Android Training (Java) Day eighth-common classes (bottom) and exceptions

Source: Internet
Author: User
Tags getmessage

  1. Math class: All the methods inside are static;

    Common methods:

    ABS Absolute

    sqrt square root

    Pow (double A, double b) A's power of B

    Max (double A, double b) Compares two numbers who are big

    Min (double A, double b) Compare two numbers who are small

    Random () returns a number from 0.0 to 1.0, but cannot fetch 1

    Long round (double a) double type data A is converted to long (rounded)

  2. Random pseudo-Random class (all methods are from Math.random () this method)

    Common construction Methods: New Random ();

    Common methods: nextint (int i) 0 to i-1 return type is int type;

    Note: When doing the project, generally use pseudo-random class, because the method inside it is more flexible, but if the analysis from the memory angle, consider choosing math.random ();

  3. Date class: Java.util.Date: Gets the current system time; System.out.println (new Date ()); At this time, according to the system format;

  4. Format--dateformat--simpledateformat; A format that represents a date

    SimpleDateFormat commonly used construction methods: New SimpleDateFormat (String regx);//date format;

    Common methods of SimpleDateFormat:

    Format (date date); return string type;

    Parse (string str); Returns the date type (parsing the given string str into a date format, which must be consistent with REGX)

    Common fields:

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  5. The calendar is an abstract class:

    Object creation: Calendar c=calendar.getinstance (); Get current system time

    Common methods:

    Get (int a); Returns the int type, a represents the field

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    SetTime (date date), no return value;

    GetTime (); Returns the date type;

    Add (the number of days in the current month, the difference in days); You can output the current time of the forward or backward date

  6. System (evolved from C + +)

    Common methods:

    ArrayCopy (5 parameters)

    Exit (int i), when i=0, indicates normal exit (Java Virtual machine), otherwise abnormal exit;

    Currenttimemillis (); it can be used to test the execution time of the program, noting that the unit is milliseconds;

  7. Exception is not normal

    Classification: Understanding between compile-time exceptions, run-time exceptions, and errors:

    Error: Called a bug, generated and thrown by a Java virtual machine, including dynamic link failure, virtual machine error, and so on, the program does not handle it.

    Exception: The parent class of all exception classes whose subclasses correspond to a wide variety of possible exception events, generally requiring the user to be declared or captured.

    Runtime Exception: A special kind of exception, such as by 0, array subscript beyond the range, it produces more frequent, processing trouble, if the display of the declaration or capture will have a significant impact on program readability and operational efficiency. As a result, systems are automatically detected and handed over to the exception handlers (which the user does not have to handle).



    < try to capture exception information >

}catch (Exception class exception object name) {

< captures exception information (Printstacktrace ();), and can also customize exception information (System.out.println (..)) >


< is bound to execute a statement block >


public void F () throws exception class name {}//similar to Try{}catch () {}, but cannot customize exception information

public void F () {

Throw exception class object; Be sure to throw exception information at this time




}catch (Exception Class 1) {

}catch (Exception Class 2) {


Exception class 2> Exception Class 1, subclass in front, parent class behind

Exception Parent class: Class Exception extends throwable{};

GetMessage (); The return value is derived from the formal parameter inside the construction method;


public void F () {


throw new Exception ("abc");

}catch (Exception e) {

System.out.println (E.getmessage ()); Output ABC



After-school exercises:

Known to have an array of int[] a={1,2,3,4,5,6,7,8,9};

(1) Use random to do one such verification code: random number 1+ random number 2 =? For example: 1+2=?

(2) Calculate its value and use scanner input value to determine whether the input value is correct;


public class yangzhengma{

public static void Main (string[] args) {

Int[] a={1,2,3,4,5,6,7,8,9};

Scanner s = new Scanner (;

Random r = new Random ();

for (int i = 0; i < 2; i++) {

A[i] = R.nextint (9);


while (true) {

System.out.print (A[0] + "+" + a[1] + "=");

if (A[0] + a[1] = = S.nextint ()) {

System.out.println ("OK");



System.out.println ("Re-transmission");





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Android Training (Java) Day eighth-common classes (bottom) and exceptions

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