在空对象引用上在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3146) 在 android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3296) 在 android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 在 android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:114) 在 android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:74) 在 android.app.ActivityThread$H.handleMessage(ActivityThread.java:1994) 在 android.os.Handler.dispatchMessage(Handler.java:106) 在 android.os.Looper.loop(Looper.java:226) 在 android.app.ActivityThread.main(ActivityThread.java:7224) 在 java.lang.reflect.Method.invoke（本机方法）在 com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:500) 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:913) 原因：java.lang.NullPointerException：尝试在空对象引用上调用虚方法 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' 在 com.example.newsplash.MainActivity.addListenerOnButtonClick(MainActivity.java:36) 在 com.example.newsplash.MainActivity.onCreate(MainActivity.java:23) 在 android.app.Activity.performCreate(Activity.java:7337) 在 android.app.Activity.performCreate(Activity.java:7328) 在 android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272) 在 android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3126) 在 android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3296) 在 android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 在 android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:114) 在 android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:74) 在 android.app.ActivityThread$H.handleMessage(ActivityThread.java:1994) 在 android.os.Handler.dispatchMessage(Handler.java:106) 在 android.os.Looper.loop(Looper.java:226) 在 android.app.ActivityThread.main(ActivityThread.java:7224) 在 java.lang.reflect.Method.invoke（本机方法）

Answer 1

您的代码

您创建了一个变量 Button order 但您从未对其进行初始化。然后，在未初始化的按钮上，您调用 .setOnClickListener，这显然无法完成。

您的堆栈跟踪：

java.lang.NullPointerException: 尝试在空对象引用
上调用虚拟方法'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)'

解决方案是对 order 使用 findViewById 并获得实际的按钮参考，然后您可以使用 .setOnClickListener

Answer 2

**是的，现在有效了。谢谢你 ** public class MainActivity 扩展 AppCompatActivity {

CheckBox Chapathi, Dosai, Biriyani, Coffee;
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    addListenerOnButtonClick();
}

public void addListenerOnButtonClick() {
    Chapathi = (CheckBox) findViewById(R.id.Chapathi);
    Biriyani = (CheckBox) findViewById(R.id.Biriyani);
    Dosai = (CheckBox) findViewById(R.id.Dosai);
    Coffee = (CheckBox) findViewById(R.id.Coffee);


    
    button = (Button) findViewById(R.id.order);
    button.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick (View view)
        {
            int totalamount = 0;
            StringBuilder result = new StringBuilder();
            result.append("Selected Items:");
            if (Chapathi.isChecked()) {
                result.append("\nChapathi 10RS");
                totalamount += 10;

            }
            if (Biriyani.isChecked()) {
                result.append("\nBiriyani is 100RS");
                totalamount += 100;
            }
            if (Dosai.isChecked()) {
                result.append("\nDosai is 10RS");
                totalamount += 10;

            }
            if (Coffee.isChecked()) {
                result.append("\n Coffee is 10RS");
                totalamount += 10;
            }
            result.append(("\nTotal"+totalamount + "RS"));
            Toast.makeText(getApplicationContext(), result.toString(), Toast.LENGTH_LONG).show();
        }


    });

}

}

启动后我添加了 addListenerOnButtonClick();在 mainactivity.java 然后应用程序崩溃

after splash i added addListenerOnButtonClick(); in mainactivity.java then app crashes

crash

android

screen

splash-screen

android-appcompat