Java Toolkit Class
The java.util package has many practical classes, interfaces, and exceptions.
Vector class, stack class, hash table, enumeration interface, calendar class, random function class, mapping interface and attribute class.
Vector class
vector is heterogeneous, which can store different objects and dynamically increase storage capacity
All vector operations are performed on Object classes. Object objects retrieved from vector space need to be transformed downward.
The number of elements is less than or equal to the capacity.
Only objects can be put in. The basic data type should use its wrapper class.
common method
method | explain |
---|---|
Vector() | Construct empty vector |
Vector(int initialCapacity) | Initializes a vector of the specified size |
Vector(int initialCapacity,int capacityIncrement) | capacityIncrement Value of each expansion: 0 Double each expansion |
add() | Add Element |
addElement() | Add to End |
capacity() | Return capacity size |
clear() | empty |
elementAt() | Returns the object element at the specified location |
elements() | Returns an object for each vector element that is allowed to access |
firstElement() | Returns the first object element |
get() | Returns the object element at the specified location |
indexOf() | Find the location of the specified object element. If it does not exist, return - 1 |
isEmpty() | Judge whether it is empty |
remove() | Clear the element of the specified position or object |
removeAllElements() | Clear All Elements |
setElementAt() | Set the specified object element at the specified location to overwrite the original |
setSize() | Set capacity size |
size() | Number of returned elements |
trimToSize() | Set the actual capacity as the internal cache capacity |
toArray() | Return vector element array |
subList() | Return vector position |
removeElementAt() | Delete the object where the index refers |
example
import java.util.Vector; public class VectorDemo { public void test(){ Vector vectTemp = new Vector(4,3); System.out.println("Number of elements: " + vectTemp.size()); System.out.println("Capacity size: " + vectTemp.capacity()); // Add Element vectTemp.add(new Integer(3)); vectTemp.addElement(new Double(4.0)); vectTemp.insertElementAt(new Float(5.0f),1); vectTemp.add("hello world"); vectTemp.add("wkk"); System.out.println("Number of elements: " + vectTemp.size()); System.out.println("Capacity size: " + vectTemp.capacity()); // toString() will output all elements as strings System.out.println(vectTemp.toString()); // 2nd element System.out.println(vectTemp.get(2)); } public static void main(String[] args) { new VectorDemo().test(); } }
Stack class
LIFO
Can receive any type of object
common method
method | explain |
---|---|
Stack() | Construct an empty stack |
empty() | Judge whether it is empty |
peek() | Get the element at the top of the stack |
pop() | Delete the element at the top of the stack and return |
push(Object item) | Push |
search(Object item) | Check whether there is a specified object in the stack |
example
import java.util.Stack; public class StackDemo { public void test(){ Stack stkTemp = new Stack(); System.out.println("Whether the stack is empty: "+stkTemp.empty()); stkTemp.push(new Integer(2)); stkTemp.push("wkk"); // View Element Location System.out.println("element wkk position:" + stkTemp.search("wkk")); System.out.println("Element 2 Position:" + stkTemp.search(new Integer(2))); // Top stack element peek System.out.println(stkTemp.peek()); System.out.println(stkTemp.peek()); //Top stack element pop System.out.println(stkTemp.pop()); System.out.println(stkTemp.pop()); } public static void main(String[] args) { new StackDemo().test(); } }
- The top element of the stack is at 1
- The position of the next element at the top of the stack is 2
Hashtable class
The Dictionary class is an abstract class that provides a unified interface for dictionary tables.
