Let’s write a java file first. File name is Gouzaohanshu.java
notice: the main() must be in the class name which is same as the file name, i.e. Gouzaohanshu
============================
package ch.allenstudy.newway01;
class Gouzao{
private int i,j;
public void set(int a, int b)
{
i = a;
j = b;
}
public void show()
{
System.out.printf(“i=%d, j=%d\n”, i, j);
}
}
public class Gouzaohanshu
{
public static void main(String[] args)
{
Gouzao gz = new Gouzao();
gz.set(3, 55);
gz.show();
}
}
========================
You see, we can assign value to i and j with gz.set() method.
While, how if I
Constructor, if translated into Chinese, its “构造函数”. But this translation is very bad. It looks like the constructor is a kind of method. But, in fact, Constructor is NOT method. The accurate translation should be “构造器”.
The different between Constructor and Method is:
Once you new an object, you will call the Constructor directly.
But for mehtod, after you new an object, only after you call it, it will be invoked.
Let’s see a sample.
Below java file (Gouzaohanshu.java) is very simple.
====================
package ch.allenstudy.newway01;
class Gouzao{
private int i,j;
public Gouzao()
{
System.out.println(“Hello Constructor!”);
}
}
public class Gouzaohanshu
{
public static void main(String[] args)
{
Gouzao gz = new Gouzao();
}
}
======================
Please notice the bold part. It is a constructor.
In class Gouzaohanshu, we don’t call any method, just a new an object. Whhat’s the result? As below:
—————–
Hello Constructor!
—————–
You can see, the code in public Gouzao() is exectued.
Now you understand that, for a Constructor, once the object is generated, they are called.
While, what use of this?
Now, if with the method way, if we want to assign values to i and j, we have to new the object, and then call the gz.set() method.
If we can assign values to i and j the same time of NEWing the object, that will be perfect.
But how to reach this with only using the way like Gouzao gz = new Gouzao(1,2)?
Change the code to:
—————————-
package ch.allenstudy.newway01;
class Gouzao {
private int i, j;
public Gouzao(int a, int b) {
i = a;
j = b;
System.out.printf(“Now i = %d, j = %d\n”, a, b);
}
}
public class Gouzaohanshu {
public static void main(String[] args) {
Gouzao gz = new Gouzao(2,3);
}
}
—————————
Result:
Now i = 2, j = 3
You see, we have reach the object — assign values to variables directly with Constructor.
Overall, why do we need constructor?
Answer: we need it to assign values to variables directly.
————————
BTw, when we write Gouzao gz = new Gouzao(2,3), this means that: you are creating a new object gz based on class Gouzao, and at the same time, you are transferring 2 values (“2″ and “3″) to the constructor.