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import java.util.*; public class Birthdays { public static void main(String[] args){ Map<Friends, String> hm = new HashMap<Friends,String>(); Friends f1 = new Friends("Charis"); Friends f2 = new Friends("Darling"); hm.put(f1, "summer 2009"); hm.put(f2, "spring 2002"); System.out.println(hm.get(f1)); Friends f3 = new Friends("Darling"); System.out.println(hm.get(f3)); } } class Friends{ String name; Friends(String n){ name = n; } }
结果:
summer 2009 null
2. enum用法
public class WeatherTest {
static Weather w;
public static void main(String[] args){
System.out.println(w.RAINY.count+" "+w.Sunny.count+" ");
}
}
enum Weather{
RAINY, Sunny;
int count = 0;
Weather(){
System.out.print("c ");
count++;
}
}
结果
c c 1 1
public class WeatherTest {
static Weather w;
public static void main(String[] args){
System.out.println(w.RAINY+" "+w.Sunny+" ");
}
}
enum Weather{
RAINY(1), Sunny(2);
private int n;
private Weather(int n){
this.n=n;
}
@Override
public String toString() {
return String.valueOf(this.n);
}
}
结果
1 2
枚举的遍历:
public class WeatherTest {
enum Weather{
RAINY(1), Sunny(2);
private int n;
private Weather(int n){
this.n=n;
}
@Override
public String toString() {
return String.valueOf(this.n);
}
}
public static void main(String[] args){
Weather[] allW = Weather.values();
for(Weather w: allW)
System.out.print(w+" ");
}
}
结果同上, 1 2
3. 数值类型题
public class Dec26 {
public static void main(String[] args){
short a1=6;
new Dec26().go(a1);
new Dec26().go(new Integer(7));
}
void go(short x){
System.out.print("S ");
}
void go(Long x){
System.out.print("L ");
}
void go(int x){
System.out.print("i ");
}
void go(Number n){
System.out.print("N ");
}
}
结果:
S N
4. switch, case
public class Humping {
public static void main(String[] args) {
String r="-";
char[] c={‘a‘,‘b‘,‘c‘,‘z‘};
for(char c1:c)
switch(c1){
case ‘a‘:
r+="a";
case ‘b‘:
r+="b";
break;
default:
r+="X";
case ‘z‘:
r+="z";
}
System.out.println(r);
}
}
结果:
-abbXzz
5. HashMap键值为空的问题
HashMap：适用于在Map中插入、删除和定位元素。
Treemap：适用于按自然顺序或自定义顺序遍历键(key)。
两者都是非线程安全，前者无排序，后者会自动排序
Ø HashMap里面存入的键值对在取出的时候是随机的，它根据键的HashCode值存储数据,根据键可以直接获取它的值，具有很快的访问速度。在Map 中插入、删除和定位元素，HashMap是最好的选择。
Ø TreeMap取出来的是排序后的键值对。插入、删除需要维护平衡会牺牲一些效率。但如果要按自然顺序或自定义顺序遍历键，那么TreeMap会更好。
本测试增加和查找功能，HashMap比TreeMap的效率要高。
import java.util.*;
public class Garage {
public static void main(String[] args) {
Map<String,String> hm=new HashMap<String,String>();
String[] k={null,"2","3",null,"5"};
String[] v={"a","b","c","d","e"};
for(int i=0;i<5;i++){
hm.put(k[i],v[i]);
System.out.print(hm.get(k[i])+" ");
}
System.out.print(hm.size()+" "+hm.values()+"\n");
}
}
结果
a b c d e 4 [c, d, b, e]
换成TreeMap报错, 所以 TreeMap键值不允许空
null和" "的区别:
import java.util.*;
public class Garage {
public static void main(String[] args) {
Map<String,String> hm=new HashMap<String,String>();
String[] k={null,"2","3"," ","5"};
String[] v={"a","b","c","d","e"};
for(int i=0;i<5;i++){
hm.put(k[i],v[i]);
System.out.print(hm.get(k[i])+" ");
}
System.out.print(hm.size()+" "+hm.keySet()+" "+hm.values()+"\n");
}
}
结果
a b c d e 5 [3, null, 2, , 5] [c, a, b, d, e]
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原文地址：http://www.cnblogs.com/wujixing/p/5534916.html
