Java basic tutorial constructor and method overloading

In methods and data members, we mentioned that objects in Java are initialized when they are created. During initialization, the object's data members are assigned initial values. We can initialize it explicitly. If we do not assign an initial value to the data member, the data member will take a default initial value based on its type.

Explicit initialization requires us to determine the initial value when writing the program, which is sometimes very inconvenient. We can use constructor to initialize objects. Constructors can initialize data members and specify specific operations. These operations are performed automatically when the object is created.

Define the constructor

The constructor is a method. Like normal methods, we define constructors in the class. The constructor has the following basic characteristics:

1. The name of the constructor is the same as the name of the class
2. The constructor has no return value

We define the constructor of the Human class:

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public class Test

{

public static void main(String[] args)

{

Human aPerson = new Human(160);

System.out .println(aPerson.getHeight());

}

}

class Human

{

/**

   * constructor

   */

Human(int h)

{

This.height = h;

System.out.println("I'm born");

}

/**

   * accessor

   */

int getHeight ()

{

return this.height;

}

int height;

}

The above program will print

I'm born
160

The constructor can receive a parameter list like a normal method. Here, the constructor Human() receives an integer as parameter. In the body of the method, we assign the integer parameter to the data member height. The constructor does two things when the object is created:

The constructor can receive a parameter list like a normal method. Here, the constructor Human() receives an integer as parameter. In the body of the method, we assign the integer parameter to the data member height. The constructor does two things when the object is created:

1. Provides an initial value for the data member this.height = h;
2. Performs specific initial operations System.out.println("I 'm born");

In this way, we can flexibly set the initial value when calling the constructor without being as constrained as display initialization.

How is the constructor called? When we create classes, we all use new Human(). In fact, we are calling the constructor of the Human class. When we do not define this method, Java will provide a blank constructor to be called when using new. But when we define a constructor, Java will call the defined constructor when creating an object. When calling, we provide a parameter of 160. You can also see from the final running results that the height of the object is indeed initialized to 160.

Priority of initialization method

In methods and data members, we can see that if we provide an explicit initial value, then the data member will adopt an explicit initial value instead of Default initial value. But if we both provide an explicit initial value and initialize the same data member in the constructor, the final initial value will be determined by the constructor. For example, in the following example:

public class Test
{
  public static void main(String[] args)
  {
    Human aPerson = new Human(160);
    System.out.println(aPerson.getHeight());
  }
 
}
 
class Human
{
  /**
   * constructor
   */
  Human(int h)
  {
    this.height = h; 
  }
 
  /**
   * accessor
   */
  int getHeight()
  {
    return this.height;
  }
 
  int height=170; // explicit initialization
}

The running result is:

160

The final initialization value of the object is consistent with the value in the construction method. Therefore:

##Build method> Explicit initial value> Default initial value

(In fact, the so-called priority is related to the execution order during initialization Related, I will go into this in depth later)

Method overloading

More than one constructor can be defined in a class, such as:

public class Test
{
  public static void main(String[] args)
  {
    Human neZha  = new Human(150, "shit");
    System.out.println(neZha.getHeight()); 
  }
 
}
 
class Human
{
  /**
   * constructor 1
   */
  Human(int h)
  {
    this.height = h;
    System.out.println("I'm born");
  }
 
  /**
   * constructor 2
   */
  Human(int h, String s)
  {
    this.height = h;
    System.out.println("Ne Zha: I'm born, " + s);
  }
 
  /**
   * accessor
   */
  int getHeight()
  {
    return this.height;
  }
 
  int height;
}

Running results:

Ne Zha: I'm born, shit
150

There are two constructors defined above, both named Human. The two constructors have different parameter lists.

When using new to create an object, Java will decide which constructor to build based on the provided parameters. For example, when building neZha, we provide two parameters: the integer 150 and the string "shit", which corresponds to the parameter list of the second build method, so Java will call the second build method.

In Java, Java will determine the method to be called based on both the method name and the parameter list. This is called method overloading. The build method can be overloaded, and ordinary methods can also be overloaded. For example, the breath() method below:

public class Test
{
  public static void main(String[] args)
  {
    Human aPerson = new Human();
    aPerson.breath(10);
  }
 
}
 
class Human
{
  /**
    * breath() 1
    */
  void breath()
  {
    System.out.println("hu...hu...");
  }
 
 
  /**
  * breath() 2
  */
  void breath(int rep)
  {
    int i;
    for(i = 0; i < rep; i++) {
      System.out.println("lu...lu...");
    }
  }
 
  int height;
}

Running results:

lu...lu...
lu...lu...
lu...lu...
lu...lu...
lu...lu...
lu...lu...
lu...lu...
lu...lu...
lu...lu...
lu...lu...

You can see that because it is provided when calling One parameter: the integer 10, so the second breath() method whose parameter list matches it is called.

Summarize

Constructor features: The same name as the class, no return value
Constructor purpose: Initialization, initial operation
Method overloading: method name + parameter list-> Which method is actually called

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