Sample fiile to test whatsapp installation on android device via Appium

WhatsApp.java

package com.sundeepmachado;

import java.io.File;
import java.net.URL;

import junit.framework.Assert;

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.remote.DesiredCapabilities;
import org.openqa.selenium.remote.RemoteWebDriver;
import org.testng.annotations.AfterMethod;
import org.testng.annotations.BeforeMethod;
import org.testng.annotations.Test;

public class WhatsApp {

	private WebDriver driver;

	@BeforeMethod
	public void setUp() throws Exception {

		// set up appium and tell from where it can install the apk file from
		// computer to device
		File appDir = new File("E:\\AppiumDemo-master\\AppiumDemo-master\\apps");
		File app = new File(appDir, "com.whatsapp.apk");
		// Very important properties you need for Appium to work, change as per
		// SDK and device name
		DesiredCapabilities capabilities = new DesiredCapabilities();
		capabilities.setCapability("device", "Android");
		capabilities.setCapability("deviceName", "Huawei MediaPad T1");
		capabilities.setCapability("platformName", "Android");
		// You need to have this sdk installed for Appium to work
		capabilities.setCapability("platformVersion", "4.3.1");
		capabilities.setCapability("app", app.getAbsolutePath());
		// The URL where the hub will start
		driver = new RemoteWebDriver(new URL("http://127.0.0.1:4723/wd/hub"),
				capabilities);

	}

	@AfterMethod
	public void tearDown() throws Exception {
		driver.quit();
	}

	@Test
	public void whatsAppinstall() {
		// Find and click the accept button
		WebElement accept = driver.findElement(By
				.id("com.whatsapp:id/eula_accept"));
		accept.click();
		// Verify nect page title after you click accept button
		Assert.assertEquals("Verify your phone number",
				driver.findElement(By.id("android:id/action_bar_title"))
						.getText());
	}

}

Can you help me how to test already installed mobile applications?

See above answer