ViewModel class 尝试显示存储库中的结果时抛出空指针

Question

我试图在有人点击 adapter 后显示来自 api 的结果，但是我遇到 null pointer 和 view model class.这是我想在 MatchDetailActivity

中显示结果的方法

    private TeamEntity entity;
    MatchViewModel viewModel;
    private MatchAdapter adapter;
    private ActivityMatchDetailBinding binding;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        binding = DataBindingUtil.setContentView(this, R.layout.activity_match_detail);

        if(getSupportActionBar()!=null)
            getSupportActionBar().hide();


        binding.displayMatch.setLayoutManager(new LinearLayoutManager(this));
        binding.displayMatch.setHasFixedSize(true);
        adapter = new MatchAdapter(this, entity);

}

 private void getTeamEvents(){
        viewModel.getTeamEvents(entity.getTeamId()).observe(this, eventsResults ->{

            if(eventsResults != null){
                adapter.setList(eventsResults);
            }
        });
    }

但我一直在这里得到一个空指针:

 viewModel.getTeamEvents(entity.getTeamId()).observe(this, eventsResults
->{

下面是我的堆栈跟踪，我们将不胜感激。

 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.sportsapp/com.sportsapp.presentation.ui.MatchDetailActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'androidx.lifecycle.LiveData com.sportsapp.presentation.viewmodels.MatchViewModel.getTeamEvents(java.lang.String)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3555)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3707)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2220)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:237)
        at android.app.ActivityThread.main(ActivityThread.java:8016)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1076)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'androidx.lifecycle.LiveData com.sportsapp.presentation.viewmodels.MatchViewModel.getTeamEvents(java.lang.String)' on a null object reference
        at com.sportsapp.presentation.ui.MatchDetailActivity.getTeamEvents(MatchDetailActivity.java:70)
        at com.sportsapp.presentation.ui.MatchDetailActivity.onCreate(MatchDetailActivity.java:54)

Answer 1

您还没有为您的 view model 赋值在任何地方添加以下行 onCreate()

viewModel = new ViewModelProvider(this).get(MyViewModel.class);

希望对您有所帮助

Answer 2

viewModel = new ViewModelProvider(this).get(MyViewModel.class);

ViewModel class 尝试显示存储库中的结果时抛出空指针

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