How to write constructor method in java

The constructor method in java is written as "class class_name {public class_name(){} public ciass_name([paramList]){} ...//class body}". The method name must be the same as the class name, and their respective The method parameters should be different.

The operating environment of this tutorial: windows7 system, java8 version, DELL G3 computer.

The constructor method is a special method of the class, used to initialize a new object of the class, and is automatically called after the object is created (new operator). Every class in Java has a default constructor and can have more than one constructor.

Java constructor methods have the following characteristics:

• The method name must be the same as the class name

• can have 0 or 1 One or more parameters

• No return value, including void

• The default return type is the object type itself

• Can only be used in conjunction with the new operator

It is worth noting that if a return value type is defined for the constructor or void is used to declare that the constructor has no return value, the compiler No error will occur, but Java will treat this so-called constructor method as an ordinary method.

At this time, you may have questions. Doesn't the constructor have no return value? Why can't it be declared with void?

Simply put, this is the syntax of Java. In fact, the constructor of a class has a return value. When the new keyword is used to call the constructor, the constructor returns an instance of the class. The instance of this class can be regarded as the return value of the constructor, so the return value of the constructor The value type is always the current class, and there is no need to define a return value type. But you must be careful not to use return in the constructor to return an object of the current class, because the return value of the constructor is implicit.

Note: The constructor cannot be modified by static, final, synchronized, abstract and native (similar to abstract). The constructor is used to initialize a new object, so it does not make sense to modify it with static. The constructor cannot be inherited by subclasses, so it is meaningless to modify it with final and abstract. Multiple threads will not create the same object with the same memory address at the same time, so there is no need to use synchronized modification. If you don’t know other keywords except static and final, they will be explained in detail later in the tutorial.

The syntax format of the constructor method is as follows:

class class_name {
    public class_name(){}    // 默认无参构造方法
    public ciass_name([paramList]){}    // 定义构造方法
    …
    // 类主体
}

In a class, the method with the same name as the class is the constructor method. Each class can have multiple constructors, but they are required to each contain different method parameters.

Example:

Construction methods mainly include no-parameter construction method and parameterized construction method. The example is as follows:

public class MyClass {
    private int m;    // 定义私有变量
    MyClass() {
        // 定义无参的构造方法
        m = 0;
    }
    MyClass(int m) {
        // 定义有参的构造方法
        this.m = m;
    }
}

This example defines There are two construction methods, namely the parameterless construction method and the parameterized construction method. Defining multiple methods with the same name with different parameters in a class is called method overloading. Both constructors have the same name as the class, MyClass. Different constructors can be called for initialization when instantiating the class.

Note: The constructor of a class is not required to be defined. If no constructor is defined in a class, Java will automatically generate a default constructor for the class. The default constructor does not contain any parameters and the method body is empty. If one or more constructors are explicitly defined in a class, Java no longer provides a default constructor.

Tip: The parameterless constructor is also called the Nullary constructor. Only constructors automatically added by the compiler are called default constructors. If you write a constructor with no parameters and no content, it is not called a default constructor (just a Nullary constructor). Although it is just a noun definition, the difference between the two should be distinguished during the certification exam.

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