Java Constructors: What\'s the Difference Between Void and Non-Void Constructors?

Constructors in Java: "Void" vs. "Non-Void"

In the Java programming language, classes can have constructors, which are special methods that are called when an instance of the class is created. Constructors serve to initialize the object's state. There are two types of constructors: "void" constructors and "non-void" constructors.

"Non-Void" Constructors

A "non-void" constructor declares a return type other than void. In other words, it returns a value, usually an instance of the class itself. When a "non-void" constructor is called, a new object is created and the returned value is assigned to the reference variable that invoked the constructor.

"Void" Constructors

"Void" constructors, on the other hand, do not declare a return type. Instead, they simply execute the constructor body and initialize the object's state. They do not return a value, and therefore cannot be used to create new objects.

Constructor Visibility Issue

In Java, constructors do not have the same access modifiers as methods. A constructor's visibility must match the visibility of the class itself. For example, if a class is declared public, then its constructor must also be declared public.

Example: Constructor Visibility Impact

Consider the following two code snippets:

Example 1: Constructor with Return Type

public class Class1 {
    public static Integer value = 0;
    public Class1() { da(); }
    public int da() {
        Class1.value += 1;
        return 5;
    }
    public static void main(String[] args) {
        Class1 h = new Class1();
        Class1 h2 = new Class1();
        System.out.println(Class1.value);
    }
}

Example 2: Constructor with Void Return Type

public class Class1 {
    public static Integer value = 0;
    public void Class1() { da(); }
    public int da() {
        Class1.value += 1;
        return 5;
    }
    public static void main(String[] args) {
        Class1 h = new Class1();
        Class1 h2 = new Class1();
        System.out.println(Class1.value);
    }
}

Observation:

In Example 1, the output is 2. This is because the constructor calls the da() method, which increments the value field by 1.

However, in Example 2, the output is 0. This is because the constructor with the void return type does not actually call the da() method. Instead, it is a method with the same name as the class, which is distinct from the constructor.

Conclusion:

In Java, constructors are not methods. They are used to initialize an object's state and do not have return types. Attempting to declare a constructor with a return type other than void will result in a compile error.

The above is the detailed content of Java Constructors: What\'s the Difference Between Void and Non-Void Constructors?. For more information, please follow other related articles on the PHP Chinese website!