How to implement Primary key OR unique key for user id in Sqlite for Android studio.

I'm new to Android Application Development. I am trying to Insert values in Database. My code is

public void onClick(View view)

{

        if(view==register)

        {
       if(user_id.getText().toString().trim().length()==0||
                    password.getText().toString().trim().length()==0||
                    email.getText().toString().trim().length()==0 || contact.getText().toString().trim().length()==0)
            {
                showMessage("Error", "Please enter all values");
                return;
            }

            db.execSQL("INSERT INTO customer VALUES('"+user_id.getText()+"','"+password.getText()+
                    "','"+email.getText()+"','"+contact.getText()+"');");
            showMessage("Success", "Record added"); // showMessage is Print function
            clearText();
        }
    }

As i inserted Duplicate user_id my application get closed. I want to display message dat User id is not available on duplicate entries. can i use try catch block ? or something else. Please suggest.