使用构造函数分配和初始化超类成员的Java程序

Java中的构造函数是一种用于初始化对象的特殊方法。每当使用 new 关键字创建对象时，至少会调用一个构造。构造函数名称必须与类名称匹配，并且不能有返回类型。如果在这种情况下类中没有可用的构造函数，则Java编译器默认提供默认构造函数（无参数构造函数）。

参数化构造函数：参数化构造函数用于用我们自己的值初始化类的字段。

例子：

Java

// Parameterized Constructor Example in Java
  
import java.io.*;
class parameterizedConstructor {
  
    // fields of the class
    String name;
    int regestrationNumber;
  
    // creating a parameterized constructor so that we can
    // initialize the value of the class
    parameterizedConstructor(String name,
                             int regestrationNumber)
    {
        System.out.println("constructor call");
        this.name = name;
        this.regestrationNumber = regestrationNumber;
    }
}
  
class GFG {
    public static void main(String[] args)
    {
        // creating our first object
        parameterizedConstructor obj1
            = new parameterizedConstructor("Nilesh",
                                           2021806);
        System.out.println("Name of the student "
                           + obj1.name);
        System.out.println("Registration Number "
                           + obj1.regestrationNumber);
  
        // creating second object
        parameterizedConstructor obj2
            = new parameterizedConstructor("Bhaskar",
                                           2021807);
        System.out.println("Name of the student "
                           + obj2.name);
        System.out.println("Registration Number "
                           + obj2.regestrationNumber);
    }
}

输出

constructor call
Name of the student Nilesh
Registration Number 2021806
constructor call
Name of the student Bhaskar
Registration Number 2021807

在类层次结构中，根据层次结构调用构造函数，首先调用父类构造函数，然后调用子类构造函数

示例 1

Java

// Java Program to Allocate and Initialize Super Class
// Members Using Constructor
  
import java.util.*;
class y {
    int b;
    int c;
  
    // parameterized constructor of class y
    y(int b, int c)
    {
        this.b = b;
        this.c = c;
        System.out.println("Hi I am parent constructor");
        System.out.println("multiplication of two number "
                           + b + " and " + c + " is "
                           + b * c);
    }
}
class x extends y {
    int a;
  
    // parameterized constructor of class x
    x(int b, int c, int a)
    {
        
          // calls constructor of y
        super(b, c);
        System.out.println(
            "Hi I am child class constructor");
        System.out.println("class field of x class is "
                           + a);
    }
}
  
class GFG {
  
    public static void main(String[] args)
    {
        // creating an object of class x
        // this will invoke the constructor of x
        // but before invoking the constructor of class x
        // it will invoke the constructor of it's parent
        // class which is y
        x obj = new x(3, 4, 5);
    }
}

输出

Hi I am parent constructor
multiplication of two number 3 and 4 is 12
Hi I am child class constructor
class field of x class is 5

示例 2

Java

// Java Program to Allocate and Initialize Super Class
// Members Using Constructor
  
import java.io.*;
class grandParent {
    int grandParentAge;
    String grandParentName;
  
    // constructor
    grandParent(int grandParentAge, String grandParentName)
    {
        this.grandParentAge = grandParentAge;
        this.grandParentName = grandParentName;
    }
}
class parent extends grandParent {
    int parentAge;
    String parentName;
  
    // parameterized constructor
    parent(int grandParentAge, String grandParentName,
           int parentAge, String parentName)
    {
        
          // calls grandparent constructor
        super(grandParentAge, grandParentName);
        this.parentAge = parentAge;
        this.parentName = parentName;
    }
}
class child extends parent {
    int childAge;
    String childName;
    
      // constructor
    child(int grandParentAge, String grandParentName,
          int parentAge, String parentName, int childAge,
          String childName)
    {
        
          // calls parent constructor
        super(grandParentAge, grandParentName, parentAge,
              parentName);
        this.childAge = childAge;
        this.childName = childName;
    }
    public void dispalyDetails()
    {
        System.out.println("Name of grand parent "
                           + grandParentName + " and he is "
                           + grandParentAge + " year old");
        System.out.println("Name of parent " + parentName
                           + " and he is " + parentAge
                           + " year old");
        System.out.println("Name of child " + childName
                           + " and he is " + childAge
                           + " year old");
    }
}
class GFG {
    public static void main(String[] args)
    {
        // creating an object of class child
        // this will invoke the constructor of child
        // but before invoking the constructor of class
        // child it will invoke the constructor of it's
        // parent class which is parent but parent is child
        // of grandparent class so, before invoking the
        // constructor of parent class it will invoke the
        // constructor of grandparent class
        child obj = new child(85, "Pan Singh", 39,
                              "M.S.Dhoni", 5, "Ziva Dhoni");
        obj.dispalyDetails();
    }
}

输出

Name of grand parent Pan Singh and he is 85 year old
Name of parent M.S.Dhoni and he is 39 year old
Name of child Ziva Dhoni and he is 5 year old