Display back button on action bar and back event

原文地址：
https://stackoverflow.com/a/37185334
I think onSupportNavigateUp() is best and Easiest way to do so
check the code below
if you want it programmatically Add this line in onCreate() method

getSupportActionBar().setDisplayHomeAsUpEnabled(true);

Note: make sure Actionbar is not null.

And Override this method

@Override
public boolean onSupportNavigateUp(){  
    finish();  
    return true;  
}

thats it
OR Non-programmatically you can add meta to the activity in manifest file as

<meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value="MainActivity" />

Other solution:

mToolbar.setNavigationOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        onBackPressed();
    }
});