Objects and object variables

Classes are templates or blueprints for constructing objects , The process of constructing objects from classes can be called instances of classes .

So how should we construct objects and understand them ？

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First step ： We create our own class ——Person class , In it, we define Person Some properties and methods of this class .

public class Person {

    //  attribute 
    public String name;
    public int age;

    //  Parameterless constructors 
    public Person() {
    }

    //  There are reference constructors （ All parameter constructor ）
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    //  Method 
    public void say(){
        System.out.println(" Can talk ");
    }

    public void create(){
        System.out.println(" Be able to invent ");
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

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  The second step ： We construct classes into objects , That is, the process of instantiation 【 a key 】.

To use objects , First, we must construct the object , And specify the initial state of the object , Then apply the method to the object .

In our Java In language , You need to use a constructor （ Constructors ） To construct a new instance . Constructors （ Constructors ） It's a special method , This method is used to initialize objects .

instructions ： The constructor name needs to be consistent with the class name . So when we need to construct a Person Class , Need to be in the constructor 1 prefix new Operators can construct New object ：

new Person(); //  In essence, the time-lapse parameter structure is called to initialize the state 
new Person("Tom", 18); //  In essence, what is called is a parameterized construct （ All the cords structure ） To initialize the State 

After we construct the object of this class , You can call all the properties and methods of this class ！

public class Test {
    public static void main(String[] args) {
        new Person().create();
        new Person("Tom", 18).say();
    }
}

/*
 test result : 
     Be able to invent 
     Can talk 
 */

  But we found that , The object we construct , Whether it is a nonparametric structure , Or a parametric structure , It's very troublesome every time , Not convenient enough . We hope that the constructed object can be used many times , Instead of using construct once , So at this point , We need to construct the object , Put it in a variable , that Object variables It was born ！

Person commonPerson = new Person();
Person Tom = new Person("Tom", 18);

  In this case , We have Object variables commonPerson and Tom These two object variables , These two object variables will reference the newly constructed object Person, One is the method of nonparametric construction , One is the method of parametric construction .

public class Test {
    public static void main(String[] args) {
        Person commonPerson = new Person();
        Person Tom = new Person("Tom", 18);

        commonPerson.say();
        Tom.create();
    }
}
/*
 test result ：
     Can talk 
     Be able to invent 
 */

object And Object variables There is an important difference between , If we define object variables in another way

Person commonPerson;
Person Tom;

To test ：

By that definition , The variable at this time commonPerson and Tom In fact, there is no reference to any object at this time , And an error will be reported indicating that it is not initialized , So can't call Person Class , It also verifies the constructor again （ Constructors ） It is used to initialize objects and construct new instances , It also echoes why the constructor name should be consistent with the class name .

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summary ： Pass the above test and understand , We can see that the object variable does not actually contain an object , It simply refers to an object . stay Java in , The value of any object variable is a reference to an object stored in another place , So we can also know ,new The return value of the operator is also a reference .