數組包裝成list
import java.util.*;
public class test {
public static void main(String[] args) throws Exception {
List<Integer> list=Arrays.asList(1,2,3,4,5,6,7,8,9);
try{
list.add(10);
}catch(Exception e){
System.out.printf("can not add\n");
}
finally{
}
try{
list.remove(9);
}catch(Exception e){
System.out.printf("can not remove\n");
}
finally{
}
List<Integer> list2=new ArrayList<>(Arrays.asList(1,2,3,4,5,6,7,8,9));
list2.add(10);
list2.remove(9);
}
}
/*
output:
can not add
can not remove
*/List、map嵌入list 、map 遍歷
import java.util.*;
public class test2 {
public void mapInList(){
List<Map> listmap = new ArrayList<Map>();
Map<Integer, String> map = new HashMap<Integer, String>();
Map<Integer, Integer> map2 = new HashMap<Integer, Integer>();
map.put(1, "aa");
map.put(2, "bb");
map.put(3, "cc");
map2.put(1, 1001);
map2.put(2, 1002);
map2.put(3, 1003);
listmap.add(map);
listmap.add(map2);
System.out.printf("map in list:\n");
for (Map m : listmap) {
Iterator<Map.Entry<Object, Object>> it = m.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<Object, Object> entry = it.next();
System.out.print("["+entry.getKey() + " " + entry.getValue()+"] ");
}
System.out.printf("\n");
}
}
public void listInMap(){
Map<Integer,List<String>> maplist=new HashMap<Integer,List<String>>();
List<String> tmp=new ArrayList<String>();
List<String> tmp2=new ArrayList<String>();
tmp.add("a");
tmp.add("bb");
tmp.add("ccc");
tmp2.add("ddd");
tmp2.add("ee");
tmp2.add("f");
maplist.put(1, tmp);
maplist.put(2, tmp2);
System.out.printf("list in map:\n");
for(Map.Entry<Integer, List<String>> entry:maplist.entrySet()){
System.out.print(entry.getKey()+"=> {");
for (String m : entry.getValue())
System.out.print("["+m+"] ");
System.out.printf("}\n");
}
}
public static void main(String[] args) {
test2 tmp=new test2();
tmp.mapInList();
tmp.listInMap();
}
}
/*output
map in list:
[1 aa] [2 bb] [3 cc]
[1 1001] [2 1002] [3 1003]
list in map:
1=> {[a] [bb] [ccc] }
2=> {[ddd] [ee] [f] }
*/Map key值覆蓋
import java.util.*;
public class test3 {
public static void main(String[] args) {
Map<Integer,String> map=new HashMap<Integer,String>();
map.put(1, "aa");
map.put(2, "bb");
map.put(3, "cc");
map.put(2, "dd");
for(Map.Entry<Integer, String> entry:map.entrySet()){
System.out.print(entry.getKey()+"=> ["+entry.getValue()+"]\n");
}
}
}
/*
output
1=> [aa]
2=> [dd]
3=> [cc]
*/泛型編程
參考文章：http://www.cnblogs.com/lwbqqyumidi/p/3837629.html
Sort list排序
import java.util.*;
class student implements Comparable<student>{
private int id;
private double result;
public student(int id,double result){
this.id=id;
this.result=result;
}
@Override
public int compareTo(student obj) {
if(this.result!=obj.result)
return (int)(obj.result-this.result);
return this.id-obj.id;
}
public String toString(){
return "["+id+":"+result+"]";
}
}
public class test4 {
public static void main(String[] args) {
List<student> tmp=new ArrayList<student>();
for(int i=0;i<20;i++){
tmp.add(new student(i+1000,(int)(Math.random()*15)));
}
Collections.sort(tmp);
for (student index : tmp) {
System.out.println(index);
}
}
}
/*
output
[1017:14.0]
[1003:13.0]
[1019:13.0]
[1008:12.0]
[1009:12.0]
[1001:11.0]
[1014:8.0]
[1011:7.0]
[1012:7.0]
[1013:7.0]
[1015:7.0]
[1002:6.0]
[1005:6.0]
[1007:6.0]
[1000:5.0]
[1004:5.0]
[1010:4.0]
[1016:4.0]
[1018:3.0]
[1006:0.0]
*/文件讀寫
import java.util.*;
import java.io.*;
public class test5 {
