Activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/test001"
        android:layout_width="wrap_content"
        android:layout_height="18dp"
        android:text="@string/test002"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp"
        android:onClick="clearId"
        android:text="クリア"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/test001" />

</android.support.constraint.ConstraintLayout>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.XXXXX.mynfc">

    <uses-feature android:name="android.hardware.nfc" android:required="true"/>
    <uses-permission android:name="android.permission.NFC"/>

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

MainActivity.java

package com.example.XXXXX.mynfc;

import android.app.PendingIntent;
import android.content.Intent;
import android.nfc.NfcAdapter;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
import java.util.Arrays;

public class MainActivity extends AppCompatActivity {

    private NfcAdapter mNfcAdapter; 

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // ボタンを設定
        Button button = findViewById(R.id.button2);

        // インスタンスを取得
        mNfcAdapter = android.nfc.NfcAdapter.getDefaultAdapter(this);
    }

    @Override
    protected void onResume() {
        super.onResume();

        // NFCかざし設定
        Intent intent = new Intent(this,this.getClass());

        intent.setFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);

        PendingIntent pendingIntent = PendingIntent.getActivity(getApplicationContext(), 0,intent, 0);

        mNfcAdapter.enableForegroundDispatch(this, pendingIntent, null, null );
    }

    @Override
    protected void onPause() {

        super.onPause();

        mNfcAdapter.disableForegroundDispatch(this); //ディスパッチを無効化する
    }

    @Override
    protected void onNewIntent(Intent intent){
        super.onNewIntent(intent);

        byte[] uid = intent.getByteArrayExtra(NfcAdapter.EXTRA_ID); // NFCのUID

        Toast.makeText(this, Arrays.toString(uid), Toast.LENGTH_SHORT).show(); // 表示

        StringBuilder tagId = new StringBuilder("ID(" + uid.length + "bytes):");

        for (int i=0; i<uid.length; i++){
             tagId.append(String.format("%02x",uid[i] & 0xff));
        }

        Toast.makeText(this, tagId, Toast.LENGTH_SHORT).show();  // 表示 その2

        TextView tv = (TextView)findViewById(R.id.test001);

        tv.setText(tagId);
    }

    public void clearId(View view) {
        TextView tv = (TextView) findViewById(R.id.test001);
        tv.setText("");
    }
}