Common class
Inner class
class Outer{
class Inner {
// The inner class also forms a. Class file
// Access the private property in outer.
// The method + ().... attribute is defined by itself
}
}яА
public static void main(String[] args) {
//Outer outer = new Outer();
//Outer.Inter inter = outer.new Inter();
//Two steps are troublesome. You can take one step directly
Outer.Inter inter = new Outer().new Inter();
inter.show(); // The method in another class is not self-contained
}
System.out.println(Outer.this.name); // If the two names access the external attribute at the same time, add Outer.this System.out.println(name);
private static cannot be included, that is, static
But you can add a constant to private final static
– about static –
public class StaticText {
private String name = "Crossing crane shadow in cold pond";
static class Inner{
private String name = "Cold moon buries the soul";
public void show(){
// static is actually separated from external classes!!!!!! important
//So it should be new again
StaticText outer = new StaticText();
System.out.println(outer.name);
System.out.println(name); }
– local internal classes –
The class in the method, that is, the class cannot be preceded by modifiers such as private, but the contents can be
Main code
package InnerClass.inClass;public class StaticText {
private String name = "Crossing crane shadow in cold pond";
public void show(){
String name2 = "Cold moon buries the soul";
class Inner{
private String name3 = "lindaiyu";
public void show2(){
System.out.println(StaticText.this.name);
}
} // Because a class executes only one method, it will not be executed in the local internal class
//So in the outer layer, instantiate the local inner class. Just. fierce!
Inner inner = new Inner();
inner.show2(); }}
Test code
package InnerClass.inClass;public class Static {
public static void main(String[] args) {
StaticText outer = new StaticText();
outer.show(); ,,//The variable object may need to be added with final to prevent it from disappearing
}
}
//Local inner class
//Use with interface
public class UsbTest {
public static void main(String[] args) {
//Local inner class
//Use with interfa interface
class ak implements Usb{ !!!!!!Connected to the interface
@Override
public void service() {
System.out.println("Good wind can send me to Qingyun");
} }
Usb / Or call ak ak = new ak(); //Object instantiation
ak.service(); //Usb usb = new Mouse(); //usb.service(); }
Anonymous inner classes directly the new interface. I didn't know about the interface before,} add; Rewrite method: alt+insert
Usb usb = new Usb(){
@Override
public void service() {
System.out.println("How do you do");
} };
usb.service();
Object class
Class class1 = s1.getClass();
Class class2 = s2.getClass();
If see if class 1 and class 2 are equal
. hashCode(), look at the hash code
toString()
System.out.println(b.toString()); //In my opinion, toString is mostly used in the text text to String input
equial()
String a = "Weibei spring tree";
String b = "Weibei spring tree";
//String b = "sunset cloud in Jiangdong";
System.out.println(a.hashCode());
System.out.println(b.hashCode());
System.out.println(a.equals(b)); // If the contents of a and b are equal, they are the same, and the hash value is also equal. Otherwise, false is not returned
//But if it is new, although the content is not equal
– finalize –
garbage collection
Use System.gc(). To notify the JVM for garbage collection
Boxing (storing the data in the stack in the heap, basic type (int a = 10; in the stack at the beginning) - "reference type") and unpacking (stacking to the stack)
int age = 10; Integer name = 111; Integer integer = age; int aaa = 111;
String conversion
int n1 = 8;int n2 = 800;
//Convert int type to String s1 = n1 + "";
// 2: Using the Interger function
String s2 = Integer.toString(n1);
// How to change to hexadecimal
String s3 = Integer.toString(n1,16);
System.out.println(s3);
System.out.println(""+n1+n2);
//"============================"
//Convert string to int
//Use Integer.parseInt
String string = "666";
int num = Integer.parseInt(string);
System.out.println(num);
//=================
//boolean can also be converted to basic type
//Only "ture" is converted to true, and others are false
String string2 = "ture";
boolean b1 = Boolean.parseBoolean(string2);
System.out.println(b1);
Integer memory area
Integer integer1 = new Integer(200);
Integer integer2 = new Integer(200);
// If they are not equal, the result is an error
//Integer is a reference variable, stored in the stack, and the memory address is compared
//So it's different
System.out.println(integer1==integer2);
Integer integer3 = 100;
Integer integer4 = 100;
System.out.println(integer3==integer4); //Equal, in buffer
//The code is not equal / / because the buffer in Integer is
-128 - 127 between;
//Beyond this range, it will be created with new. Become different
Integer integer5 = 200;
Integer integer6 = 200;
System.out.println(integer5==integer6);
String
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String name = "hello"; name = "zhangsan"; String name2 = "zhangsan";
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String str = new String("java");
String str2 = new String("java"); //Two are created, one in the heap and one in the string pool,
//Like others, str, this basic type is on the stack
System.out.println(str == str2);// ==The comparison is that the addresses are not equal
System.out.println(str.equals(str2));// Equals compares the content and equals
length() charAt() contains()
String string = "java Is the best language in the world";
//Length
System.out.println(string.length());
//charAt() returns the character of a position
System.out.println(string.charAt(string.length()-1));
//contains() determines whether a string is included. If yes, true is returned
System.out.println(string.contains("java"));
