Android Studio：制作一个随机数生成器应用程序，不显示随机数？

Question

I have EditViews to prompt the user for a min and max. The data entered is passed into an Activity function and a random number is generated. That random number is passed onto another Activity to be displayed. When I enter a min number and a max number and then click the button that says generate, I get directed to a blank screen. However, I'm expecting randValue to be displayed.

MainActivity.java

public class MainActivity extends AppCompatActivity {
public final static String RAND_NUM = "com.example.random.MESSAGE";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

public void generateNum(View view) {
    Intent intent = new Intent(this, DisplayNumActivity.class);

    EditText minInput = (EditText) findViewById(R.id.min_num);
    EditText maxInput = (EditText) findViewById(R.id.max_num);

    int minNum = Integer.parseInt(minInput.getText().toString());
    int maxNum = Integer.parseInt(maxInput.getText().toString());
    Random rand = new Random();
    int randNum = rand.nextInt(maxNum - minNum) + minNum;
    intent.putExtra(RAND_NUM, randNum);
    startActivity(intent);

  }
}

DisplayNumActivity.java

public class DisplayNumActivity extends AppCompatActivity {

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

    Intent intent = getIntent();
    String randValue = intent.getStringExtra(MainActivity.RAND_NUM);

    TextView textView = new TextView(this);
    textView.setTextSize(30);
    textView.setText(randValue);

    ViewGroup layout = (ViewGroup) findViewById(R.id.activity_display_num);
    layout.addView(textView);

  }
}

activity_main.xml

<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:orientation="horizontal"
    tools:context="com.example.random.MainActivity">

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Min number"
    android:id="@+id/textView"
    android:layout_marginTop="40dp"
    android:layout_alignParentTop="true"
    android:layout_alignLeft="@+id/textView2"
    android:layout_alignStart="@+id/textView2" />

<EditText
    android:id="@+id/min_num"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:hint="min"
    android:inputType="number"
    android:layout_marginTop="32dp"
    android:layout_below="@+id/textView"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Max number"
    android:id="@+id/textView2"
    android:layout_marginTop="43dp"
    android:layout_below="@+id/min_num"
    android:layout_centerHorizontal="true" />

<EditText
    android:id="@+id/max_num"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:hint="max"
    android:inputType="number"
    android:minWidth="500dp"
    android:layout_centerVertical="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_text"
    android:onClick="generateNum"
    android:id="@+id/button"
    android:layout_below="@+id/max_num"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="43dp" />

Answer 1

OK, two things:

1) In DisplayNumActivity, change

String randValue = intent.getStringExtra(MainActivity.RAND_NUM);

to this:

int randValue = intent.getIntExtra(MainActivity.RAND_NUM, 0);

Here we are reading in an int value from the intent instead of a String. In your main activity you saved an int here, not a String, the String here is just the "key" in the key-value pair. The zero is just a default in case it can't find your int.

2) To display the number you have to first change that int back to a String in order to set it in a TextView and then we display it to your relative layout like this:

String stringRandValue = Integer.toString(randValue);
View relativeLayout =  findViewById(R.id.activity_display_num);;
TextView textView = new TextView(this);
textView.setTextSize(30);
textView.setText(stringRandValue);
((RelativeLayout)relativeLayout).addView(textView);

That should work.

Answer 2

I think you have to specify exactly which layout you want to add your textview to.

Like: RelativeLayout relativelayout = (RelativeLayout)findViewById(R.id.relativelayoutID);

Casting to viewgroup seems bit heavy to me. Please try suggested way and let me know if it changes anything for you.

Edit : There is one more thing which I feel might be the culprit in this case and that is missing layout params for your text View.

Example: //create params like this

LayoutParams layoutparams = new RelativeLayout.LayoutParams( LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT );

//set layout params like this

textView.setLayoutParams(layoutparams);

Note - replace the parent layout type according to your xml where this textView is been added.

Answer 3

Replace this

String randValue = intent.getStringExtra(MainActivity.RAND_NUM);

With this

String randValue = intent.getIntExtra(MainActivity.RAND_NUM, default_int_value);