java - 尝试与 web3j (Android) 取得平衡，出现空指针异常

Question

努力让任何东西在android上的web3j中工作，尝试这个例子(我已经替换了token和addr)，我使用的地址确实有一些rinkeby ETH。在手机上测试，当我加载此类/Activity 时，应用程序崩溃，代码位于 oncreate 方法中。在 list 中打开互联网权限，并在构建gradle中编译android web3j。

 Web3j web3 = Web3jFactory.build(new 
 HttpService("https://rinkeby.infura.io/token"));

    EthGetBalance ethGetBalance = null;
    try {
        ethGetBalance = web3
                .ethGetBalance("addr",DefaultBlockParameterName.LATEST)
                .sendAsync()
                .get();
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    }

    BigInteger wei = ethGetBalance.getBalance();

错误

06-30 02:15:47.115 18904-18904/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.test.user.test, PID: 18904
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.user.test/com.test.user.test.balance}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2607)
    at android.app.ActivityThread.access$900(ActivityThread.java:174)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1325)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:146)
    at android.app.ActivityThread.main(ActivityThread.java:5756)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
    at dalvik.system.NativeStart.main(Native Method)
 Caused by: java.lang.NullPointerException
    at com.test.user.test.balance.onCreate(balance.java:43)
    at android.app.Activity.performCreate(Activity.java:5619)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1093)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2512)

错误引用的第43行是我提交的代码中的最后一行代码。当我注释掉该行应用程序时，它不会崩溃，只会收到一些(我认为是全部)警告。

Answer 1

您的 ethGetBalance 返回空值。我认为您输入了无效地址。这就是为什么方法web3j.ethGetBalance 失败。使用有效地址尝试此操作。

    Web3j web3 = Web3jFactory.build(new HttpService("https://ropsten.infura.io/your-token"));

    String strAddress="0x24b********************"; //strAddress is your wallet address
    EthGetBalance ethGetBalance=
    web3.ethGetBalance(strAddress,DefaultBlockParameterName.LATEST).sendAsync().get();

    BigInteger wei = ethGetBalance.getBalance();

ethGetBalance.getBalance() 用于获取以 wei 为单位的以太币余额。要在您的钱包中获取实际代币，请使用此转换方法。

  java.math.BigDecimal tokenValue = Convert.fromWei(String.valueOf(wei), Convert.Unit.ETHER);
  String strTokenAmount = String.valueOf(tokenValue);