class HeavyTask {
    private TestListener listener;
    // interface 設定
    interface TestListener {
        void onSuccess(int result);
    }
    // listener
    void setListener(TestListener listener) {
        this.listener = listener;
    }

    void taskStart() {
        int sum = 1;
        int i= 0;

        for ( ; i<  20; ){
            sum += sum;
            i++;
        }

        if(listener != null) {
            // 計算が終わったら結果をlistenerに渡す
            listener.onSuccess(sum);
        }
    }
}