Java singleton design pattern example

//Singleton design pattern - example class //Eager initialization public class SingletonObject {             //Our static variable holding an instance of an existing singleton class        private static SingletonObject singletonObject;              static {          // Initialize "singletonObject" only once          singletonObject = new SingletonObject();        }             //The class constructor method is also private. We can not create the class instance outside of this class        private SingletonObject() {               //...constructor codes        }             //The method that returns the single instance of the SingletonObject class        public static SingletonObject getInstance() {               return singletonObject;        }        //Prevent cloning of class by throwing CloneNotSupportedException        @Override        public Object clone() throws CloneNotSupportedException {               throw new CloneNotSupportedException();        } }

//Singleton design pattern - example class //Lazy initialization public class SingletonObject {             //Our static variable holding an instance of an existing singleton class        private static SingletonObject singletonObject = null;              //The class constructor method is also private. We can not create the class instance outside of this class        private SingletonObject() {               //...constructor codes        }             //The method that returns the single instance of the SingletonObject class        public static SingletonObject getInstance() {           if(singletonObject == null) {                  synchronized (SingletonObject.class) {                         //double-checked locking                         if(singletonObject == null) {                               singletonObject = new SingletonObject();                         }                  }                             }               return singletonObject;        }        //Prevent cloning of class by throwing CloneNotSupportedException        @Override        public Object clone() throws CloneNotSupportedException {               throw new CloneNotSupportedException();        } }

Java builder design pattern example

import java.util.List; public class Student {     private String name;     private int age;               //optional     private List<String> language; //optinal     public static class Builder {         private String name;         private int age;         private List<String> language;         public Builder (String name) {             this.name = name;         }         public Builder age(int age) {             this.age = age;             return this;         }         public Builder language(List<String> language) {             this.language = language;             return this;         }         public Student build() {             return new Student(this);         }     }     private Student(Builder builder) {         name = builder.name;         age = builder.age;         language = builder.language;     }     @Override     public String toString() {         return "Student{" + "name='" + name + '\'' + ", age=" + age + ", language=" + language +'}';     } }

import java.util.Arrays; public class TestStudent {     public static void main(String[] args) {        Student s1 = new Student.Builder("eda").build();                Student s2 = new Student.Builder("eda").age(28).language(Arrays.asList("japanese","english")).build();         System.out.println("s1--> " + s1);         System.out.println("s2--> " + s2);     } }

Output:

s1--> Student{name='eda', age=0, language=null}

s2--> Student{name='eda', age=28, language=[japanese, english]}


Quoting from Joshua Bloch's Second Edition of Effective Java:
"In summary, the Builder pattern is a good choice when designing classes
whose constructors or static factories would have more than a handful of
parameters, especially if most of those parameters are optional."