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java自学笔记(1)

unadlib 2023-01-13 阅读 174

  java的ArrayList类型,就是一个数组; 我们可以通过java.util的 iterator进行迭代.

import java.util.ArrayList;
import java.util.Iterator;

public class ArrayListIterator {
public static void main(String[] args){
ArrayList list = new ArrayList();
list.add("a");
list.add("b");
list.add("c");
System.out.println("the list adds a,b,c");

Iterator it = list.iterator();
System.out.print("iterator results is:");

while(it.hasNext()){
System.out.print(it.next() + ",");
};

System.out.println();

list.clear();
System.out.println("you has been remove list values");
}
}

 

java自学笔记(1)_java

 

  利用FileOutputStreamObjectOutputStream把object保存到指定的文件中,

  再次利用FileInputStreamObjectInputStream打开我们上次保存的文件,

  这里的new 的Box继承了Serializable;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

public class Box implements Serializable {
private static final long serialVersionUID = -3450064362986273896L;

private int width;
private int height;

public static void main(String[] args) {
Box myBox=new Box();
myBox.setWidth(50);
myBox.setHeight(30);
try {
FileOutputStream fs=new FileOutputStream("E:\\foo.txt");
ObjectOutputStream os=new ObjectOutputStream(fs);
os.writeObject(myBox);
os.close();

FileInputStream fi=new FileInputStream("E:\\foo.txt");
ObjectInputStream oi=new ObjectInputStream(fi);
Box box=(Box)oi.readObject();
oi.close();
System.out.println(box.height+","+box.width);
} catch (Exception e) {
e.printStackTrace();
}
}

public int getWidth() {
return width;
}
public void setWidth(int width) {
this.width = width;
}
public int getHeight() {
return height;
}
public void setHeight(int height) {
this.height = height;
}
}

 

java自学笔记(1)_System_02

 

  利用java.text.SimpleDateFormat 格式化日期

public class Datetime {

//会输出当前的时间;
public static void main(String args[]){
java.util.Date current=new java.util.Date();
java.text.SimpleDateFormat sdf=new java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String c=sdf.format(current);
System.out.println(c);
}

}

 

java自学笔记(1)_java_03

 

  HashMap和Iterator的使用:

import java.util.HashMap;
import java.util.Iterator;

public class DmHashMap {
public static void main (String[] args) {
System.out.println("****HashMap****");

HashMap hm = new HashMap();
hm.put("a", "0");
hm.put("b", "1");
hm.put("c", "2");
hm.put("d", "3");

//相当于js中数组的indexOf方法, 返回的是布尔值;
System.out.println( hm.containsKey("a") );

//相当于js中数组的indexOf方法;
System.out.println( hm.get("a") );

//返回一个对象
System.out.println( hm.entrySet() );

Iterator it = hm.entrySet().iterator();

while( it.hasNext() ) {
System.out.println( it.next() );
};

//获取所有的key
it = hm.keySet().iterator();

while( it.hasNext() ) {
System.out.println( it.next() );
};

//获取所有的value
it = hm.values().iterator();

while( it.hasNext() ) {
System.out.println( it.next() );
};
};
};

 

  循环的顺序是从后面往前面

java自学笔记(1)_java_04

 

  DecimalFormat是格式化数字以后小数点的工具方法:

import java.text.DecimalFormat;
public class DoubleFormate {
public static void main(String[] args) {
java.text.DecimalFormat df=new java.text.DecimalFormat("0.00");
double d1=123456789.123456;
double d2=987654321.987654321;

System.out.println("format1_d1="+df.format(d1));
System.out.println("format1_d2="+df.format(d2));

DecimalFormat dff = new DecimalFormat("#,##0.00");
System.out.println("format2_d1="+dff.format(d1));
System.out.println("format2_d2="+dff.format(d2));
}
}

java自学笔记(1)_数组_05

 

  GregorianCalendar是很强大的格式化时间工具方法:

import java.util.*;

public class ShowDate {

public static void main(String[] args) {
Calendar calendar = new GregorianCalendar();
Date trialTime = new Date();
calendar.setTime(trialTime);

