CompletableFuture 回调测试

示例：

  CompletableFuture<TransactionReceipt> receipt = contract.set(new BigInteger(value)).sendAsync();
 // 异步
        receipt.thenAccept((result)->{
            System.out.println("result:"+result);
        });
//        等待 同步
        receipt.whenComplete((t,u)->{
            System.out.println("result:"+t);
            System.out.println("result1:"+u);
        }).exceptionally(f->{
            System.out.println("-----exception:"+f.getMessage());
            return null;
        }).get();

//          同步
        receipt.thenAccept((t)->{
            System.out.println("result:"+t);
        }).get();