一 , 集合的体会（Collection , list, set ,map)
1,遍历list的方法：
第一种方法：for-each方法
public class ListTest{
public static void main(String[] args){
List<String> list = new ArrayList<String>();
long t1,t2;
for(int j = 0; j < 10000000; j++)
{
list.add("aaaaaa" + j);
}
for(String tmp:list)
{
//System.out.println(tmp);
}
}
第二种方法：for-变量的方法
public class ListTest{
public static void main(String[] args){
List<String> list = new ArrayList<String>();
long t1,t2;
for(int j = 0; j < 10000000; j++)
{
list.add("aaaaaa" + j);
}
for(int i = 0; i < list.size(); i++)
{
list.get(i);
//System.out.println(list.get(i));
}
}
t2=System.currentTimeMillis();
}
第三种方法 ：迭代遍历方法
public class ListTest{
public static void main(String[] args){
List<String> list = new ArrayList<String>();
long t1,t2;
for(int j = 0; j < 10000000; j++)
{
list.add("aaaaaa" + j);
}
Iterator<String> iter = list.iterator();
t1=System.currentTimeMillis();
while(iter.hasNext())
{
iter.next();
//System.out.println(iter.next());
}
t2=System.currentTimeMillis();
// for (Iterator i = list.iterator(); i.hasNext();) {
//System.out.println(i.next());
//}
}
}
2，遍历Set的方法：
1.迭代遍历：
Set<String> set = new HashSet<String>();
Iterator<String> it = set.iterator();
while (it.hasNext()) {
String str = it.next();
System.out.println(str);
}
2.for循环遍历：
for (String str : set) {
System.out.println(str);
}
优点还体现在泛型 假如 set中存放的是Object
Set<Object> set = new HashSet<Object>();
for循环遍历：
for (Object obj: set) {
if(obj instanceof Integer){
int aa= (Integer)obj;
}else if(obj instanceof String){
String aa = (String)obj
}
}
3.Map 的遍历
//循环遍历map的方法
public class CircleMap {
public static void main(String[] args) {
Map<String, Integer> tempMap = new HashMap<String, Integer>();
tempMap.put("a", 1);
tempMap.put("b", 2);
tempMap.put("c", 3);
// 遍历方法一 hashmap entrySet() 遍历
System.out.println("方法一");
Iterator it = tempMap.entrySet().iterator();
while (it.hasNext()) {
Map.Entry entry = (Map.Entry) it.next();
Object key = entry.getKey();
Object value = entry.getValue();
System.out.println("key=" + key + " value=" + value);
}
System.out.println("");
// JDK1.5中,应用新特性For-Each循环
// 遍历方法二
System.out.println("方法二");
for (Map.Entry<String, Integer> entry : tempMap.entrySet()) {
String key = entry.getKey().toString();
String value = entry.getValue().toString();
System.out.println("key=" + key + " value=" + value);
}
System.out.println("");
// 遍历方法三 hashmap keySet() 遍历
System.out.println("方法三");
for (Iterator i = tempMap.keySet().iterator(); i.hasNext();) {
Object obj = i.next();
System.out.println(obj);// 循环输出key
System.out.println("key=" + obj + " value=" + tempMap.get(obj));
}
for (Iterator i = tempMap.values().iterator(); i.hasNext();) {
Object obj = i.next();
System.out.println(obj);// 循环输出value
}
System.out.println("");
// 遍历方法四 treemap keySet()遍历
System.out.println("方法四");
for (Object o : tempMap.keySet()) {
System.out.println("key=" + o + " value=" + tempMap.get(o));
}
System.out.println("11111");
// java如何遍历Map <String, ArrayList> map = new HashMap <String,
// ArrayList>();
System.out.println("java 遍历Map <String, ArrayList> map = new HashMap <String, ArrayList>();");
Map<String, ArrayList> map = new HashMap<String, ArrayList>();
Set<String> keys = map.keySet();
Iterator<String> iterator = keys.iterator();
while (iterator.hasNext()) {
String key = iterator.next();
ArrayList arrayList = map.get(key);
for (Object o : arrayList) {
System.out.println(o + "遍历过程");
}
}
System.out.println("2222");
Map<String, List> mapList = new HashMap<String, List>();
for (Map.Entry entry : mapList.entrySet()) {
String key = entry.getKey().toString();
List<String> values = (List) entry.getValue();
for (String value : values) {
System.out.println(key + " --> " + value);
}
}
}
}
Q：Comparable和Comparator区别
A:调用java.util.Collections.sort(List list)方法来进行排序的时候，List内的Object都必须实现了Comparable接口。
java.util.Collections.sort(List list，Comparator c)，可以临时声明一个Comparator 来实现排序。
Collections.sort(imageList, new Comparator() {
public int compare(Object a, Object b) {
int orderA = Integer.parseInt( ( (Image) a).getSequence());
int orderB = Integer.parseInt( ( (Image) b).getSequence());
return orderA - orderB;
}
});
如果需要改变排列顺序
改成return orderb - orderA 即可。