How to share text to WhatsApp from my app?

You can use intent to do so. No need to use Whatsapp API. Hope that I have not misunderstood your question. Hope that helps, thanks.

Intent whatsappIntent = new Intent(Intent.ACTION_SEND);
whatsappIntent.setType("text/plain");
whatsappIntent.setPackage("com.whatsapp");
whatsappIntent.putExtra(Intent.EXTRA_TEXT, "The text you wanted to share");
try {
    activity.startActivity(whatsappIntent);
} catch (android.content.ActivityNotFoundException ex) {
    ToastHelper.MakeShortText("Whatsapp have not been installed.");
}

If the user does not have the Whatsapp app in their device then the user will get the ActivityNotFoundException

Then, you should move the user to download the app first.

public void shareViaWhatsApp() {
        Intent whatsappIntent = new Intent(Intent.ACTION_SEND);
        whatsappIntent.setType("text/plain");
        whatsappIntent.setPackage("com.whatsapp");
        whatsappIntent.putExtra(Intent.EXTRA_TEXT, "Application of social rating share with your friend");
        try {
            Objects.requireNonNull(getActivity()).startActivity(whatsappIntent);
        } catch (android.content.ActivityNotFoundException ex) {
            startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("http://play.google.com/store/apps/details?id=com.whatsapp")));
        }
    }

There are two ways to integrate with WhatsApp:

• Through a custom URL scheme

• Through Android's intent system.

If you have a website and want to open a WhatsApp chat with a pre-filled message, you can use our custom URL scheme to do so. Opening whatsapp://send?text= followed by the text to send, will open WhatsApp, allow the user to choose a contact, and pre-fill the input field with the specified text.

Like most social apps on Android, WhatsApp listens to intents to share media and text. Simply create an intent to share text, for example, and WhatsApp will be displayed by the system picker:

Intent sendIntent = new Intent();
sendIntent.setAction(Intent.ACTION_SEND);
sendIntent.putExtra(Intent.EXTRA_TEXT, "This is my text to send.");
sendIntent.setType("text/plain");
startActivity(sendIntent);

However, if you prefer to share directly to WhatsApp and bypass the system picker, you can do so by using setPackage in your intent:

sendIntent.setPackage("com.whatsapp");

This would simply be set right before you call startActivity(sendIntent);

Please refer below link Official WhatsApp Page: https://www.whatsapp.com/faq/en/android/28000012,

If you want to share some text to specific WhatsApp contact, Please refer below code.

private void openWhatsApp() {
String smsNumber = "7****"; //without '+'
try {
    Intent sendIntent = new Intent("android.intent.action.MAIN");
    //sendIntent.setComponent(new ComponentName("com.whatsapp", "com.whatsapp.Conversation"));
    sendIntent.setAction(Intent.ACTION_SEND);
    sendIntent.setType("text/plain");
    sendIntent.putExtra(Intent.EXTRA_TEXT, "This is my text to send.");
    sendIntent.putExtra("jid", smsNumber + "@s.whatsapp.net"); //phone number without "+" prefix
    sendIntent.setPackage("com.whatsapp");
    startActivity(sendIntent);
} catch(Exception e) {
    Toast.makeText(this, "Error/n" + e.toString(), Toast.LENGTH_SHORT).show();
 }

}

For more details please refer below link Send text to specific contact (whatsapp)