java8 Date LocalDate LocaDateTime 互相转化

java 8中 java.util.Date 类新增了两个方法，分别是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());
}

这两个方法使我们可以方便的实现将旧的日期类转换为新的日期类，具体思路都是通过Instant当中介，然后通过Instant来创建LocalDateTime（这个类可以很容易获取LocalDate和LocalTime），新的日期类转旧的也是如此，将新的先转成LocalDateTime，然后获取Instant，接着转成Date，具体实现细节如下：

    @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);
    }

输出结果
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);
    }

输出结果
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);
    }



输出结果
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);
    }

输出结果
date: Mon Nov 19 09:46:30 CST 2018
LocalDate: 2018-11-19