Java 如何防止文本视图中显示null？

对安卓来说非常陌生，我真的被这件事难住了

因此，我让我的

main活动

从第二个活动

CallAnActivity

接收一些字符串

我打算将字符串传递回主活动，它们将显示在

TextView

中。但是在我输入任何信息之前，我希望这个文本视图不包含任何内容，并且看起来是空白的

但是当第一次启动

main活动

时，文本视图

displayMessageActivity

显示多个

null

值。由于有三个字符串被发送，它只会将null这个词重复三次

main活动的代码为：

    //callAnActivityButton to open second activity of app
    Button callAnActivityButton = findViewById(R.id.callActivityButton);
    callAnActivityButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick( View v ) {
            Intent intent = new Intent(MainActivity.this, CallAnActivity.class);
            startActivity(intent);
        }
    });

    Intent intent = getIntent();
    String emailAddress = intent.getStringExtra("emailAddress") + "\n";
    String emailSubject = intent.getStringExtra("emailSubject") + "\n";
    String emailBody = intent.getStringExtra("emailBody");

    //TextView to display text from second activity
    TextView displayMessageText = findViewById(R.id.displayMessageText);
    displayMessageText.setText(emailAddress + emailSubject + emailBody);

第二个活动的代码是，
CallanaActivity
：

    public void sendText(){

    String emailAddress = mEditTextAddress.getText().toString();
    String emailSubject = mEditTextSubject.getText().toString();
    String emailBody = mEditTextBody.getText().toString();

    Intent intent = new Intent(CallAnActivity.this, MainActivity.class);
    intent.putExtra("emailAddress", emailAddress);
    intent.putExtra("emailSubject", emailSubject);
    intent.putExtra("emailBody", emailBody);
    startActivityForResult(intent, REQUEST_CODE);
}

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

    mEditTextAddress = findViewById(R.id.emailAddressText);
    mEditTextSubject = findViewById(R.id.emailSubjectText);
    mEditTextBody = findViewById(R.id.emailBodyText);

    Button sendTextButton = findViewById(R.id.sendMailButton);
    sendTextButton.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick (View v) {
            sendText();
        }
    });
}

只需在
setText
调用中使用
if
块：-

if (intent.getStringExtra("emailAddress") != null) {
    displayMessageText.setText(emailAddress + emailSubject + emailBody);
}

更改您的内容，如下所示。你的getExtra语法是错误的

String emailAddress = intent.getExtras().getStringExtra("emailAddress") + "\n";  
String emailSubject = intent.getExtras().getStringExtra("emailSubject") + "\n";
String emailBody = intent.getExtras().getStringExtra("emailBody"); 

更改您的内容，如下所示。您的getExtra语法错误，字符串emailAddress=intent.getExtras（）.getStringExtra（“emailAddress”）+“\n”；字符串emailSubject=intent.getExtras（）.getStringExtra（“emailSubject”）+“\n”；字符串emailBody=intent.getExtras（）.getStringExtra（“emailBody”）；