在Android Studio中单击按钮后，很难在ListView中显示随机leap年的列表

Question

On my MainActivity.java, I have written code for creating a list of Leap Year till 2015 (inclusive) from 1800 (inclusive). However, I am stuck at the part on where a user clicks 'Generate' button and List of Leap Year is displayed in ListView .
I believe the real problem is with my populate() method because that's only part I am not so sure of. Because I haven't dealt with ArrayAdapter or ArrayList much so far.
NOTE: I don't get any compiling or run time error, I am missing some logical part. The app launches succesfully, but Generate Button doesn't do anything visibly.

Refer to my code Below: MainActivity.java

package net.androidbootcamp.gregorianspeeddating;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;

import java.util.Random;

public class MainActivity extends AppCompatActivity {
    Button btnGen, btnMan;
    ListView listView;
    Integer randomNum;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    listView = (ListView) findViewById(R.id.yearsListView);

    //Button Generate that creates a ListView of Leap Year on MainActivity
    btnGen = (Button) findViewById(R.id.btnGen);
    btnGen.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Random r = new Random();
            randomNum = r.nextInt(2016 - 1800) + 1800;

            if (isLeapYear(randomNum)) {
                populate();
            }


        }
    });
    // Button Manipulate that takes user to another activity
    btnMan = (Button) findViewById(R.id.btnMan);
    btnMan.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(MainActivity.this, Manipulate.class));
        }
    });


}

// Returns true if given Integer is a Leap Year.
public boolean isLeapYear(Integer year) {
    boolean b = (year % 4 == 0) && (year % 100 != 0) || (year % 400 == 0);
    if (!b) {
        return false;
    }
    return true;
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

// Method To Populate ListView with random Leap Year
public void populate() {
    ArrayAdapter<Integer> adapter = new ArrayAdapter<>(this,  android.R.layout.simple_list_item_1, randomNum);
    listView.setAdapter(adapter);
    }
}

content_main.xml which contains Button 'Generate' and 'ListView' --> This is displayed in activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    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"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="net.androidbootcamp.gregorianspeeddating.MainActivity"
    tools:showIn="@layout/activity_main">

<ListView
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:id="@+id/yearsListView"
    android:layout_weight="1"
    android:dividerHeight="13dp"
    android:divider="@android:color/transparent"/>

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Generate"
    android:id="@+id/btnGen"
    android:layout_alignParentBottom="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_marginLeft="28dp"
    android:layout_marginStart="28dp"
    android:layout_marginBottom="60dp" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Manipulate"
    android:id="@+id/btnMan"
    android:layout_marginRight="28dp"
    android:layout_marginEnd="33dp"
    android:layout_alignTop="@+id/btnGen"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true" />
</RelativeLayout>

Answer 1

In order to display a list of random leap years, you need to give your listview a list of years rather than a single year. In the onClick method of your Button you randomly generate one number between 1800 and 2015 rather than generating a list of random numbers.

What you have to do is:

1. Instead of having Integer randomNum; , use List<Integer> randomNums;
2. Use a variable int count = 20; // use a variable count to keep track of the number of leap years to generate

3. In the onClick method of the button, use

   Random rand = new Random(); randomNums = new ArrayList<Integer>(); while(randomNums.size != count){ randomNums.add(rand.nextInt(2016 - 1800) + 1800); } populate()

   This while loop can go on and on until you find the required count of random numbers.

4. Finally in the populate method use the array randomNums

   ArrayAdapter<Integer> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, randomNums);

All the best :)

Answer 2

I haven't run your code yet, but I can see one problem in your onClick handler. In this condition:

if (isLeapYear(randomNum)) {
    populate();
}

you are really only calling populate if the random year that was generated is a leap year. From your code it looks like you are simply generating one random number and then creating a list with 0 or 1 entries (0 if it's not a leap year and 1 if it is). So the chances of you having any items in that list is only 1 in 4.

It would help if you post your desired outcome. For example: are you trying to list ALL years starting in 1800 and ending in 2015 that are leap years in order? In that case, why do you need to generate a random number?