toCharArray() returns the array corresponding to the string
indexOf() returns the first occurrence of a string
lastIndexOf() returns the last occurrence of a string
System.out.println(Arrays.toString(string.toCharArray()));
System.out.println(string.indexOf("java"));
System.out.println(string.indexOf("java",4));
System.out.println(string.lastIndexOf("java"));
trim() removes spaces before and after the string
toUpperCase() toLowerCase() changes the string from uppercase to lowercase or vice versa
endsWith() startsWith() determines whether it starts or ends with a character
String sss = " hello,WOrld ";
System.out.println(sss.trim());
System.out.println(sss.toUpperCase());
System.out.println(sss.toLowerCase());
String yyy = "hello,java";
System.out.println(yyy.endsWith("java"));
System.out.println(yyy.startsWith("hello"));
Replacea() replaces the string
split() splits the string
System.out.println(yyy.replace("hello","hi"));
String say = "java is the best language of ,prongram";
String[] arry = say.split("[] ,]+");//Important, [,]: indicates a space or comma, and a plus sign indicates several
for(String x:arry)
System.out.println(x);
equals() compares the contents. There are only two results: equal or unequal, false or true
equalsIgnoreCase() ignores case for comparison
compareTo(): compare sizes
String s1 = "hello"; String s1 = "HELLO"; System.out.println(s1.equalsIgnoreCase(s2));//Ignore case comparison String s3 = "abc"; String s4 = "advsdg"; System.out.println(s3.compareTo(s4));// The comparison code value is - 23, which is different from the previous one String s3 = "abc";String s4 = "abcsdg"; //Compare the string length as before, which is - 3 System.out.println(s3.compareTo(s4));
Change the first letter of the string to uppercase
public static void main(String[] args) {
String str = "this is text";
String[] array = str.split(" ");
for (String x :array)
System.out.println(x);
for(int i =0;i<array.length;i++){
char first = array[i].charAt(0); //Convert the first character to uppercase
char upperfirst = Character.toUpperCase(first);
String news=upperfirst+array[i].substring(1);
//substring(1) cut the string from the first position
System.out.println(news);
}
Use of StringBuffer and StringBuilder (more efficient)
- append add
- insert()
- replace()
- delete(0，str.length); delete
- reverse() reverse
StringBuffer sb = new StringBuffer();
//append add
sb.append("java World first");
System.out.println(sb);
sb.append("java not bad");
System.out.println(sb);
sb.append("java Really fragrant");
//insert()
sb.insert(0,"I'm at the front");
System.out.println(sb);
//replace()
sb.replace(0,5,"hello");
//delete(); delete
sb.delete(0,5);
System.out.println(sb);}
Can test computer performance
long start = System.currentTimeMillis(); //Time function
String string = "";
// Using StringBuffer and StringBuilder will greatly reduce the time
//This computer even has a card
for(int i=0;i<99999;i++) {
string += i;
}
long end = System.currentTimeMillis();
System.out.println("Time use"+(end-start));
BigDecimal
You can mention double instead
double a=1.0;double b=0.9;
// In the double operation, approximate values appear
// Higher precision operation is required,,
BigDecimalSystem.out.println(a+b);
double result = (0.4+0.5)/0.9;
System.out.println(result);
BigDecimal bd1 = new BigDecimal("1.0");
BigDecimal bd2 = new BigDecimal("0.9");
BigDecimal bd3 = bd1.multiply(bd2);
BigDecimal bd4 = bd1.add(bd2);
//BigDecimal bd5 = bd1.divide(bd2);
//Round BigDecimal
r5 = bd1.divide(bd1,2,BigDecimal.ROUND_UP);System.out.println(r5);
Date
//today Date date1 = new Date(); System.out.println(date1); System.out.println(date1.toLocaleString()); //Yesterday, today minus milliseconds Date date2 = new Date(date1.getTime()-(60*60*24*1000)); System.out.println(date2); // Method after before System.out.println(date1.after(date2)); System.out.println(date1.before(date2)); // Compare compareTo System.out.println(date1.compareTo(date2)); // Compare equality equals System.out.println(date1.equals(date2));}
Calender
public static void main(String[] args) {
// Create an object because the class is of protected type
Calendar calendar1 = new Calendar.getInstance(); System.out.println(calendar1.getTime().toLocaleString()); System.out.println(calendar1.getTimeInMillis());
// Get time information
//year
int year = calendar1.get(Calendar.YEAR);
//month
int month = calendar1.get(Calendar.MONTH);
//day
int day = calendar1.get(Calendar.DAY_OF_MONTH);
//hour
int hour = calendar1.get(Calendar.HOUR);
//minute
int minute = calendar1.get(Calendar.MINUTE);
//second
int second = calendar1.get(Calendar.SECOND);
//Modification time
Calendar calender2 = Calendar.getInstance();
calender2.set(Calendar.DAY_OF_MONTH,5); System.out.println(calender2.getTime().toLocaleString());
//add method modification time
calender2.add(Calendar.HOUR,-1); System.out.println(calender2.getTime().toLocaleString());
//Supplementary maximum value
int max = calender2.getActualMaximum(Calendar.DAY_OF_MONTH);
int min = calender2.getActualMinimum(Calendar.DAY_OF_MONTH);
}
}
SimpleDateFormat
Time conversion
public static void main(String[] args) throws Exception{
//Convert date to string
SimpleDateFormat sdf = new SimpleDateFormat("yy/MM/dd");
Date date4 = new Date();
String str = sdf.format(date4);
System.out.println(str);
//Convert string to date
Date date2 = sdf.parse("18/05/01");
System.out.println(date2);
}
System, time, garbage collection
public static void main(String[] args) {
int[] a = {1,2,3,4,5,6,7,8,9};
// b the initial array cannot be empty. new is required
int[] b= new int[9];
System.arraycopy(a,0,b,0,a.length);
for(int c:b)
System.out.println(c);
String result = " ";
long start = System.currentTimeMillis();
for(int i=0;i<99;i++) {
result += i ;
}
long end = System.currentTimeMillis();
System.out.println(end-start); new Student("zhangsan",87);
System.gc();
System.exit(0);}