Kotlin, Practically Speaking - https://creativedrewy.github.io/KotlinPracticallySpeaking/

ChainExample.kt

fun <A> chain(vararg funcs: (A) -> Unit): (A) -> Unit {
    return fun(a: A) {
        funcs.forEach { it(a) }
    }
}

fun logThenDoSomething() {
    chain({ Log.v("TAG", "Log this first, yo!")}, ::functionInMyClass)
}

CompanionSlide1.java

public class GroupEditActivity extends AppCompatActivity {

    public static final String GROUP_ID_EXTRA = "GROUP_ID_EXTRA";

    public static Intent createIntentForActivity(Context ctx, int id) {
        Intent intent = new Intent(ctx, GroupEditActivity.class);
        intent.putExtra(GROUP_ID_EXTRA, id);
        return intent;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.group_details);
    }
}

CompanionSlide2a.kt

class GroupEditActivity : AppCompatActivity() {
    public override fun onCreate(savedInstanceState: Bundle?) { ... }

    companion object {
        val GROUP_ID_EXTRA = "GROUP_ID_EXTRA"

        fun createIntentForActivity(ctx: Context, id: Int): Intent {
            val intent = Intent(ctx, GroupEditActivity::class.java)
            intent.putExtra(GROUP_ID_EXTRA, id)
            return intent
        }
    }
}

CompanionSlide2b.java

public class MyJavaActivity extends AppCompatActivity {
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        GroupEditActivity.Companion
            .createIntentForActivity(this, GroupEditActivity.Companion.getGROUP_ID_EXTRA(), 2);
    }
}

CompanionSlide3a.kt

companion object {
    const val GROUP_ID_EXTRA = "GROUP_ID_EXTRA"

    @JvmStatic
    fun createIntentForActivity(ctx: Context, extraFlag: String, id: Int): Intent {
        val intent = Intent(ctx, GroupEditActivity::class.java)
        intent.putExtra(extraFlag, id)
        return intent
    }
}

CompanionSlide3b.java

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    GroupEditActivity.createIntentForActivity(this, GroupEditActivity.GROUP_ID_EXTRA, 2);
}

JavaActivity.java

public class ConvertMeActivity extends AppCompatActivity {
    private Button myButton;

    @Inject protected MyViewModelFactory viewModelFactory;

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

        myButton = findViewById(R.id.my_button);
        myButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Snackbar.make(v, "hi!", Snackbar.LENGTH_SHORT).show();
            }
        });

        MyViewModel myViewModel = ViewModelProviders.of(this, viewModelFactory).get(MyViewModel.class);
        Boolean someBool = myViewModel.getHasListBeenModified();
    }

    private BottomNavigationView.OnNavigationItemSelectedListener listener = new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            switch (item.getItemId()) {
                case R.id.navigation_home:
                    myButton.setText(R.string.title_home);
                    return true;
                case R.id.navigation_dashboard:
                    myButton.setText(R.string.title_dashboard);
                    return true;
            }
            return false;
        }
    };
}

KotlinActivityBetter.kt

class ConvertMeActivity : AppCompatActivity() {
    private lateinit var myButton: Button

    @Inject internal lateinit var viewModelFactory: MyViewModelFactory

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_convert_me)

        myButton = findViewById(R.id.my_button)
        myButton.setOnClickListener { Snackbar.make(it, "hi!", Snackbar.LENGTH_SHORT).show() }

        val myViewModel = ViewModelProviders.of(this, viewModelFactory).get(MyViewModelFactory::class.java)
        val someBool = myViewModel.hasListBeenModified
    }

    private val listener = BottomNavigationView.OnNavigationItemSelectedListener {
        return@OnNavigationItemSelectedListener when (it.itemId) {
            R.id.navigation_home -> {
                myButton.setText(R.string.title_home)
                true
            }
            R.id.navigation_dashboard -> {
                myButton.setText(R.string.title_dashboard)
                true
            }
            else -> false
        }
    }
}

KotlinActivityDefault.kt

class ConvertMeActivity : AppCompatActivity() {
    private var myButton: Button? = null

    @Inject protected val viewModelFactory: MyViewModelFactory? = null

    override fun onCreate(savedInstanceState: Bundle) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_convert_me)

        myButton = findViewById(R.id.my_button)
        myButton!!.setOnClickListener { v -> Snackbar.make(v, "hi!", Snackbar.LENGTH_SHORT).show() }

        val myViewModel = ViewModelProviders.of(this, viewModelFactory).get(MyViewModel::class.java)
        val someBool = myViewModel.hasListBeenModified
    }

    private val listener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
        when (item.itemId) {
            R.id.navigation_home -> {
                myButton!!.setText(R.string.title_home)
                return@OnNavigationItemSelectedListener true
            }
            R.id.navigation_dashboard -> {
                myButton!!.setText(R.string.title_dashboard)
                return@OnNavigationItemSelectedListener true
            }
        }
        false
    }
}

PackageFunction.kt

package com.yammer.droid.convenience

//This file is CalculatorFunctions.kt

fun addFiveToNumber(input: Int): Int {
  return 5 + input
}