My second Android application

 实现功能：Activity之间的切换与传递值

MainActivity：

        val greetingText = findViewById<TextView>(R.id.printer)
        val inputField = findViewById<EditText>(R.id.inputer)
        val submitButton = findViewById<Button>(R.id.btn)
        submitButton.setOnClickListener {
            val message = inputField.text.toString()    //获取输入内容
            if (message == "") {    //当输入内容为空
                //Toast为系统提示，第一个参数为Context(Activity亦为一种Context)，
                //第二个参数为提示文本，第三个参数为显示时长
                Toast.makeText(
                    this,
                    "You haven't entered anything!",
                    Toast.LENGTH_SHORT
                ).show()
            } else {
                greetingText.text = "Welcomed! " + message
                inputField.text.clear()

                val intent = Intent(this, SecondActivity::class.java)
                intent.putExtra("USER", message)    //向下一个Activity传递数据
                startActivity(intent)
            }
        }

SecondActivity： 

        //获取上一个Activity传递的数据
        val name = intent.getStringExtra("USER")
        val textView = findViewById<TextView>(R.id.secTxt)
        textView.text = "This is the second activity!\nWelcome $name!"