java - 在我的应用程序的第二个 Activity 中显示 "Happy birthday null"

Question

主要 Activity `公共(public)类 MainActivity 扩展 AppCompatActivity {

private Object Context;
public static final String MSG="com.mp.exampleapp.ORDER";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

}

public void createBirthdayCard(View view) {
    EditText text = (EditText)findViewById(R.id.nameInput);
    String str = text.getText().toString();

    Toast.makeText(this, "Button was clicked "+str,Toast.LENGTH_LONG).show();
    getIntent().putExtra(MSG,str);
    startActivity(new Intent(this,birthdaygreeting.class)) ;

    ImageView image=(ImageView) findViewById(R.id.cake);
    image.setImageResource(R.drawable.wish);
}

`第二个 Activity

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


     Intent intent = getIntent();
    String msg = "HAPPY BIRTHDAY "+intent.getStringExtra(MainActivity.MSG);
    // receive the value by getStringExtra() method
    // and key must be same which is send by first activity
    TextView textView= (TextView)findViewById(R.id.textView2);
   textView.setText(msg);

}

运行此程序后 - 单击按钮后 - 显示的消息是“生日快乐 null”。我不知道出了什么问题。

Answer 1

出了什么问题？

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您传递 put extra 的地方位于 getIntent() 中。这意味着您正在为上一个 Activity 提供额外的 Intent 。在这种情况下，当您返回并获得该额外的 Intent 时，您将获得它。但不会在下一个 Activity 中获得。现在您得到 null，因为该 Activity 不存在该额外内容。所以，它为空。

如何解决？

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而不是使用这个

public void createBirthdayCard(View view) {
    EditText text = (EditText)findViewById(R.id.nameInput);
    String str = text.getText().toString();
    Toast.makeText(this, "Button was clicked "+str,Toast.LENGTH_LONG).show();
    getIntent().putExtra(MSG,str);
    startActivity(new Intent(this,birthdaygreeting.class)) ;

    ImageView image=(ImageView) findViewById(R.id.cake);
    image.setImageResource(R.drawable.wish);
}

用这个

public void createBirthdayCard(View view) {
    EditText text = (EditText)findViewById(R.id.nameInput);
    String str = text.getText().toString();
    Toast.makeText(this, "Button was clicked "+str,Toast.LENGTH_LONG).show();
    startActivity(new Intent(this,birthdaygreeting.class).putExtra(MSG,str)) ;

    ImageView image=(ImageView) findViewById(R.id.cake);
    image.setImageResource(R.drawable.wish);
}

我怎么知道它会对我有帮助？

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这会对你有所帮助，因为在你给出的新 Intent 中，正在给出额外的东西。因此，该值正在被发送。