第一种方法：最简单实用的

Intent intent = new Intent(MainActivity.this,SecondActivity.class);
startActivity(intent);

方法二：一般不会用这个吧

Intent intent=new Intent();
ComponentName componentName = new ComponentName(MainActivity.this,SecondActivity.class);
intent.setComponent(componentName);
startActivity(intent);

跳转系统浏览器

Intent intent=new Intent();
intent.setAction(Intent.ACTION_VIEW);
Uri url = Uri.parse("https://baidu.com");
intent.setData(url);
startActivity(intent);

打开图库

Intent intent=new Intent();
intent.setAction(Intent.ACTION_GET_CONTENT);
intent.setType("image/*");
startActivity(intent);

发短信

Intent intent=new Intent();
intent.setAction(Intent.ACTION_SEND);
intent.setType("text/plain");
intent.putExtra(Intent.EXTRA_TEXT,"I am boy");
startActivity(intent);

打电话

 Intent intent=new Intent();
 intent.setAction(Intent.ACTION_VIEW);
Uri url = Uri.parse("tel:10086");
 intent.setData(url);
startActivity(intent);