1. Why do you need packaging? Wrapper class action is?
Because the java language is object-oriented language, but the basic data types in the java is not object-oriented, but in actual use is often necessary basic data into objects, easy to operate. For example, when storing data sets, you can store objects
effect:
[1] and the presence of basic data types as corresponding to the class type, easy operation relates to the subject
[2] contains basic data type associated with each property and the related method of operation
2. The string "123" into the digital basic types of options?
Integer class static method to convert
Integer.parseInt(“123”);
3. Auto boxing and auto-unboxing mean? Illustration.
Automatic boxing and unboxing automatically refers to the basic data types and automatic packaging interconversion
237. [machine] to complete the classroom teacher Integer test, and write their own test code class Double
public class TestDouble {
public static void main(String[] args) {
Double int1 = new Double(10);
Double int2 = Double.valueOf(20);
double a = int1.doubleValue();
Double int3 = Double.parseDouble("334");
Double int4 = new Double("999");
String str1 = int3.toString();
}
}
238. Why String class is called immutable character sequence? From the String class source code analysis, explanation is given.
Strings are constants, their values can not be changed after creation, String objects are immutable, so you can share
String class underlying structure is an array of char value, and this array can be modified with the final
239. [machine] equals method with the String class equals method of Object What is the relationship? And read more of the String class equals method source code analysis of its internal processes.
String class equals method of Object class overrides the equals method for comparing two objects have the same contents String
Source String class equals method of analysis:
If the memory address (reference) two String objects are the same, then return true
If two String object memory address (reference) are not the same, then the type determination, if it is not of type String, so direct returns false, if it is of type String downcast is performed, is converted into String type, and then change the char type variable arrays, compare the contents of two arrays are the same corresponding position, if the same returns true, if different returns false
240.String class trim () method is what role?
Returns a copy of string, remove the spaces before and after the string
241. The results "hamburger" .substring (4, 8) returns?
urge
[242.] Analysis of the machine of the following code, and memory configuration shown in FIG, gives the results for each printed character
Explanation.
String s = "abc";
String ss = "abc";
String s3 = "abc" + "def"; // here compilers optimized!
String s4 = "abcdef";
String s5 = ss+"def";
String s2 = new String("abc");
System.out.println(s==ss);
System.out.println(s3==s4);
System.out.println(s4==s5);
System.out.println(s4.equals(s5));
System.out.println (s == ss); the result is true
System.out.println (s3 == s4); the result is true
System.out.println (s4 == s5); the result is false
System.out.println (s4.equals (s5)); the result is true
243. [machine] practice commonly used method of the String class
Length of string:
System.out.println(“hello”.length());
Remove the spaces before and after the string:
System.out.println(“ hello world ”.trim());
Interception substring:
System.out.println(“helloworld”.substring(0,4));
244.StringBuffer and StringBuilder links are? The difference is?
contact:
StringBuffer and StringBuilder are variable sequence of characters, the underlying structure of the array is an array of char
the difference:
StringBuffer: jdk1.0 version, thread-safe, but low efficiency
StringBuilder: jdk1.5 version, thread-safe, but efficient
The following code 245. What are the consequences? How many objects produced during operation? Modify the code to use StringBuilder.
String s = "";
for(int i=0;i<10000;i++){
s +=i;
}
It will lead to a large number of copies string objects remain in memory, reducing efficiency
It will produce 10,001 objects during operation
StringBuffer sb=new StringBuffer();
for(int i=0;i<10000;i++){
sb.append(i);
}
246. How computer time is represented?
Obtained from 1970-1-10: 0: 0 milliseconds to the current elapsed time, date or time and then converted to
247.System.currentTimeMillis () What does it mean?
Returns the current time in milliseconds
248.Date d = new Date () indicates the current time it is?
The precise millisecond
249. [machine] we use SimpleDateFormat classes to implement with time into a string. That two commonly used
A method? And write code, illustration
// string transfer date
SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
sdf.parse("1990-3-3");
// Date to String
SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
sdf.format(new Date());
250. The March 3, 1990 is represented by a Calendar, and come to this day is the first day of the year? The Day
Period increased by 35 days, is the day? Use the code to illustrate.
public static void main(String[] args) throws ParseException {
Calendar c=new GregorianCalendar();
SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
c.setTime(sdf.parse("1990-3-3"));
System.out.println(c.get(Calendar.DAY_OF_YEAR));
c.add(Calendar.DATE, 35);
System.out.println(c.get(Calendar.YEAR)+"年"
+(c.get(Calendar.MONTH)+1)+"月"+c.get(Calendar.DATE));
}
251. [machine] to write test code usage Date / SimpleDateFormat / Calendar of.