public static void main(String[] args) {
try{
BufferedWriter fileBuf = new BufferedWriter(new FileWriter("C:\\Users\\wk950\\Desktop\\test5.txt"));
BufferedReader buf = new BufferedReader(new InputStreamReader(System.in));
for(String str;;){
str = buf.readLine();
fileBuf.write(str);
fileBuf.newLine();
if(str.equals(""))break;
}
buf.close();
fileBuf.close();
}catch(IOException e){
System.out.printf("error");
}
}
}比較字符串string stringbuffer StringBuilder
public class test6 {
public void testString(int loopNum){
String teststr=new String();
long start=System.currentTimeMillis();
for(int i=0;i<loopNum;i++){
teststr+="a";
}
long end=System.currentTimeMillis();
System.out.printf("String Total Time: %10d\n",end-start);
}
public void testStringbuffer(int loopNum){
StringBuffer teststr=new StringBuffer();
long start=System.currentTimeMillis();
for(int i=0;i<loopNum;i++){
teststr.append("a");
}
long end=System.currentTimeMillis();
System.out.printf("Stringbuffer Total Time: %10d\n",end-start);
}
public void testStringBuilder(int loopNum){
StringBuilder teststr=new StringBuilder();
long start=System.currentTimeMillis();
for(int i=0;i<loopNum;i++){
teststr.append("a");
}
long end=System.currentTimeMillis();
System.out.printf("StringBuilder Total Time: %10d\n",end-start);
}
public static void main(String[] args) {
test6 tmp=new test6();
int num=100000;
tmp.testString(num);
tmp.testStringbuffer(num);
tmp.testStringBuilder(num);
}
}
/*
output
String Total Time: 4393
Stringbuffer Total Time: 7
StringBuilder Total Time: 5
*/實現遞歸展示文件夾
import java.io.*;
public class test7 {
private static int[] dirlist=new int[255];
public static void main(String[] args) {
File dir=new File("C:\\Users\\wk950\\Desktop\\001");
getAllFiles(dir,0);
}
public static void getLevel(int level,int flage)
{
for(int l=0;l<level-1;l++)
{
if(dirlist[l]==0)
System.out.printf("│ ");
else System.out.printf(" ");
}
if(flage>1){
System.out.printf("├───");
dirlist[level-1]=0;
}
else{
System.out.printf("└───");
dirlist[level-1]=1;
}
}
public static void getAllFiles(File dir,int level)
{
File[] files=dir.listFiles();
for(int i=0;i<files.length;i++)
{
if(files[i].isDirectory())
{
getLevel(level+1,files.length-i);
System.out.println(files[i].getName());
getAllFiles(files[i],level+1);
}
else
{
getLevel(level+1,files.length-i);
System.out.println(files[i].getName());
}
}
}
}
/*
├───.classpath
├───.project
├───.settings
│ └───org.eclipse.jdt.core.prefs
├───.vs
│ └───001
│ └───v14
│ └───.suo
├───001
│ ├───bin
│ │ ├───Debug
│ │ │ ├───001.exe
│ │ └───Release
│ ├───Form1.Designer.vb
│ ├───Form1.resx
│ ├───Form1.vb
│ ├───My Project
│ │ ├───Application.Designer.vb
│ │ ├───Application.myapp
│ └───obj
│ └───Debug
│ ├───001.exe
│ ├───001.pdb
│ ├───TempPE
│ │ └───My Project.Resources.Designer.vb.dll
│ ├───_001.Form1.resources
├───001.sln
├───bin
│ ├───adapter.class
│ ├───collectiontest
│ │ ├───collectiontest.class
│ ├───gui
│ │ ├───test1$1.class
│ ├───listtest
│ │ ├───student.class
│ │ ├───test.class
│ ├───number.class
│ ├───show
│ ├───teststatic
│ │ └───testStatic.java
│ └───throwstest
│ └───throwstest.java
└───test5.txt
*/計算日期之差
import java.text.*;
import java.util.*;
public class test8 {
public static void main(String[] args) {
Date date = new Date();
SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String dateString = formatter.format(date);
System.out.println(dateString);
}
}
/*
output
2016-11-06 17:12:34
*/
