@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;
}