I'm new in programming and I can't understand why there are errors on every `public void onClick(View v) { " and warning on "(new OnClickListener);`

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

    //mana lapu parslegshnas
    Button matematikaslapa = (Button) findViewById(R.id.poga1);
    matematikaslapa.setOnClickListener(new OnClickListener);

    @Override
    public void onClick(View v) {
        Intent myIntent = new Intent(v.getContext(), matematikaslapa.class);
        v.getContext().startActivity(myIntent);
    }

    Button fizikaslapa = (Button) findViewById(R.id.poga2);
    fizikaslapa.setOnClickListener(new OnClickListener);

    @Override
    public void onClick(View v) {
        Intent myIntent = new Intent(v.getContext(), fizikaslapa.class);
        v.getContext().startActivity(myIntent);
    }

    Button kimijaslapa = (Button) findViewById(R.id.poga3);
    kimijaslapa.setOnClickListener(new OnClickListener);

    @Override
    public void onClick(View v) {
        Intent myIntent = new Intent(v.getContext(), kimijaslapa.class);
        v.getContext().startActivity(myIntent);
    }
    //beigas

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
}

```