The Singleton design is one of the must-known design pattern if you prepare for your technical interviews (Big IT companies have design questions apart from coding questions). The Singleton Pattern allow one class to have only one instance at any time. You can delay the instantiation to the point when it is needed for the first time.

Below shows the Lazy Singleton Design Pattern in Java. We use the keyword volatile to tell the Java (Java Virtual Machine) that at any time, reading the member field should be made directly from/to the memory location i.e. no cache should be used.

And We need to define the private constructor to avoid external instantiation via the constructor.

Lastly, the syncrhonized keyword in Java solves the racing conditions if multithreading access occurs.

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package com.company.singleton;
 
public class LazySingleton {
    // the instance will not be cached
    private static volatile LazySingleton instance = null; 
 
    // private constructor - avoid Instantiation
    private LazySingleton() {
    }
 
    public static synchronized LazySingleton getInstance() {
        if (instance == null) {
            instance = new LazySingleton();
        }
        return instance;
    }
}

package com.company.singleton;

public class LazySingleton {
    // the instance will not be cached
    private static volatile LazySingleton instance = null; 

    // private constructor - avoid Instantiation
    private LazySingleton() {
    }

    public static synchronized LazySingleton getInstance() {
        if (instance == null) {
            instance = new LazySingleton();
        }
        return instance;
    }
}

Java

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