Java 获得；“强制关闭”；用我的第一个HelloWorld登录android代码

我试着跟着实验室学习

这是密码

NameGetter.java

public class NameGetter extends Activity implements OnClickListener {

    EditText name;
    Button submit;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.setContentView(R.layout.name_getter);
        name = (EditText) this.findViewById(R.id.editText1);
        submit = (Button) this.findViewById(R.id.button1);
        submit.setOnClickListener(this);
    }

    public void onClick(View v) {
        String userName = name.getText().toString();
        Intent submitIntent = new Intent(NameGetter.this, HelloWorld.class);
        submitIntent.putExtra("username", userName);
        this.startActivity(submitIntent);
    }
}

public class HelloWorld extends Activity {

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView tv = (TextView) this.findViewById(R.id.textViewHello);

        Bundle name = this.getIntent().getExtras();

        if(name != null) {
            String nameEntered = name.getString("username");
            if(nameEntered != null) {
                tv.setText(nameEntered);
            } else {
                tv.setText("nameEntered is null");
            }
        } else {
            tv.setText("name is null");
        }
    }
}

HelloWorld.java

public class NameGetter extends Activity implements OnClickListener {

    EditText name;
    Button submit;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.setContentView(R.layout.name_getter);
        name = (EditText) this.findViewById(R.id.editText1);
        submit = (Button) this.findViewById(R.id.button1);
        submit.setOnClickListener(this);
    }

    public void onClick(View v) {
        String userName = name.getText().toString();
        Intent submitIntent = new Intent(NameGetter.this, HelloWorld.class);
        submitIntent.putExtra("username", userName);
        this.startActivity(submitIntent);
    }
}

public class HelloWorld extends Activity {

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView tv = (TextView) this.findViewById(R.id.textViewHello);

        Bundle name = this.getIntent().getExtras();

        if(name != null) {
            String nameEntered = name.getString("username");
            if(nameEntered != null) {
                tv.setText(nameEntered);
            } else {
                tv.setText("nameEntered is null");
            }
        } else {
            tv.setText("name is null");
        }
    }
}

我可以在编辑文本框中输入文本，当我按下提交按钮时，我得到一个强制关闭。LogCat说：

java.lang.RuntimeException:无法启动活动组件信息{com.myPackageHello.com/com.myPackageHello.com.HelloWorld}:java.lang.NullPointerException`

编辑：谢谢大家！这是

this.setContentView（R.layout.main）行
 插入

setContentView(R.layout.<layout_id>);

请注意，在NameGetter活动中，您将内容视图设置为：

this.setContentView(R.layout.name_getter);

但在你的地狱世界活动中，你还没有这样做。在super.onCreate之后和findViewById之前添加该行。祝你好运
 jsut在应用程序标记内的androidmanifest.xml中声明HelloWorld活动，如下所示

        <activity
            android:name=".HelloWorld"
            android:label="@string/app_name" >
        </activity>


您在maanifest中声明了HelloWorld吗？setContentView（R.layout.）在哪里；HelloWorld？？我想您假设两个活动之间共享相同的UI，这是错误的。两个活动使用它们自己的UI，因此您必须调用setContentView（R.layout.name\u getter）；在HelloWorld类中，位于super.onCreate（）之后。如果是，则将抛出ActivityNotFoundException而不是NPE。