java - 无法确定 NullPointerException 的原因

Question

我有下面这个购物车类，它被编码为显示购物车的内容。产品以线性布局显示，在 textViews 中包含产品信息和相应的按钮以删除产品，并在线性布局中水平排列。 View 元素是动态创建的。在调用此类之前，我的应用程序运行良好。然后应用程序崩溃，LogCat 在 "lb.addView(ll)" 行给出了一个 Java.lang.NullPointerException 错误。我不知道为什么会这样。有人请帮我解决这个问题。

import android.app.Activity;
import android.app.ActionBar.LayoutParams;
import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;

public class Secondscreen extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.secondscreen); 

        //TextView showCartContent          = (TextView) findViewById(R.id.showCart);
        final    Button       thirdBtn    = (Button) findViewById(R.id.third);
        final    LinearLayout lb          = (LinearLayout) findViewById(R.id.linearMain);
        final    Controller   aController = (Controller) getApplicationContext();
        final    int          cartSize    = aController.getCart().getCartSize();
        //String                showString  = "";

        if(cartSize > 0){   
            LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT); 

            for(int i = 0; i < cartSize; i++) {
                // Get product data from product data arraylist
                String pName  = aController.getCart().getProducts(i).getProductName();
                int    pPrice = aController.getCart().getProducts(i).getProductPrice();
                String pDisc  = aController.getCart().getProducts(i).getProductDesc();

                // Create LinearLayout to view elements
                LinearLayout ll = new LinearLayout(this);
                ll.setOrientation(LinearLayout.HORIZONTAL);             

                TextView product = new TextView(this);
                product.setText(" " + pName + "    ");

                // Add textView to LinearLayout
                ll.addView(product);

                TextView productdesc = new TextView(this);
                product.setText(" " + pDisc + "    ");

                // Add textView to LinearLayout
                ll.addView(productdesc);

                TextView price = new TextView(this);
                price.setText("  $" + pPrice + "     ");

                // Add textView to LinearLayout
                ll.addView(price);

                final Button btn = new Button(this);
                btn.setId(i + 1);
                btn.setText("Remove from Cart");

                // set the layoutParams on the button
                btn.setLayoutParams(params);
                final int index = i;

                //Create click listener for dynamically created button
                btn.setOnClickListener(new OnClickListener() {
                    public void onClick(View v) {
                        // Get product instance for index
                        ModelProducts tempProductObject = aController.getCart().getProducts(index);
                        aController.getCart().removeProducts(tempProductObject);
                        Toast.makeText(getApplicationContext(),"Products Removed \n Now Cart size: "+ aController.getCart().getCartSize(), Toast.LENGTH_LONG).show();
                    }
                });

                // Add button to LinearLayout
                ll.addView(btn);

                // Add LinearLayout to XML layout
                lb.addView(ll);
            }
        }

        thirdBtn.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                if(cartSize > 0) {
                    Intent i = new Intent(getBaseContext(), Thirdscreen.class);
                    startActivity(i);
                } else
                    Toast.makeText(getApplicationContext(), "Shopping cart is empty.", Toast.LENGTH_LONG).show();
            }
        });
    } 

    @Override
    protected void onDestroy() {
        super.onDestroy();
    }
}

Answer 1

很可能你的 lb 是空的，因为调用

final LinearLayout lb          = (LinearLayout) findViewById(R.id.linearMain);

没有像你预期的那样通过 id 找到它。