// PRint out a bunch of interesting things
System.out.println("ERA: " + calendar.get(Calendar.ERA));
System.out.println("YEAR: " + calendar.get(Calendar.YEAR));
System.out.println("MONTH: " + calendar.get(Calendar.MONTH));
System.out.println("WEEK_OF_YEAR: " + calendar.get(Calendar.WEEK_OF_YEAR));
System.out.println("WEEK_OF_MONTH: " + calendar.get(Calendar.WEEK_OF_MONTH));
System.out.println("DATE: " + calendar.get(Calendar.DATE));
System.out.println("DAY_OF_MONTH: " + calendar.get(Calendar.DAY_OF_MONTH));
System.out.println("DAY_OF_YEAR: " + calendar.get(Calendar.DAY_OF_YEAR));
System.out.println("DAY_OF_WEEK: " + calendar.get(Calendar.DAY_OF_WEEK));
System.out.println("DAY_OF_WEEK_IN_MONTH: " + calendar.get(Calendar.DAY_OF_WEEK_IN_MONTH));
System.out.println("AM_PM: " + calendar.get(Calendar.AM_PM));
System.out.println("HOUR: " + calendar.get(Calendar.HOUR));
System.out.println("HOUR_OF_DAY: " + calendar.get(Calendar.HOUR_OF_DAY));
System.out.println("MINUTE: " + calendar.get(Calendar.MINUTE));
System.out.println("SECOND: " + calendar.get(Calendar.SECOND));
System.out.println("MILLISECOND: " + calendar.get(Calendar.MILLISECOND));
System.out.println("ZONE_OFFSET: " + (calendar.get(Calendar.ZONE_OFFSET)/(60*60*1000)));
System.out.println("DST_OFFSET: " + (calendar.get(Calendar.DST_OFFSET)/(60*60*1000)));

System.out.println("Current Time, with hour reset to 3");
calendar.clear(Calendar.HOUR_OF_DAY); // so doesn't override
calendar.set(Calendar.HOUR, 3);
System.out.println("ERA: " + calendar.get(Calendar.ERA));
System.out.println("YEAR: " + calendar.get(Calendar.YEAR));
System.out.println("MONTH: " + calendar.get(Calendar.MONTH));
System.out.println("WEEK_OF_YEAR: " + calendar.get(Calendar.WEEK_OF_YEAR));
System.out.println("WEEK_OF_MONTH: " + calendar.get(Calendar.WEEK_OF_MONTH));
System.out.println("DATE: " + calendar.get(Calendar.DATE));
System.out.println("DAY_OF_MONTH: " + calendar.get(Calendar.DAY_OF_MONTH));
System.out.println("DAY_OF_YEAR: " + calendar.get(Calendar.DAY_OF_YEAR));
System.out.println("DAY_OF_WEEK: " + calendar.get(Calendar.DAY_OF_WEEK));
System.out.println("DAY_OF_WEEK_IN_MONTH: " + calendar.get(Calendar.DAY_OF_WEEK_IN_MONTH));
}
}

java自学笔记(1)_System_06

 

  使用for循环,循环ArrayList中保存的HashMap数据:

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class sortDemo {
public static void main(String[] args) {
List<Map<String, String>> list = new ArrayList<Map<String, String>>();
Map<String, String> map1 = new HashMap<String, String>();
map1.put("FINISH_DATA=2013-03-09", "BEGIN_DATA=2013-03-03");
Map<String, String> map2 = new HashMap<String, String>();
map2.put("FINISH_DATA=2013-01-19", "BEGIN_DATA=2013-01-13");
Map<String, String> map3 = new HashMap<String, String>();
map3.put("FINISH_DATA=2013-01-12", "BEGIN_DATA=2013-01-06");
Map<String, String> map4 = new HashMap<String, String>();
map4.put("FINISH_DATA=2013-01-05", "BEGIN_DATA=2012-12-30");
list.add(map1);
list.add(map2);
list.add(map3);
list.add(map4);
for (Map<String, String> m : list) {
for (Map.Entry<String, String> en : m.entrySet()) {
System.out.println(en.getKey() + " , " + en.getValue());
}
}
}
}