Hashtable is a set of key value pairs and a concrete implementation of the abstract Dictionary class. It can store any object.
common method
method | explain |
---|---|
Hashtable() | Construct an empty hash table |
clear() | empty |
containsKey() | Whether the object is a key in the table |
containsValue() | Whether the object is a value in the table |
elements() | Returns the object that allows access to each object element |
get() | Returns the corresponding value according to the specified key |
put(Object key, Object value) | Put a set of key value pairs into the table |
keys() | Returns the object that allows access to each key |
size() | Number of keys |
remove() | Delete the corresponding key value pair according to the specified key |
Each key corresponds to a unique key value pair, and the key in a hash table cannot be duplicate
Any object can act as a key, and any object can also act as a value.
example
import java.util.Hashtable; public class HashtableDemo { public void test(){ Hashtable hshTemp = new Hashtable(); hshTemp.put("001",new Integer(1)); hshTemp.put("002",new Integer(2)); // Number of elements System.out.println("Number of elements: " + hshTemp.size()); // Get value System.out.println(hshTemp.get("001")); // Judge whether there is a value 1 System.out.println("Valuable 1: " + hshTemp.containsValue(new Integer(1))); // Hashtable to String System.out.println(hshTemp); } public static void main(String[] args) { new HashtableDemo().test(); } }
Enumeration interface
Enumeration corresponds to a collection of objects, which is generally implemented with Vector, Stack, Hashtable or other classes.
Use their elements() method to get a collection of elements.
iterator
Common methods:
method | explain | |
---|---|---|
hasMoreElements() | Determine whether the object has multiple elements | |
nextElement() | Get the next object in the object collection |
example
import java.util.Vector; import java.util.Enumeration; public class EnumerationDemo { public void test(){ Vector vctTemp = new Vector(3,2); Enumeration enumTemp ; // Add String for(int intTemp = 1;intTemp < 4;intTemp++){ String string = "character string" + intTemp; vctTemp.addElement(string); } enumTemp = vctTemp.elements(); while(enumTemp.hasMoreElements()){ String strElement = (String)enumTemp.nextElement(); System.out.println("Element value: " + strElement); } } public static void main(String[] args) { new EnumerationDemo().test(); } }
Enumeration is an interface, so you cannot use the new() method to define an instance
Calendar class
Working principle: obtain the system time, time zone and other data from the local computer system to obtain specific time information.
Common Attributes
attribute | explain |
---|---|
YEAR | year |
MONTH | month |
DAY_OF_MONTH | The day of the month |
DAY_OF_WEEK | The day of the week |
DAY_OF_YEAR | The current day |
WEEK_OF_YEAR | Week of the year |
Calendar() | Constructor, but protected, cannot be directly new |
getInstance() | Get an instance of the calendar class |
get(int field) | Get the value of an attribute. The field is the attribute defined in the calendar |
set(int field) | Set the value of a property |
getTime() | Return the date corresponding to the calendar (date) |
Clear(int field) | Clear the value of a property of the calendar class |
example
import java.util.Calendar; public class CalendarDemo { public void test(){ Calendar cldTemp = Calendar.getInstance(); // Print basic calendar information System.out.println("Year:" + cldTemp.get(Calendar.YEAR)); System.out.println("month: " + cldTemp.get(Calendar.MONTH)); System.out.println("day: " + cldTemp.get(Calendar.DATE)); // Specific calendar information System.out.println("upper/afternoon: " + cldTemp.get(Calendar.AM_PM)); System.out.println("hour: " + cldTemp.get(Calendar.HOUR)); System.out.println("minute: " + cldTemp.get(Calendar.MINUTE)); System.out.println("second: " + cldTemp.get(Calendar.SECOND)); System.out.println("millisecond: " + cldTemp.get(Calendar.MILLISECOND)); // Convert to Array System.out.println("array: " + cldTemp.toString()); } public static void main(String[] args) { new CalendarDemo().test(); } }
- The whole calendar is stored in an array
- The constructor of the calendar class is protected, so it cannot be directly created with new, but obtained with getInstance()
Random class
The Random class is a pseudorandom number generator.
