Use OnClickListeners for All Categories - onCreate method in MainActivity.java

onCreate method in MainActivity.java

// Find the View that shows the numbers category
TextView numbers = (TextView) findViewById(R.id.numbers);

// Set a click listener on that View
numbers.setOnClickListener(new View.OnClickListener() {
   // The code in this method will be executed when the numbers View is clicked on.
   @Override
   public void onClick(View view) {
       Intent numbersIntent = new Intent(MainActivity.this, NumbersActivity.class);
       startActivity(numbersIntent);
   }
});

Note: If you don't want to create a Intent class, you can simply call the Intent constructor INSIDE the startActivity() method, like this:

TextView numbers = findViewById(R.id.numbers);

    numbers.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View v){
            startActivity(new Intent(MainActivity.this, NumbersActivity.class));
        }
    });
}

This is actually nice