Here is  the complete java file Serial_monitor.java first download it and compare it to these small code segments.
https://content.instructables.com/F3T/71R7/IJUCQVC1/F3T71R7IJUCQVC1.java

Below are detailled description of the code:

First are  created all  UI elements and library variables

    Button btOpen, btClose,  btWrite;
    EditText etWrite;
    TextView tvRead;
    Spinner spBaud;
    CheckBox cbAutoscroll;

    Physicaloid mPhysicaloid; 		// initialising library

Second, in onCreate method we can initialise those variables we have created

        btOpen  = (Button) findViewById(R.id.btOpen);
        btClose = (Button) findViewById(R.id.btClose);
        btWrite = (Button) findViewById(R.id.btWrite);
        etWrite = (EditText) findViewById(R.id.etWrite);
        tvRead  = (TextView) findViewById(R.id.tvRead);
        spBaud = (Spinner) findViewById(R.id.spBaud);
        cbAutoscroll = (CheckBox)findViewById(R.id.autoscroll);

        mPhysicaloid = new Physicaloid(this); // setting the context for library

Now we can display Required UI elements on screen, calling a method to do this

That method takes boolean argument which specifies weather app is connected to Arduino or not

setEnabledUi(false); // not connected to Arduino so false

//setEnabledUi method to set UI elements on screen
private void setEnabledUi(boolean on) {
        if(on) {                            // if connected to device
            btOpen.setEnabled(false);       //hide open button (already opened)
            spBaud.setEnabled(false);       //hide baudrate selector
            cbAutoscroll.setEnabled(false); // hide autoscroll
            btClose.setEnabled(true);       // display close button
            btWrite.setEnabled(true);       // display send button
            etWrite.setEnabled(true);       // display edittext field
        } else {                            // if not connected to device
            btOpen.setEnabled(true);        //display open button
            spBaud.setEnabled(true);        //display baudrate selector
            cbAutoscroll.setEnabled(true);  //display autoscroll
            btClose.setEnabled(false);      // hide close button (already closed)
            btWrite.setEnabled(false);      // hide send button
            etWrite.setEnabled(false);      // hide edittext field
        }
    }

Now we displayed everything needed, and we can add actions to our buttons.

listener method for open button, In this method we cover baudrate setting, autoscroll option and connecting to device.

 public void onClickOpen(View v) {
	// setting the baudrate based on spinner
        String baudtext = spBaud.getSelectedItem().toString(); 	// get the text from spinner
	//switch statement to check for baud rate
        switch (baudtext) {
            case "300 baud":
                mPhysicaloid.setBaudrate(300);
                break;
            case "1200 baud":
                mPhysicaloid.setBaudrate(1200);
                break;
            case "2400 baud":
                mPhysicaloid.setBaudrate(2400);
                break;
            case "4800 baud":
                mPhysicaloid.setBaudrate(4800);
                break;
            case "9600 baud":
                mPhysicaloid.setBaudrate(9600);
                break;
            case "19200 baud":
                mPhysicaloid.setBaudrate(19200);
                break;
            case "38400 baud":
                mPhysicaloid.setBaudrate(38400);
                break;
            case "576600 baud":
                mPhysicaloid.setBaudrate(576600);
                break;
            case "744880 baud":
                mPhysicaloid.setBaudrate(744880);
                break;
            case "115200 baud":
                mPhysicaloid.setBaudrate(115200);
                break;
            case "230400 baud":
                mPhysicaloid.setBaudrate(230400);
                break;
            case "250000 baud":
                mPhysicaloid.setBaudrate(250000);
                break;
            default: 		// default is 9600
                mPhysicaloid.setBaudrate(9600);
        }

        if(mPhysicaloid.open()) { 	// tries to connect to device and if device was connected
            setEnabledUi(true);

            if(cbAutoscroll.isChecked()) { 	// if auto scroll was selected
                tvRead.setMovementMethod(new ScrollingMovementMethod());
            }   

	   // read listener, When new data is received from Arduino add it to Text view
            mPhysicaloid.addReadListener(new ReadLisener() {
                @Override
                public void onRead(int size) {
                    byte[] buf = new byte[size];
                    mPhysicaloid.read(buf, size);
                    tvAppend(tvRead, Html.fromHtml("<font color=blue>" + new String(buf) + "</font>"));// add data to text viiew
                }
            });

        } else {
		//Error while connecting
            	Toast.makeText(this, "Cannot open", Toast.LENGTH_LONG).show();
        }
    }

tvAppend method to write received data to Textview

Handler mHandler = new Handler();
    private void tvAppend(TextView tv, CharSequence text) {
        final TextView ftv = tv;
        final CharSequence ftext = text;
        mHandler.post(new Runnable() {
            @Override
            public void run() {
                ftv.append(ftext); 	// add text to Text view
            }
        });
    }

Listener for close button

public void onClickClose(View v) {	//when close button is pressed
        if(mPhysicaloid.close()) { 	//close the connection to arduino
            mPhysicaloid.clearReadListener();	//clear read listener
            setEnabledUi(false);	// set UI accordingly
        }
    }

Listeener for Send button

 public void onClickWrite(View v) { 	// when send button is prressed
        String str = etWrite.getText().toString()+"\r\n";	//get text from EditText
        if(str.length()>0) {
            byte[] buf = str.getBytes();	//convert string to byte array
            mPhysicaloid.write(buf, buf.length);	//write data to arduino
        }
    }

That’s all everything is completed just connect your phone and run the program it should install and open.