前言

android studio 已經有內建的日期元件了，我們知將他引用即可

操作

第一步：在局文件創一個 button 和 textview

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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/data"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/databtn"
        android:layout_width="168dp"
        android:layout_height="76dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

第二步：把佈局文件與主程式連結

 package com.example.data;

import androidx.appcompat.app.AppCompatActivity;

import android.app.DatePickerDialog;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.EditText;
import android.widget.TextView;

import java.util.Calendar;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    private TextView data;
    private String online_date;
    private Button databtn;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        data =  findViewById(R.id.data);
        databtn =  findViewById(R.id.databtn);
        databtn.setOnClickListener(this);
    }

第三步：當按鈕被按下時，顯示日歷

@Override
    public void onClick(View view) {
        Calendar c = Calendar.getInstance();
        new DatePickerDialog(MainActivity.this, new DatePickerDialog.OnDateSetListener() {

            @Override
            public void onDateSet(DatePicker view, int year, int monthOfYear, int dayOfMonth) {
                // TODO Auto-generated method stub
                data.setText(year+"/"+(monthOfYear+1)+"/"+dayOfMonth);
                online_date=year+"/"+(monthOfYear+1)+"/"+dayOfMonth;
            }
        }, c.get(Calendar.YEAR), c.get(Calendar.MONTH), c.get(Calendar.DAY_OF_MONTH)).show();
        data.setText(online_date);
    }