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What is a construction method?

Construction method with parameters

1.3.1 What is a construction method?

In Java language, when creating an object, you need to do the corresponding initialization work for the created object, such as assigning memory addresses to the attributes of the object and setting the initial values of the attributes of the object. This is done by a special method in Java called a constructor.

In every Java class, a constructor is provided. If we do not define constructors in this class, then the compiler automatically calls implicit constructors in this class.

Syntax rules for constructing methods:

Method name must be the same as class name

2. There is no return type, and void cannot be written.

Example 7 gives an example of a constructor in which the object attributes name and age are given initial values.

Example 7

package org.hopetech.l01;

public class Employee4 {

private String name; //employee name

private int age; //employee age

public Employee4(){

this.name = "Luo Zhiguo";

this.age = 30;

}

/**

* Employees introduce themselves

*/

public void selfIntroduction() {

System.out.println("I am" + name + ", I have" + age + "this year! ");

}

}

Objects may not call constructor methods, which are specifically used to create objects. Therefore constructor methods must be called by the new keyword. Employee4 employee = new Employee4();

Example 8

package org.hopetech.l01;

public class Employee3Test {

public static void main(String[] args) {

//call constructor to instantiate employee class

Employee4 employee = new Employee4();

//invoke employee self-introduction method

employee.selfIntroduction();

}

}

When instantiating the object, we have already initialized the object properties by assigning default values to the instance object properties through the constructor.

If we want to explicitly assign initial values to instance object properties, that is, we instantiate the class into multiple objects, and the property values of each object are not different. At this point, we need to use construction methods with parameters.

1.3.2 Construction methods with parameters

Constructors with parameters can initialize the properties of an object by passing parameters through the constructor when instantiating the object.

Example 9

package org.hopetech.l01;

public class Employee5 {

private String name; //employee name

private int age; //employee age

public Employee5(String name,int age){

this.name = name;

this.age = age;

}

/**

* Employees introduce themselves

*/

public void selfIntroduction() {

System.out.println("I am" + name + ", I have" + age + "this year! ");

}

}

Obviously, we explicitly assign initial values to individual instance properties of the Employee5 employee class through the parameterized constructor method.

When instantiating an object using a parameterized constructor, the values passed must match exactly the constructor's parameters in number, order, and data type.

example 10

package org.hopetech.l01;

public class Employee5Test {

public static void main(String[] args) {

//instantiate an employee using parameter-based construction methods

Employee5 employee1 = new Employee5("Li Dong",24);

Employee5 employee2 = new Employee5("Luo Zhiguo",30);

//invoke employee self-introduction method

employee1.selfIntroduction();

employee2.selfIntroduction();

}

}

By invoking the construction method with parameters, the initialization of object attributes is completed at the same time of object creation, and the operation of object initialization is simplified.

We may have different initialization behaviors for classes in different situations. So we might have to write multiple constructors. What should we do about that? We will learn an important concept below: method overloading.

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