// Date of usage
public static void main(String[] args) {
Date date1 = new Date();
System.out.println(date1.toString());
}
// SimpleDateFormat usage
SimpleDateFormat s1 = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
String daytime = s1.format(new Date());
// Calendar of usage
GregorianCalendar calendar2 = new GregorianCalendar();
calendar2.set(Calendar.YEAR, 2009);
252. [machine] to imitate the teacher's written, visual calendar to complete the development program.
public class AppMain {
public static void main(String[] args) throws ParseException {
System.out.println ( "Please enter the date (format: 2010-3-3):");
Scanner scanner = new Scanner(System.in);
String dateString =scanner.nextLine();
System.out.println ( "date that you just entered is:" + dateString);
String[] str = dateString.split("-");
int year = Integer.parseInt(str[0]);
int month = new Integer(str[1]);
int day = new Integer(str[2]);
Calendar c = new GregorianCalendar(year,month-1, day);
c.set(Calendar.DATE, 1);
// week: 1-7 day one hundred twenty-three thousand four hundred fifty-six
int dow = c.get(Calendar.DAY_OF_WEEK);
System.out.println ( "Day \ t a \ t two \ t three \ t four \ t five \ t six");
for(int i=0;i<dow-1;i++){
System.out.print("\t");
}
int maxDate = c.getActualMaximum(Calendar.DATE);
for(int i=1;i<=maxDate;i++){
StringBuilder sBuilder = new StringBuilder();
if(c.get(Calendar.DATE)==day){
sBuilder.append(c.get(Calendar.DATE)+"*\t");
}else{
sBuilder.append(c.get(Calendar.DATE)+"\t");
}
System.out.print(sBuilder);
if(c.get(Calendar.DAY_OF_WEEK)==Calendar.SATURDAY){
System.out.print("\n");
}
c.add(Calendar.DATE, 1);
}
}
253.File class to represent a catalog?
can
254.File class method with mkdir mkdirs, what's the difference?
mkdir: you can create a folder in the directory already exists
mkdirs: You can create a folder in the directory does not exist
255. File class using an analog implementation of DOS dir and dir / s command
public static void main(String[] args) {
File file = new File("e:/教学1");
showTree(file, 1);
}
public static void showTree(File file, int level){
File[] files = file.listFiles();
for (File f : files) {
for(int i = 0; i < level; i++){
System.out.print("-");
}
if (f.isDirectory()) {
System.out.println(f.getName());
showTree(f, level+1);
}else{
System.out.println(f.getName());
}
}
}
256. [machine] using a recursive algorithm, based on the tree structure of the code of the teacher, showing a complete directory tree
import java.io.File;
public class FileTree {
public static void main(String[] args) {
File f = new File("d:/1005班");
printFile(f, 0);
}
static void printFile(File file,int level){
for (int i = 0; i < level; i++) {
System.out.print("-");
}
System.out.println(file.getName());
if(file.isDirectory()){
File[] files = file.listFiles();
for (File temp : files) {
printFile(temp, level+1);
}
}
}
}
257. When using enumeration? What is the definition of enumeration?
When it is desired defining a set of constants , the enum
Enum definition:
You can only take one particular value of
Use the keyword enum
All enumerated types implicitly inherit from java.lang.Enum
258. [machine] manually define an enumeration representing the twelve months of the English month.
public enum Month {
JANUARY,FEBRUARY,MARCH,APRIL,MAY,JUNE,JULY,AUGUST,SEPTEMBER,OCTOBER,NOVEMBER,DECEMBER
}
259. The result of the expression [machine] switch statement can enumerate it? Imitate classroom code to write an example to show.
public static String testEnum(Month m){
String str=null;
switch (m) {
case JANUARY:
str = "January";
break;
case FEBRUARY:
str = "February";
break;
default:
break;
}
return str;
}