Calendar 得到前一天当前时间

@Test
    public void test(){
        //因为Calendar的构造方法是私有的，所以实例化一个Calendar对象用getInstance方法
        Calendar calendar = Calendar.getInstance();
        calendar.add(Calendar.DATE, -1);    //得到前一天
        Date date = calendar.getTime();
        DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        System.out.println(df.format(date));
        }

这是一个非常好的实例，易于理解，能够帮助我们轻松掌握Calendar的使用方法。可以看点源码，底层使用Map集合实现的

public Map<String, Integer> getDisplayNames(int field, int style, Locale locale) {
        if (!checkDisplayNameParams(field, style, ALL_STYLES, LONG, locale,
                                    ERA_MASK|MONTH_MASK|DAY_OF_WEEK_MASK|AM_PM_MASK)) {
            return null;
        }

        // ALL_STYLES
        if (style == ALL_STYLES) {
            Map<String,Integer> shortNames = getDisplayNamesImpl(field, SHORT, locale);
            if (field == ERA || field == AM_PM) {
                return shortNames;
            }
            Map<String,Integer> longNames = getDisplayNamesImpl(field, LONG, locale);
            if (shortNames == null) {
                return longNames;
            }
            if (longNames != null) {
                shortNames.putAll(longNames);
            }
            return shortNames;
        }

        // SHORT or LONG
        return getDisplayNamesImpl(field, style, locale);
    }

    private Map<String,Integer> getDisplayNamesImpl(int field, int style, Locale locale) {
        DateFormatSymbols symbols = DateFormatSymbols.getInstance(locale);
        String[] strings = getFieldStrings(field, style, symbols);
        if (strings != null) {
            Map<String,Integer> names = new HashMap<String,Integer>();
            for (int i = 0; i < strings.length; i++) {
                if (strings[i].length() == 0) {
                    continue;
                }
                names.put(strings[i], i);
            }
            return names;
        }
        return null;
    }