This gist will help you to use Crashlytics like a pro.

build.gradle

android {
    defaultConfig {
         manifestPlaceholders = [crashlyticsApiKey: "514bec26d15bb8f67dc8129f0eda3cabf79fXXXX"]
    }
    buildTypes {
        debug {
            
        }
        staging {
            manifestPlaceholders = [crashlyticsApiKey: "848bec26d15bb8f67dc8129f0eda3cabf79fXXXX"]
        }
        release {
            manifestPlaceholders = [crashlyticsApiKey: "427bec26d15bb8f67dc8129f0eda3cabf79fXXXX"]
        }
    }
}


  <meta-data
       android:name="io.fabric.ApiKey"
       android:value="${crashlyticsApiKey}" />

build.gradle(app)

  android {
    defaultConfig {
         buildConfigField 'Boolean', 'enableCrashlytics', 'false'
    }
    buildTypes {
        debug {
            buildConfigField 'Boolean', 'enableCrashlytics', 'false'
        }
        staging {
            buildConfigField 'Boolean', 'enableCrashlytics', 'true'
        }
        release {
            buildConfigField 'Boolean', 'enableCrashlytics', 'true'
        }
    }
}
  
if (BuildConfig.enableCrashlytics)
      Fabric.with(this, new Crashlytics());

android {
    buildTypes {
        debug {
          // Disable fabric build ID generation for debug builds
          ext.enableCrashlytics = false
          ...
          
// Set up Crashlytics, disabled for debug builds
Crashlytics crashlyticsKit = new Crashlytics.Builder()
    .core(new CrashlyticsCore.Builder().disabled(BuildConfig.DEBUG).build())
    .build();

// Initialize Fabric with the debug-disabled crashlytics.
Fabric.with(this, crashlyticsKit);

CrashlyticsDemoActivity.java

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;

import com.crashlytics.android.Crashlytics;

public class CrashlyticsDemoActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sample);

Crashlytics.setUserIdentifier("12345");
Crashlytics.setUserEmail("jaisonfdo@gmail.com");
Crashlytics.setUserName("Jaison");

        try {
            String value=null;
            int x=Integer.parseInt(value);

        } catch (Exception e) {
        e.printStackTrace();
Crashlytics.log("Test from sample activity");
Crashlytics.logException(e);
        }
    }
}