Instance Once

InstanceOnceDemo.java

package demo;

public class InstanceOnceDemo {

    public static void main(String[] args) {

        var c1 = new MyClass(2);
        System.out.println(c1.calc());
        System.out.println(c1.calc());

        var c2 = new MyClass(3);
        System.out.println(c2.calc());
        System.out.println(c2.calc());
    }

    static class MyClass {

        private final int seed;

        private final ClassValue<Integer> cache;

        public MyClass(int seed) {
            this.seed = seed;
            this.cache = new ClassValue<>() {
                @Override
                protected Integer computeValue(Class<?> type) {
                    return compute();
                }
            };
        }

        public int calc() {
            return this.cache.get(MyClass.class);
        }

        private int compute() {
            System.out.println("Compute");
            return 42 * seed;
        }

    }

}