java - Android findViewById 时出现 NullPointerException

Question

当我运行我的应用程序时，我在 log-cat 中收到此错误:

Caused by: java.lang.NullPointerException
at com.myfirstapp.myfirstapp.MainActivity.onCreate(MainActivity.java:52)
at android.app.Activity.performCreate(Activity.java:5312)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1111)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2552)
... 11 more

这是第 52 行:

String message = editTextInput.getText().toString();

到目前为止，我已经确定 NPE 必须是在定义 EditText(或任何与此相关的 View )时:

EditText editText = (EditText) findViewById(R.id.input_text);

但是，当我定义一个没有以下内容的 EditText 时，我不会得到 NPE:findViewById() 像这样的 View :

TextView desc = new TextView(this);

这是整个 onCreate() 方法:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }

    EditText editText = (EditText) findViewById(R.id.input_text);
    Button submit = (Button) findViewById(R.id.submit);
    LinearLayout layout = (LinearLayout) findViewById(R.id.layout_main);
    Intent intent = new Intent(this, testActivity.class);
    final TextView desc = new TextView(this);
    final TextView title = new TextView(this);

    String message = editText.getText().toString();

    submit.setVisibility(View.VISIBLE);
    submit.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            editText.setVisibility(View.GONE);}});

    desc.setTextSize(20);
    desc.setText(message);
    desc.setGravity(Gravity.LEFT | Gravity.TOP);
    desc.setPadding(5, 5, 5, 5);
    desc.setTextColor(getResources().getColor(R.color.black));

    title.setTextSize(10);
    title.setText(R.string.title_activity_dictionary);
    title.setGravity(Gravity.LEFT | Gravity.TOP);
    title.setPadding(5, 5, 5, 5);
    title.setTextColor(getResources().getColor(R.color.black));

    layout.addView(desc);
    layout.addView(title);
}

我不明白的是，当我明确定义 View 时，为什么会有 NPE？

Answer 1

确保您在布局中可用的 findviewbyid 方法中传递了正确的 id。

否则。只需清理并构建项目即可。