Pattern · Intro

Singleton

Guarantee a class has exactly ONE instance and give the whole program a single point of access to it.

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step 1 / 8
1/2One instance, ever
Config
- instance: Config «static»
- Config()
+ getInstance(): Config «static»
One instance, ever
 1class Config {
 2    private static Config instance;
 3    private Config() { }                 // constructor is private
 4    static Config getInstance() {
 5        if (instance == null)
 6            instance = new Config();      // created once, lazily
 7        return instance;
 8    }
 9}
State
constructorprivate
✦

The constructor is PRIVATE — no other code can write `new Config()`. That's how the class controls its own instantiation.