android - 尝试在空对象引用上调用虚拟方法 'boolean java.lang.String.isEmpty()'

Question

请帮助...每当我尝试转到 foodDetailActivity.. 它会将页面返回到注册登录 Activity ？当我调试 fooddetail Activity 时..它显示下面提到的 logcat..

public class FoodList extends AppCompatActivity {

    RecyclerView recyclerView;
    RecyclerView.LayoutManager layoutManager;

    FirebaseDatabase database;
    DatabaseReference foodList;
    String categoryId="";

    FirebaseRecyclerAdapter<Food,FoodViewHolder> adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_food_list);

        database=FirebaseDatabase.getInstance();
        foodList=database.getReference("Foods");

        recyclerView=findViewById(R.id.recycler_food);
        recyclerView.setHasFixedSize(true);
        layoutManager=new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);

        if(getIntent() !=null)
            categoryId=getIntent().getStringExtra(CategoryId);
        if(!categoryId.isEmpty() && categoryId !=null)
        {
            loadListFood(categoryId);
        }
    }

    private void loadListFood(String categoryId) {
        adapter=new FirebaseRecyclerAdapter<Food, FoodViewHolder>(Food.class,R.layout.food_item,FoodViewHolder.class,
                foodList.orderByChild("MenuId").equalTo(categoryId)) {
            @Override
            protected void populateViewHolder(FoodViewHolder viewHolder, Food model, int position) {

                viewHolder.food_name.setText(model.getName());
                Picasso.with(getBaseContext()).load(model.getImage())
                        .into(viewHolder.food_image);

                final Food local=model;
                viewHolder.setItemClickListener(new ItemClickListener() {
                    @Override
                    public void onClick(View view, int position, boolean isLongClick) {
                        Intent foodDetail=new Intent(FoodList.this,FoodDetail.class);
                      foodDetail.putExtra(FoodId,adapter.getRef(position).getKey());
                        startActivity(foodDetail);
                    }
                });
            }
        };
        Log.d("TAG",""+adapter.getItemCount());
        recyclerView.setAdapter(adapter);
    }
}

这是我的 FoodDetail Activity ...

public class FoodDetail extends AppCompatActivity {

    TextView food_name,food_price,food_description;
    ImageView food_image;
    CollapsingToolbarLayout collapsingToolbarLayout;
    FloatingActionButton btnCart;
    ElegantNumberButton numberButton;

    String foodId="";

    FirebaseDatabase database;
    DatabaseReference foods;
    Food currentFood;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_food_detail);

        database = FirebaseDatabase.getInstance();
        foods = database.getReference("Foods");

        numberButton = findViewById(R.id.number_button);
        btnCart = findViewById(R.id.btnCart);
        btnCart.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                new Database(getBaseContext()).addToCart(new Order(
                        foodId,
                        currentFood.getName(),
                        numberButton.getNumber(),
                        currentFood.getPrice(),
                        currentFood.getDiscount()
                ));

                Toast.makeText(FoodDetail.this, "Added to Cart", Toast.LENGTH_SHORT).show();
            }
        });

        food_description = findViewById(R.id.food_description);
        food_name = findViewById(R.id.food_name);
        food_price = findViewById(R.id.food_price);
        food_image = findViewById(R.id.img_food);

        collapsingToolbarLayout = findViewById(R.id.collapsing);

        collapsingToolbarLayout.setExpandedTitleTextAppearance(R.style.ExpandedAppbar);
        collapsingToolbarLayout.setCollapsedTitleTextAppearance(R.style.CollapsedAppbar);

        if (getIntent() != null)
            foodId = getIntent().getStringExtra(FoodId);

        if(!foodId.isEmpty())
            getDetailFood(foodId);

    }

    private void getDetailFood(String foodId) {

        foods.child(foodId).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                currentFood=dataSnapshot.getValue(Food.class);

                //set image

                Picasso.with(getBaseContext()).load(currentFood.getImage()).into(food_image);
                collapsingToolbarLayout.setTitle(currentFood.getName());

                food_price.setText(currentFood.getPrice());
                food_name.setText(currentFood.getName());

                food_description.setText(currentFood.getDescription());
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });
    }
}

它在“if(!foodId.isEmpty())”中显示错误？？

它显示下面提到的 logcat..

02-14 11:49:21.238 6803-6803/com.example.rajatraturi.myrestaur E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.rajatraturi.myrestaur, PID: 6803 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.rajatraturi.myrestaur/com.example.rajatraturi.myrestaur.FoodDetail}: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.isEmpty()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2423) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2483) at android.app.ActivityThread.access$900(ActivityThread.java:153) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1349) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5441) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.isEmpty()' on a null object reference at com.example.rajatraturi.myrestaur.FoodDetail.onCreate(FoodDetail.java:81) at android.app.Activity.performCreate(Activity.java:6303) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2376) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2483)  at android.app.ActivityThread.access$900(ActivityThread.java:153)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1349)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:148)  at android.app.ActivityThread.main(ActivityThread.java:5441)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628)  02-14 11:49:21.292 6803-6803/com.example.rajatraturi.myrestaur I/Process: Sending signal. PID: 6803 SIG: 9

帮我解决这个问题..我没明白是怎么回事？？

Answer 1

错误日志表明代码正在尝试对 NULL 对象执行 isEmpty() 方法。我看到您有 2 个地方调用 isEmpty()。在 not empty 检查之前，首先检查对象是否为 not null。像这样:

public class FoodList extends AppCompatActivity {
    .....

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        .....

        // check for not null first
        if (categoryId !=null && !categoryId.isEmpty())
        {
            loadListFood(categoryId);
        }
    }

}


public class FoodDetail extends AppCompatActivity {
    ....

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        .....

        // check for not null first
        if (foodId != null && !foodId.isEmpty())
            getDetailFood(foodId);

    }

祝你好运!