java8 Date LocalDate LocaDateTime 互相轉化

java 8中 java.util.Date 類新增了兩個方法，分別是from(Instant instant)和toInstant()方法

// Obtains an instance of Date from an Instant object.
public static Date from(Instant instant) {
    try {
        return new Date(instant.toEpochMilli());
    } catch (ArithmeticException ex) {
        throw new IllegalArgumentException(ex);
    }
}

// Converts this Date object to an Instant.
public Instant toInstant() {
    return Instant.ofEpochMilli(getTime());
}

這兩個方法使我們可以方便的實現將舊的日期類轉換為新的日期類，具體思路都是通過Instant當中介，然后通過Instant來創建LocalDateTime（這個類可以很容易獲取LocalDate和LocalTime），新的日期類轉舊的也是如此，將新的先轉成LocalDateTime，然后獲取Instant，接着轉成Date，具體實現細節如下：

    @Test
    public void method3(){
        LocalDateTime localDateTime  = LocalDateTime.now();
        System.out.println("localDateTime: "+localDateTime);
        Instant instant =  localDateTime.atZone(ZoneId.systemDefault()).toInstant();
        Date date = Date.from(instant);
        System.out.println("date: "+date);
    }

輸出結果
localDateTime: 2018-11-19T10:22:00.712
date: Mon Nov 19 10:22:00 CST 2018

    @Test
    public void method4(){
        LocalDate localDate = LocalDate.now();
        System.out.println("localDate: "+localDate);
        Instant instant =  localDate.atStartOfDay(ZoneId.systemDefault()).toInstant();
        Date date = Date.from(instant);
        System.out.println("date: "+date);
    }

輸出結果
localDate: 2018-11-19
date: Mon Nov 19 00:00:00 CST 2018

    @Test
    public void method5(){
        LocalDateTime localDateTime = LocalDateTime.now();
        LocalDate localDate = localDateTime.toLocalDate();
        System.out.println(localDate);
    }
    
    @Test
    public void method6(){
        LocalDate localDate = LocalDate.now();
        LocalDateTime localDateTime = localDate.atStartOfDay();
        System.out.println(localDateTime);
    }

    @Test
    public void method1(){
        Date d1 = new Date();
        System.out.println("date: "+d1);
        Instant instant =  d1.toInstant();
        LocalDateTime local1 = LocalDateTime.ofInstant(instant, ZoneId.systemDefault());
        System.out.println("LocalDateTime: "+local1);
    }



輸出結果
date: Mon Nov 19 09:37:14 CST 2018
LocalDateTime: 2018-11-19T09:37:14.063

    @Test
    public void method2(){
        Date d1 = new Date();
        System.out.println("date: "+d1);
        Instant instant =  d1.toInstant();
        LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneId.systemDefault());
        LocalDate ld = ldt.toLocalDate();
        System.out.println("LocalDate: "+ld);
    }

輸出結果
date: Mon Nov 19 09:46:30 CST 2018
LocalDate: 2018-11-19