back button functionality in android app

I have to develop one android app. Here I wish to go back means I have used keyboard button only.but am shouldn't use keyboard back button. I wish to use the back button function for all activities.

For example:

If I have to click on the customize back button means I should able to go to the previous screen. How can I do?

MainActivity :

next.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                Intent intent=new Intent(MainActivity.this,Second.class);
                startActivity(intent);
            }
        });

SecondActivity :

pre.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                Intent intent = new Intent();
                finish();
            }
        });