空指针，底部导航的活动意图中的上下文

这个应用程序有两个活动，我想使用底部导航视图在它们之间切换。在 switch 语句中声明意图会引发空指针异常。MainActivity.java 更改为 SpecialsActivity.java。第二个活动是PizzaActivity。底部导航由 BottomNavigationBuilder.java 控制。

堆栈跟踪示例：

W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference

我尝试过：

Intent PizzaIntent = new Intent(this, PizzaActivity.class);
                                    //Intent PizzaIntent = new Intent(getCallingActivity(), PizzaActivity.class);
                                    //Intent PizzaIntent = new Intent( getBaseContext(),PizzaActivity.class);
                                    //Intent PizzaIntent = new Intent(BottomNavigationBuilder.this, PizzaActivity.class);
                                    //Intent PizzaIntent = new Intent(getApplicationContext(), PizzaActivity.class);
                                    startActivity(PizzaIntent);

底部导航生成器.java

public class BottomNavigationBuilder extends AppCompatActivity {
    private Context context;
    private BottomNavigationView bottomNavigation;
    public BottomNavigationBuilder(Context context, BottomNavigationView findViewById) {
        this.context = context;
        this.bottomNavigation = findViewById;
    }
    public BottomNavigationBuilder setBaseConfig() {
        setTextVisible();
        setSelectedListener();
        return this;
    }
    private void setTextVisible() {
        bottomNavigation.setLabelVisibilityMode(LabelVisibilityMode.LABEL_VISIBILITY_LABELED);
    }
    public void displayToast(String message) {
        Toast.makeText(this.context, message,
                Toast.LENGTH_SHORT).show();
    }
    private void setSelectedListener() {
        BottomNavigationView.OnNavigationItemSelectedListener navigationItemSelectedListener =
                new BottomNavigationView.OnNavigationItemSelectedListener() {
                    String url;
                    @Override
                    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                        switch (item.getItemId()) {
                            case R.id.action_specials:
                                displayToast("Specials clicked");
                                break;
                            case R.id.action_pizza:
                                displayToast("Pizza clicked");
                                try {
                                    Intent PizzaIntent = new Intent(this, PizzaActivity.class);
                                    //Intent PizzaIntent = new Intent(getCallingActivity(), PizzaActivity.class);
                                    //Intent PizzaIntent = new Intent( getBaseContext(),PizzaActivity.class);
                                    //Intent PizzaIntent = new Intent(BottomNavigationBuilder.this, PizzaActivity.class);
                                    //Intent PizzaIntent = new Intent(getApplicationContext(), PizzaActivity.class);
                                    startActivity(PizzaIntent);
                                } catch (Exception e) {
                                    e.printStackTrace();
                                }

                                break;
                            case R.id.action_stromboli:
                                displayToast("Stromboli clicked");
                                break;
                            case R.id.action_salad:
                                displayToast("Salad clicked");
                                break;
                            case R.id.action_drinks:
                                displayToast("Drinks clicked");
                                break;
                            default:
                                // none
                        }
                        return true;
                    }
                };
        bottomNavigation.setOnNavigationItemSelectedListener(navigationItemSelectedListener);
    }
    public BottomNavigationView getBottomNavigation() {
        return bottomNavigation;
    }
}

特价活动.java

public class SpecialsActivity extends AppCompatActivity {
    private BottomNavigationView bottomNavigation;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_specials);
        setItemId();
        setView();
    }
    private void setItemId() {
        bottomNavigation = findViewById(R.id.bottom_nav);
    }
    private void setView() {
        initNavigationView();
    }
    private void initNavigationView() {
        bottomNavigation = new BottomNavigationBuilder(this, bottomNavigation)
                .setBaseConfig()
                .getBottomNavigation();
    }
}

不能直接实例化活动。不能直接在它们之间传递组件。您甚至不需要为此进行活动。

删除活动扩展：

public class BottomNavigationBuilder {
    //...
}

然后只需使用您的context引用来获取任何需要上下文的内容（new Intent(context, PizzaActivity.class)、context.startActivity()等）。

在

将bottomNavigation传递给构建器后，也无需重新分配。这是同一个实例，因此对构建器内部bottomNavigation所做的任何更改也会在外部bottomNavigation。

您收到此错误的原因可能是因为您放置此语句的位置。目前，它像：

@Override
public boolean onNavigationItemSelected(@NonNull MenuItem item) {
    //.....
    Intent PizzaIntent = new Intent(this, PizzaActivity.class);

尝试通过执行以下操作将 Intent 构造函数中的 this 关键字更改为ApplicationContext：

//Enter your Application's context below.
Intent PizzaIntent = new Intent(context, PizzaActivity.class);

现在，Intent应该在onNavigationItemSelected内部正常工作。 context来自类的构造函数。但是，由于您的类扩展AppCompatActivity，您也可以执行getApplicationContext()。如果您有任何问题或问题，可以在下面发布。

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