c.setText(Integer.parseInt(a.getText()) + Integer.parseInt(b.getText())+"");

c.setText(Integer.parseInt(a.getText()) + Integer.parseInt(b.getText())+"");
c.setText(Integer.parseInt(a.getText()) + Integer.parseInt(b.getText())+"");

c.setText(Integer.parseInt(a.getText()) + Integer.parseInt(b.getText())+"");
+""
把前面的东西转化成字符串
c.setText(Integer.parseInt(a.getText()) + Integer.parseInt(b.getText())+"");
等价于
c.setText(String.valueOf(Integer.parseInt(a.getText()) + Integer.parseInt(b.getText())));
个人推荐用前面的方法,因为String.valueOf(参数)里面的参数如果为null会异常,但是null+""不会异常.