Detailed explanation of instance methods of vector class

This article mainly introduces the detailed explanation of the use of Java vector class and related information of examples. Friends in need can refer to the following

Basic operation examples

VectorApp.java

import java.util.Vector; 
import java.lang.*; 
import java.util.Enumeration; 
public class VectorApp 
{ 
 public static void main(String args[]) 
 { 
 Vector v1 = new Vector(); 
 Integer integer1= new Integer(1); 
 //加入为字符串对象 
 v1.addElement("one"); 
 //加入的为integer的对象 
 v1.addElement(integer1); 
 v1.addElement(integer1); 
 v1.addElement("two"); 
 v1.addElement(new Integer(2)); 
 v1.addElement(integer1); 
 v1.addElement(integer1); 
 //转为字符串并打印 
 System.out.println("The Vector v1 is:\n\t"+v1); 
 //向指定位置插入新对象 
 v1.insertElement("three",2); 
 v1.insertElement(new Float(3.9),3); 
 System.out.println("The Vector v1(used method 
 insertElementAt()is:\n\t)"+v1); 
 //将指定位置的对象设置为新的对象 
 //指定位置后的对象依次往后顺延 
 v1.setElementAt("four",2); 
 System.out.println("The vector v1 cused method setElmentAt()is:\n\t"+v1); 
 v1.removeElement(integer1); 
 //从向量对象v1中删除对象integer1 
 //由于存在多个integer1,所以从头开始。 
 //找删除找到的第一个integer1. 
 Enumeration enum = v1.elements(); 
 System.out.println("The vector v1 (used method removeElememt()is"); 
 while(enum.hasMoreElements()) 
 System.out.println(enum.nextElement()+""); 
 System.out.println(); 
 //使用枚举类(Enumeration)的方法取得向量对象的每个元素。 
 System.out.println("The position of Object1(top-to-botton):"+v1.indexOf(integer1)); 
 System.out.println("The position of Object1(tottom-to-top):"+v1.lastIndexOf(integer1)); 
 //按不同的方向查找对象integer1所处的位置 
 v1.setSize(4); 
 System.out.println("The new Vector(resized the vector)is:"+v1); 
 //重新设置v1的大小，多余的元素被抛弃 
 } 
}

Run result:

E:\java01>java VectorApp 
The vector v1 is:[one,1,1,two,2,1,1] 
The vector v1(used method insetElementAt()) is: 
[one,1,three,3.9,1,two,2,1,1] 
The vector v1(used method setElementAt()) is: 
[one,1,four,3.9,1,two,2,1,1] 
The vector v1(useed method removeElement()) is: 
one four 3.9 1 two 2 1 1 
The position of object1(top-to-botton):3 
The position of object1(botton-to-top):7 
The new Vector(resized the vector) is: 
[one,four,3.9,1]

1x expansion of Vertor

Do you still remember that ArrayList expands to 0.5 times the size of array each time? Vector is slightly different from ArrayList when performing expansion operations

protected int capacityIncrement;//用于指定每次扩容的容量
private void grow(int minCapacity) {
 // overflow-conscious code
 int oldCapacity = elementData.length;
 int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
   capacityIncrement : oldCapacity);//如不指定capacityIncrement，默认扩容的容量为原数组的容量
 if (newCapacity - minCapacity < 0)
 newCapacity = minCapacity;
 if (newCapacity - MAX_ARRAY_SIZE > 0)
 newCapacity = hugeCapacity(minCapacity);
 elementData = Arrays.copyOf(elementData, newCapacity);
}

Careful friends can find that there is an additional capacityIncrement

variable in Vector, which is used to specify the increment of each expansion. If you do not specify this variable, you can find in grow that the Vector is expanded to 1 times the size of the original array by default

ThreadSafety

Vertor is thread-safe!

Another conspicuous thing in the Vertor source code is that most methods have the synchronized keyword. Everyone knows that this keyword is used for thread synchronization, so the Vector class is a thread safe!

But even if all its methods are modified to be synchronized, it does not mean that synchronization will never be needed when calling it:

private static Vector<Integer> vector=new Vector<Integer>();
public static void main(String[] args) { 
 while(true)
 {
 for(int i=0;i<10;i++)
 {
 vector.add(i);
 }
 Thread removeThread=new Thread(new Runnable(){
 @Override
 public void run()
 {
 for(int i=0;i<vector.size();i++)
 {
 vector.remove(i);
 }
 }
 });
 Thread printThread=new Thread(new Runnable(){
 @Override
 public void run()
 {
 for(int i=0;i<vector.size();i++)
 {
 System.out.println(vector.get(i));
 }
 }
 }); 
 removeThread.start();
 printThread.start();
 while(Thread.activeCount()>20); 
 }
}

When everyone runs this code After running for a short period of time, you will find an

ArrayIndexOutOfBoundsException exception. Here, the Vector's get, remove, size methods are synchronized modified, but in a multi-threaded environment , if no additional synchronization measures are taken on the method side, this code is still unsafe. If one thread deletes the element with serial number i, and another thread accesses this i, it will directly throw an exception back, so this code is guaranteed to Security needs to be modified by adding synchronized inside run.

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