[Read Effective Java] Chapter 2 Item 4 "Force uninstantiation with a private constructor"

Force non-instantiation with private constructor

Utility classes (classes consisting of static methods and static fields) are not designed to be instantiated. Despite this, I often see things that can be instantiated, so let's use a private constructor to make it absolutely impossible to instantiate.

Glossary

Default constructor

If no constructor is defined, a constructor called the default constructor is automatically created.

Sample code

By explicitly writing a private constructor, access from the outside is disabled and the generation of the default constructor is suppressed. AssertionError is not strictly necessary, but it is insured.

//Utility classes that cannot be instantiated
public class UtilityClass {
    //Suppress the default constructor to prevent instantiation
    private UtilityClass() {
        throw new AssertionError();
    }
    … //The rest is omitted
}

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[Read Effective Java] Chapter 2 Item 5 "Avoid the creation of unnecessary objects" https://qiita.com/Natsukii/items/ec64efddce85ffd59fa8