Constructor trong java

In this tutorial, we will learn about the constructor of an object, how to create them, what is the difference between the mặc định constructor and the constructor we define & final what is the overloaded constructor?

Definition of the constructor

In Java, the constructor is a special method, which is used to initialize & return the object of the class to lớn which it is defined. The constructor will have sầu the same name as the class it defines, and they vày not define a return value.

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For example:


package com.diywkfbv.com;public class Student private String name; private int age; public Student(String name) this.name = name;

In the example above sầu, we have defined a constructor for the Student object, as you can see that it has no return type & that its name is the same as the class to lớn which it is defined.

When an object is initialized by calling its constructor with the new operator, it invokes the superclass constructor and all instance variables are initialized with their mặc định values. Bachồng to lớn the above sầu example, when initializing the Student object by calling:


package com.diywkfbv.com;public class Example public static void main(String<> args) Student student = new Student("Khanh");

then the value of the age variable in the Student object will have a default value of type int to lớn 0.

Default constructor

If in an object we do not define a constructor, by mặc định, Java will add a default constructor to our object.

For example, I defined the Student object as follows:


package com.diywkfbv.com;public class Example public static void main(String<> args) Student student = new Student();

Obviously, although I vày not define any constructors in the Student object, I can still initialize this object.

The default constructor does not contain any parameters, & when called to initialize an object, it also calls the constructor of the superclass và initializes the instance variables.

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In the case, we have defined a constructor for the object:


package com.diywkfbv.com;public class Student private String name; private int age; public Student(String name) this.name = name;

Java will not automatically add the default constructor, và then if we try khổng lồ initialize the object using the default constructor, then the compile error will happen:

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Defines a constructor

We can define a constructor for any object so that all properties of the object include methods, instance variables can be called according to lớn our definition.

For example:


package com.diywkfbv.com;public class Student private String name; private int age; public Student(String name) this.name = name; age = 30;

We can use four access modifiers to define a constructor, & thus we can limit the scope of object access from other objects.

What happens if we define a constructor with a return value type? As such, Java treats it as a normal method rather than as a constructor.

For example:


package com.diywkfbv.com;public class Student private String name; private int age; public void Student(String name) this.name = name; age = 30;

The Student object now has only a mặc định constructor added by Java. If we initialize the Student object with a name parameter, we get an error.

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Overloaded constructor

What is an overloaded constructor? That is when we define multiple constructors for an object and each constructor will have different parameters in terms of both the parameter number & the data type of the parameter.

For example:


package com.diywkfbv.com;public class Student private String name; private int age; public Student(int age) this.age = age; public Student(String name) this.name = name; public Student(String name, int age) this.name = name; this.age = age;

The rule to define an overloaded constructor is:

Constructors, as they say, must be defined using different parameters in terms of both the number of arguments and the type of the parameter.The constructors are not defined differently only in the access modifier.

In the case where the object has multiple constructors và we want khổng lồ hotline this constructor from another constructor then we must use the từ khoá this:


package com.diywkfbv.com;public class Student private String name; private int age; public Student(int age) this.age = age; public Student(String name) this.name = name; public Student(String name, int age) this(name); this.age = age;

This statement to điện thoại tư vấn this other constructor must be in the first line of the constructor, if it is after any statement, the code will be compiled error: