java - 无法使用通过 Intent 传递的字符串更改 TextView 的文本

Question

我正在做一个非常简单的登录屏幕，用户在其中输入用户名和密码，单击登录按钮，然后输入显示在下一个屏幕上。

每次尝试更改 textview 值时，我都会得到一个 NullPointerException。我在谷歌上搜索了很长时间，但一无所获，而且它必须是我完全想念的简单东西。

以下是我的代码:

public class LoggedIn extends Activity{
    String username = "";
    String password = "";

    TextView name;
    TextView pwd;

    Button infoDisp;

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

        Intent intent = getIntent();
        name = (TextView)findViewById(R.id.username);
        pwd = (TextView)findViewById(R.id.password);

        username = intent.getStringExtra("nameInfo");
        password = intent.getStringExtra("passInfo");

        name.setText(username);
        pwd.setText(password);

    }
}

编辑:我将包更改为上面的 Intent ，这也是其余代码(第一个 Activity )

public class LoginActivity extends Activity {

    Button loginBtn;
    Button registerBtn;
    EditText username;
    EditText pword;
    static String name;
    static String pass;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        username = (EditText) this.findViewById(R.id.username);
        pword = (EditText) this.findViewById(R.id.password);
        loginBtn = (Button) this.findViewById(R.id.loginBtn);

        loginBtn.setOnClickListener(new OnClickListener() {

            public void onClick(View arg0){
                Intent intent = new Intent(LoginActivity.this, LoggedIn.class);
                name = username.getText().toString();
                pass = pword.getText().toString();
                intent.putExtra("nameInfo", name);
                intent.putExtra("passInfo", pass);
                startActivity(intent);
            }
        });

    }

我没有故意检查值是否为 null 的错误，因为这应该只是一个快速运行和完成的事情。我想只要我在每个 EditText 中输入一些内容，那么字符串就不能为空，我就不会有问题......对吧？

堆栈跟踪:

FATAL EXCEPTION: main
Process: com.example.loginscreen, PID: 1677
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.loginscreen/com.example.loginscreen.LoggedIn}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2176)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2226)
    at android.app.ActivityThread.access$700(ActivityThread.java:135)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1397)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:4998)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:777)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:593)
    at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
    at com.example.loginscreen.LoggedIn.onCreate(LoggedIn.java:30)
    at android.app.Activity.performCreate(Activity.java:5243)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)
    ... 11 more

Aaa 和 xml 文件: Activity 登录:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".LoginActivity" >

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:text="Login:"
    android:textAppearance="?android:attr/textAppearanceMedium" />

<TextView
    android:id="@+id/textView2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/textView1"
    android:layout_below="@+id/textView1"
    android:layout_marginTop="38dp"
    android:text="Password:"
    android:textAppearance="?android:attr/textAppearanceMedium" />

<EditText
    android:id="@+id/username"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_above="@+id/textView2"
    android:layout_alignParentTop="true"
    android:layout_marginLeft="20dp"
    android:layout_toRightOf="@+id/textView1"
    android:ems="10" />

<EditText
    android:id="@+id/password"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/textView2"
    android:layout_alignParentRight="true"
    android:layout_toRightOf="@+id/textView2"
    android:ems="10"
    android:inputType="textPassword" >

    <requestFocus />
</EditText>

<Button
    android:id="@+id/loginBtn"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/textView2"
    android:layout_below="@+id/textView2"
    android:layout_marginTop="26dp"
    android:text="Login" />

<Button
    android:id="@+id/registerBtn"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/loginBtn"
    android:layout_alignBottom="@+id/loginBtn"
    android:layout_alignLeft="@+id/textView3"
    android:layout_marginLeft="46dp"
    android:text="Register" />

<CheckBox
    android:id="@+id/remPass"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/loginBtn"
    android:layout_below="@+id/loginBtn"
    android:layout_marginTop="19dp"
    android:text="Remember Password" />

<TextView
    android:id="@+id/textView3"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/remPass"
    android:layout_marginTop="28dp"
    android:layout_toRightOf="@+id/textView1"
    android:text="Forgot Password" />

登录:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".LoginActivity" >

<TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:text="Logged In!"
    android:textAppearance="?android:attr/textAppearanceLarge" />

<TextView
    android:id="@+id/textView2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_marginTop="56dp"
    android:text="Username: "
    android:textAppearance="?android:attr/textAppearanceLarge" />

<TextView
    android:id="@+id/textView3"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/textView2"
    android:layout_below="@+id/textView2"
    android:layout_marginTop="56dp"
    android:text="Password: "
    android:textAppearance="?android:attr/textAppearanceLarge" />

<TextView
    android:id="@+id/recUsername"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/textView2"
    android:layout_marginLeft="33dp"
    android:layout_toRightOf="@+id/textView2"
    android:editable="true"
    android:text=" "
    android:textAppearance="?android:attr/textAppearanceLarge" />

<TextView
    android:id="@+id/recPassword"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/textView3"
    android:layout_alignRight="@+id/textView4"
    android:editable="true"
    android:text=" "
    android:textAppearance="?android:attr/textAppearanceLarge" />

<Button
    android:id="@+id/infoBtn"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/textView1"
    android:layout_alignRight="@+id/textView1"
    android:layout_below="@+id/textView3"
    android:layout_marginTop="62dp"
    android:text="Press to Display Info" />

Answer 1

问题是因为您发现 TextView 的 id 是错误的..所以只需更改

    username = (TextView) this.findViewById(R.id.username);
    pword = (TextView) this.findViewById(R.id.password);

到

    username = (TextView) this.findViewById(R.id.recUsername);
    pword = (TextView) this.findViewById(R.id.recPassword);