 

java自学笔记(1)_java_07

 

  java里面有三种数组ArrayList, Vector, 以及LinkedList;

import java.util.Date;
import java.util.ArrayList;
import java.util.Vector;
import java.util.Iterator;
import java.util.LinkedList;

public class TestIterator{
public static void main(String[] args) {
ArrayList a = new ArrayList();
a.add("China");
a.add("USA");
a.add("Korea");
Iterator it = a.iterator();
while(it.hasNext()){
String country = (String)it.next();
System.out.println(country);
};

Vector v = new Vector();
v.addElement(new Date());
v.addElement(new Date(200008755554L));
it = v.iterator();
while(it.hasNext()){
Date time = (Date)it.next();
System.out.println(time);
};

LinkedList l = new LinkedList();
l.add("qihao");
l.add("nono");
l.add("hehe");
l.add("yeah");
Iterator ite = l.iterator();
while( ite.hasNext() ){
String i = (String) ite.next();
System.out.println( i );
};
}
}

 

java自学笔记(1)_数组_08

 

  String是不变类,用String修改字符串会新建一个String对象,如果频繁的修改,将会产生很多的String对象,开销很大.因此java提供了一个StringBuffer类,这个类在修改字符串方面的效率比String高了很多。

public class UsingStringBuffer {
public static void testFindStr() {
StringBuffer sb = new StringBuffer();
sb.append("This is a StringBuffer");
System.out.println("sb.indexOf(\"is\")=" + sb.indexOf("is"));
System.out.println("sb.indexOf(\"is\")=" + sb.indexOf("is", 3));
System.out.println("sb.lastIndexOf(\"is\") = " + sb.lastIndexOf("is"));
System.out.println("sb.lastIndexOf(\"is\", 1) = "
+ sb.lastIndexOf("is", 1));
}

public static void main (String[] args) {
UsingStringBuffer us = new UsingStringBuffer();
us.testFindStr();
us.testSubStr();
us.testCharAtStr();
us.testAppend();
us.testDelete();
us.testInsert();
}

public static void testSubStr() {
StringBuffer sb = new StringBuffer();
sb.append("This is a StringBuffer");
System.out.print("sb.substring(4)=" + sb.substring(4));
System.out.print("sb.substring(4,9)=" + sb.substring(4, 9));
}

public static void testCharAtStr() {
StringBuffer sb = new StringBuffer("This is a StringBuffer");
System.out.println(sb.charAt(sb.length() - 1));
}

public static void testAppend() {
StringBuffer sb = new StringBuffer("This is a StringBuffer!");
sb.append(1.23f);
System.out.println(sb.toString());
}

public static void testDelete() {
StringBuffer sb = new StringBuffer("This is a StringBuffer!");
sb.delete(0, 5);
sb.deleteCharAt(sb.length() - 1);
System.out.println(sb.toString());
}
public static void testInsert() {
StringBuffer sb = new StringBuffer("This is a StringBuffer!");
sb.insert(2, 'W');
sb.insert(3, new char[] { 'A', 'B', 'C' });
sb.insert(8, "abc");
sb.insert(2, 3);
sb.insert(3, 2.3f);
sb.insert(6, 3.75d);
sb.insert(5, 9843L);
sb.insert(2, true);
System.out.println("testInsert: " + sb.toString());
}

public static void testReplace() {
StringBuffer sb = new StringBuffer("This is a StringBuffer!");
sb.replace(10, sb.length(), "Integer");
System.out.println("testReplace: " + sb.toString());
}

public static void reverseStr() {
StringBuffer sb = new StringBuffer("This is a StringBuffer!");
System.out.println(sb.reverse());
}
}

java自学笔记(1)_java_09

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