面試題 2. 實現 Singleton模式

• 題目：設計一個類，我們只能生成該類的一個實例。
• 單例模式：確保一個類只有一個實例，並提供了一個全局訪問點。

Code

• 1.餓漢模式

  //餓漢模式
  public class Singleton1 {
      //私有化構造方法
      private Singleton1(){}
      private static Singleton1 instance = new Singleton1();
      public static Singleton1 getInstance1(){
          return instance;
      }
  }

• 2.餓漢模式、變種

  //餓漢模式、變種
  public class Singleton2 {
      //私有化構造方法
      private Singleton2(){}
      private static  Singleton2 instance = null;
      static{
          instance = new Singleton2();
      }
      public static Singleton2 getInstance2(){
          return instance;
      }
  }

• 3.懶漢、線程不安全

  //懶漢、線程不安全
  public class Singleton3 {
      //私有化構造方法
      private Singleton3(){}
      private static Singleton3 instance = null;
      public static Singleton3 getInstance3(){
          if(instance==null){
              instance = new Singleton3();
          }
          return instance;
      }
  }
   
  

• 4.懶漢、線程安全

  //懶漢、線程安全
  public class Singleton4 {
      //私有化構造方法
      private Singleton4(){}
      private static Singleton4 instance;
      public static synchronized Singleton4 getInstance4(){
          if(instance==null){
              instance = new Singleton4();
          }
          return instance;
      }
  }

• 5.靜態內部類

  //靜態內部類
  public class Singleton5 {
      //私有化構造方法
      private Singleton5(){}
      //靜態內部類
      private static class SingletonHandler{
          private static final Singleton5 INSTANCE = new Singleton5();
      }
      public static Singleton5 getInstance5(){
          return SingletonHandler.INSTANCE;
      }
  }

• 6.枚舉

  //枚舉
  public enum Singleton6 {
      INSTANCE;  
      public void whateverMethod() {  
  
      }  
  }

• 7.雙重校驗鎖

  //雙重校驗鎖
  public class Singleton7 {
      //私有化構造方法
      private Singleton7(){}
      private volatile static Singleton7 instance;
      public static Singleton7 getInstance7(){
          if(instance==null){
              synchronized(Singleton7.class){
                  if(instance==null){
                      instance = new Singleton7();
                  }
              }
          }
          return instance;
      }
  }

• 測試類

  public class Test {
      public static void main(String[] args) {
          Singleton1 instance1 = Singleton1.getInstance1();
          Singleton1 instance2 = Singleton1.getInstance1();
          System.out.println("單例模式一:"+(instance1==instance2));
  
          Singleton2 instance3 = Singleton2.getInstance2();
          Singleton2 instance4 = Singleton2.getInstance2();
          System.out.println("單例模式二:"+(instance3==instance4));
  
          Singleton3 instance5 = Singleton3.getInstance3();
          Singleton3 instance6 = Singleton3.getInstance3();
          System.out.println("單例模式三:"+(instance5==instance6));
  
          Singleton4 instance7 = Singleton4.getInstance4();
          Singleton4 instance8 = Singleton4.getInstance4();
          System.out.println("單例模式四:"+(instance7==instance8));
  
          Singleton5 instance9 = Singleton5.getInstance5();
          Singleton5 instance10 = Singleton5.getInstance5();
          System.out.println("單例模式五:"+(instance9==instance10));
  
          Singleton6 instanceOne = Singleton6.INSTANCE;
          Singleton6 instanceTwo = Singleton6.INSTANCE;
          System.out.println("單例模式五:"+(instanceOne==instanceTwo));
  
          Singleton7 instance11 = Singleton7.getInstance7();
          Singleton7 instance12 = Singleton7.getInstance7();
          System.out.println("單例模式七:"+(instance11==instance12));
      }
  }

  結果

  單例模式一:true
  單例模式二:true
  單例模式三:true
  單例模式四:true
  單例模式五:true
  單例模式五:true
  單例模式七:true

【劍指offer】面試題 2. 實現 Singleton模式