java_concurrency3.java

class Foo { 
    private Helper helper = null;
    
    public Helper getHelper() {
        if (helper == null) 
            helper = new Helper();
        return helper;
    }
}





////////////////////////////////////////////////





class Foo { 
    private Helper helper = null;
    
    public Helper synchronized getHelper() {
        if (helper == null) 
            helper = new Helper();
        return helper;
    }
}





////////////////////////////////////////////////





class Foo {
    private Helper helper = null;
 
    public Helper getHelper() {
        if (helper == null) {
            synchronized (this) {
                if (helper == null) {
                    helper = new Helper();
                }
            }
        }
        return helper;
    }
}





////////////////////////////////////////////////





class Foo {
    private volatile Helper helper = null;
 
    public Helper getHelper() {
        if (helper == null) {
            synchronized (this) {
                if (helper == null) {
                    helper = new Helper();
                }
            }
        }
        return helper;
    }
}







/////////////////// Using Guava //////////////////////

Supplier.memoize( () -> new Helper() );