请使用下面的代码来实现这一点。

String contact = "+92 3122804640"; // use country code with your phone number
        String url = "https://api.whatsapp.com/send?phone=" + contact;
        try {
            PackageManager pm = getApplicationContext().getPackageManager();
            pm.getPackageInfo("com.whatsapp", PackageManager.GET_ACTIVITIES);
            Intent i = new Intent(Intent.ACTION_VIEW);
            i.setData(Uri.parse(url));
            startActivity(i);
        } catch (PackageManager.NameNotFoundException e) {
            Toast.makeText(MainActivity.this, "Whatsapp app not installed in your phone", Toast.LENGTH_SHORT).show();
            e.printStackTrace();
        }

我使用的是：

String phone = "+8801510101010";

            PackageManager packageManager = MainActivity.this.getPackageManager();
            Intent i = new Intent(Intent.ACTION_VIEW);

            try {
                String url = "https://api.whatsapp.com/send?phone="+ phone;
                i.setPackage("com.whatsapp");
                i.setData(Uri.parse(url));
                if (i.resolveActivity(packageManager) != null) {
                    MainActivity.this.startActivity(i);
                }
            } catch (Exception e){
                e.printStackTrace();
            }