Inheritance initialization order in Java-javaTutorial-php.cn

Inheritance initialization order in Java

黄舟

Release： 2017-01-18 15:41:09

Original

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1. Initialization sequence of inheritance

1. Initialize the parent class, and then initialize the subclass.

2. First perform the initialization of the properties in the object, and then perform the initialization in the constructor.

2. Below, we use a simple example to illustrate this problem:

Parent class: Animal code:

package com.bluesky;  
  
public class Animal {  
public Animal(){  
  
    System.out.println("动物类的构造方法执行了！");  
  
}  
}

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Subclass: Dog code:

package com.bluesky;  
  
public class Dog extends Animal {  
      
    public Dog(){  
        System.out.println("子类的构造函数执行了！");  
         
    }  
  
}

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Test class: Test code:

package com.bluesky;  
  
public class Test {  
  
    public static void main(String[] args) {  
        Dog dog = new Dog();  
  
    }  
  
}

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Execution result:

The above description is to execute the parent class construction method first, and then execute the subclass construction method.

Next, let’s add a little code to see the execution order of attributes and constructors

Parent class: Animal code:

package com.bluesky;  
  
public class Animal {  
      
    public int age=10;  //如果构造方法先执行，属性后执行，则age的值应该为：10；  
                        //因为后面赋值会覆盖前面的赋值  
      
    public Animal(){  
    
    age=20;               //同理  
    System.out.println("动物类的构造方法执行了！");  
      
  
}  
}

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Subclass: Dog class code:

package com.bluesky;  
  
public class Dog extends Animal {  
       
    public String name="Jack";   //如果构造方法先执行，属性后执行，则name的值应该为：Jack；  
                                 //因为后面赋值会覆盖前面的赋值  
      
    public Dog(){  
        name="Rose";  
        System.out.println("子类的构造函数执行了！");  
         
    }  
  
}

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Test class: Test code:

package com.bluesky;  
  
public class Test {  
  
    public static void main(String[] args) {  
        Dog dog = new Dog();  
        System.out.println("dog.age="+dog.age);  
        System.out.println("dog.name="+dog.name);  
  
    }  
  
}

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Running results:

Summary: Regarding the initialization sequence of inheritance in Java:
First initialize the properties of the parent class object, and then initialize the constructor of the parent class.
Then initialize the properties of the subclass object, and finally initialize the constructor of the subclass.

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