1. Homework
problem:
Original code:
Test.java:
package test3;
public class Test {
public static void main(String[] args) {
Ch c[]=new Ch[10];
for(int i=0;i<10;i++)
{
c[i]=new Ch();
System.out.println(c[i].gettime());
}
}
}
Ch.java
package test3;
public class Ch {
private static int time=0;
public Ch()
{
time++;
}
public int gettime()
{
return time;
}
}
Screenshot:
Description:
Declared as static fields of the class can not guarantee time with the declaration of an object is changed, the constructor allow time + 1, represents the creation of an object.
2.
Hands-on brain:
1) Question:
When the class Foo write a constructor, the system will no longer generate a default constructor with parameters have been due Foo int initValue, but the main function is called there is no reference, so the error will occur.
2)
package text2;
public class test {
public static void main(String[] args) {
ob ob1=new ob();
System.out.println(ob1.value);
}
}
package text2;
public class ob {
public int value=100;
{
value=200;
}
public ob()
{
value=300;
}
}
Test Results:
Description: constructor highest priority, followed by the initialization block, is finally defined.
3)
Original code:
package text2;
Root class
{
static {
System.out.println ( "Root static initialization block");
}
{
System.out.println ( "normal initialization block the Root");
}
public Root ()
{
System.out.println ( " Root no argument constructor ");
}
}
class Mid the extends Root
{
static {
System.out.println (" Mid static initialization block ");
}
{
System.out.println (" Mid general initialization block ") ;
}
public Mid ()
{
System.out.println ( "Mid no argument constructor");
}
public Mid (String MSG)
{
// call the same class constructor overloads by the this
the this ();
the System .out.println ( "Mid constructor parameters with which the parameter values:" + MSG);
}
}
Leaf the extends Mid class
{
static {
System.out.println ( "Leaf static initialization block");
}
{
System.out.println ( "Leaf normal initialization block");
}
public Leaf ()
{
// call the super parent class has a constructor parameter string of
super ( "Java initialization sequence demo");
System.out.println ( "Leaf performed constructor");
}
}
public class test
{
public static void main(String[] args)
{
new Leaf();
}
}
operation result:
Explanation: The first starts to perform static initialization block from the parent class, then normal initialization block and begin from the parent class constructor.
4)
Original code:
package text2;
public class test
{
public static void main(String[] args)
{
ob c=new ob();
c.shuchu();
}
}
package text2;
public class ob {
private int a=1;
private static int b=2;
public static void shuchu()
{
System.out.println("实例a="+new ob().a);
System.out.println("静态b="+b);
}
}
Output:
