ListView上一个和下一个按钮

这是我的密码。但它不会停止。当Listview最后一个索引号为10时,就崩溃了。如何在listView结束时停止“下一步”按钮。

btnNext.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            txtOne.setText(letter[lastView+1]);
            txtTwo.setText(letterDetails[lastView+1]);
            txtThree.setText(letterB[lastView+1]);

            lastView++;

            if (lastView == letter.length){
                lastView=0;
                Toast.makeText(getApplicationContext(), lastView+" ", Toast.LENGTH_SHORT).show();
            }

        }
    });
    btnPrevious.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            txtOne.setText(letter[lastView-1]);
            txtTwo.setText(letterDetails[lastView-1]);
            txtThree.setText(letterB[lastView-1]);
            lastView--;
        }
    });