android - Intent 中的空指针异常

Question

我正在尝试调用一个 Activity，但它在第 17 行显示空指针异常。我刚刚开始学习 Android Studio。请帮我解决这个问题，并解释我哪里出错了。

public class activity2 extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        Button B2 = (Button) findViewById(R.id.B2);
        B2.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                Intent ladoo = new Intent(activity2.this,MainActivity.class);
                startActivity(ladoo);
                finish();
            }
        });
        super.onCreate(savedInstanceState);
    }
}

Answer 1

除了 super.onCreate() 放置的问题，问题是你没有调用 SetContentView()，所以没有 active inflated查看引用您的 Button 对象。

public class activity2 extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState); //add this here
        setContentView(R.layout.activity_activity2); //reference whatever the layout.xml file is named for this Activity

        Button B2 = (Button) findViewById(R.id.B2);
        B2.setOnClickListener(new View.OnClickListener()
        {
            @Override
            public void onClick(View v)
            {
                Intent ladoo = new Intent(activity2.this,MainActivity.class);
                startActivity(ladoo);
                finish();
            }
        });
        //super.onCreate(savedInstanceState); //remove from here
    }
}

参见 Documentation Here