common method
method | explain |
---|---|
Random | Construction method |
nextBoolean() | Returns a boolean variable true/false |
nextDouble() | Returns a double number 0.0 - 1.0 |
nextFloat() | 0.0 - 1.0 |
nextInt() | |
nextLong() | |
nextGaussian() | Generate a double precision random number 0.0 - 1.0 from a pseudo discrete Gaussian distribution |
example
import java.util.Random; public class RandomDemo { public void test() { Random rndTemp = new Random(); System.out.println("Boolean: " + rndTemp.nextBoolean()); System.out.println("double: " + rndTemp.nextDouble()); System.out.println("float: "+ rndTemp.nextFloat()); System.out.println("long: " + rndTemp.nextLong()); System.out.println("int: " + rndTemp.nextInt()); // Integer within 10 System.out.println("int(10 within): " +rndTemp.nextInt(10)); } public static void main(String[] args) { new RandomDemo().test(); } }
- nextInt(n) returns an integer of [0, n) ->n>0
Map Interface
The Map interface defines a framework for implementing basic mapping data structures.
The Hashtable class implements the Map interface.
The Map interface defines the method of storing and retrieving information on the basis of indexes. The keys in the Map interface can be of any type.
Common methods:
It is basically for the key and the object corresponding to the key
method | explain |
---|---|
clear() | Clear All |
containsKey() | Whether there is a specified key |
containsValue() | Whether there is a specified value |
entrySet() | Returns the collection corresponding to the mapping object |
hashCode() | Returns the hash code of the mapping object itself |
isEmpty() | Is it empty |
keySet() | Collection of return keys |
put() | Add Key Value Pair |
putAll(Map t) | Add all the mappings in the known mapping t to this mapping |
remove() | Delete mapping by key |
size() | Number of returned maps |
values() | Collection of return values |
Map is an interface that needs to be implemented.
The HashMap class implements the Map interface.
example
import java.util.HashMap; import java.util.Set; import java.awt.Rectangle; public class HashMapDemo { public void test(){ HashMap hmTemp = new HashMap(); // Construct arrays and pre store key objects Object objectKeyTemp [] = new Object[3]; hmTemp.put("small",new Rectangle(0,0,5,5)); hmTemp.put("midium",new Rectangle(0,0,15,15)); hmTemp.put("large",new Rectangle(0,0,30,30)); // number System.out.println("number: " + hmTemp.size()); // Whether the key small is included System.out.println("Does it contain small key: " + hmTemp.containsKey("small")); System.out.println("Does it contain Rectangle(0,0,15,15): "+hmTemp.containsValue(new Rectangle(0,0,15,15))); // Returns the collection of keys in the map Set setTemp = hmTemp.keySet(); objectKeyTemp = setTemp.toArray(); for (int i = 0; i < objectKeyTemp.length; i++) { if(objectKeyTemp[i] instanceof String) { String s = (String) objectKeyTemp[i]; System.out.println("key Value of: " + s); } } } public static void main(String[] args) { new HashMapDemo().test(); } }
Properties Class
Mainly used to read Java configuration files
In Java, the configuration file is usually a. properties file, which is configured as a key value pair.
The Properties class is a subclass of the Hashtable class. It has all the methods of the Hashtable class object
common method
method | explain |
---|---|
getProperty() | Returns the attribute value based on the specified key or value |
load(InputStream inStream) | Read the key and its specified object from the specified input stream for loading |
propertyNames() | Return the key in the property object to the enumeration interface |
setProperty(String key,String value) | Set a set of attribute object pairs in the attribute object, where the key and value are of String type |
store(OutputStream out,String header) | Load the attribute object in the attribute object into the specified output stream, and specify the name at the same time |
example
Configuration file: file.properties
age = 25 name = wkk
package com.wkk; import java.io.FileInputStream; import java.io.FileNotFoundException; import java.util.Enumeration; import java.util.Properties; import java.io.IOException; public class PropertiesDemo { public void test(){ Properties ppTemp = new Properties(); try{ ppTemp.load(new FileInputStream("src/com/wkk/file.properties")); }catch (FileNotFoundException e){ e.printStackTrace(); }catch(IOException e){ e.printStackTrace(); } Enumeration content = ppTemp.propertyNames(); while(content.hasMoreElements()){ String key = (String)content.nextElement(); String value =ppTemp.getProperty(key); System.out.println(key + ":" + value); } } public static void main(String[] args) { new PropertiesDemo().test(); } }