Trying to get balance with web3j (Android), geting nullpointer exception

Question

Struggling getting anything to work in web3j on android, trying this example (I have replaced token and addr) the address I used does have some rinkeby ETH in it. Testing on phone, application crashes when I load up this class/activity, code is in the oncreate method. Have internet permission turned on in the manifest, and compile android web3j in the build gradle.

 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();

The error

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)

Line 43 referenced in error is the last line of code in my submitted code. When I comment out that line app doesn't crash and just get a few (what I assume are all) warnings.

Answer 1

Your ethGetBalance return a null value .i think you put a invalid address. thats why method web3j.ethGetBalance failes. try this with valid address.

    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() is used to get your ether balance in wei. To get actual token in your wallet use this Conversion method.

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

Answer 2

ethGetBalance = web3.ethGetBalance(public_address, DefaultBlockParameterName.LATEST).sendAsync().get(); BigInteger wei = ethGetBalance.getBalance(); Log.e("ether balance", "wei" + wei); This is working fine in my app which is available on playstore

https://play.google.com/store/apps/details?id=multi.erc20.devgenesis.token.wallet

Answer 3

The problem is that the ethGetBalance is null, when you want to access it, because the method web3j.ethGetBalance fails.

You are sending "addr" as an address, this is not a valid Ethereum address.

A valid Ethereum address has a format similar that begins with: 0x and then has 40 elements (digits 0-9 or letters A to F). To see if your address exists, just try to find it in the website https://etherscan.io/

Try with a correct Ethereum address and it should work.

EthGetBalance ethGetBalance = web3j
                    .ethGetBalance(userAddress, DefaultBlockParameterName.LATEST)
                    .sendAsync()
                    .get();


BigInteger wei = ethGetBalance.getBalance();