You can use Whatsapp intent to do so.
Note :- WhatsApp does not support messages with both pictures and text,so use below code to share text.
Share Text on Whatsapp
Intent whatsappIntent = new Intent(Intent.ACTION_SEND);
whatsappIntent.setType("text/plain");
whatsappIntent.setPackage("com.whatsapp");
whatsappIntent.putExtra(Intent.EXTRA_TEXT, "Hello World");
try {
activity.startActivity(whatsappIntent);
} catch (android.content.ActivityNotFoundException ex) {
ToastHelper.MakeShortText("Whatsapp have not been installed.");
}
Share Image on Whatsapp
Intent whatsappIntent = new Intent(android.content.Intent.ACTION_SEND);
whatsappIntent.setType("image/*");
whatsappIntent.putExtra(Intent.EXTRA_STREAM, Uri.parse("file://" + file));//add image path
startActivity(Intent.createChooser(share, "Share image using"));
Update
Whatsapp now support text (consider as image caption) with image as
Intent whatsappIntent = new Intent(android.content.Intent.ACTION_SEND);
whatsappIntent.setType("image/*");
whatsappIntent.putExtra(Intent.EXTRA_TEXT, "Hello World");
whatsappIntent.putExtra(Intent.EXTRA_STREAM, Uri.parse("file://" + file)); //add image path
startActivity(Intent.createChooser(share, "Share image using"));
try {
activity.startActivity(whatsappIntent);
} catch (android.content.ActivityNotFoundException ex) {
Toast.makeText(activity, "Whatsapp have not been installed.", Toast.LENGTH_SHORT).show();
}