java - 使用我的第一个 HelloWorldLogin android 代码获取 "Force Close"

Question

我试图通过这里的实验室https://sites.google.com/site/androidcoursearchive/labs/lab-1来学习

这是代码

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);
    }
}

HelloWorld.java

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 EditText 框中输入文本，当我按下提交按钮时，我会强制关闭。 LogCat 内容如下:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.mypackagerhello.com/com.mypackagerhello.com.HelloWorld}: java.lang.NullPointerException`

编辑:谢谢大家! HelloWorld.java 中缺少 this.setContentView(R.layout.main); 行。

Answer 1

请注意，在您的 NameGetter Activity 中，您设置内容 View 如下:

this.setContentView(R.layout.name_getter);

但在您的 HelloWorld Activity 中您还没有这样做。将该行添加到 super.onCreate 之后和 findViewById 之前。祝你好运!