Question

我的屏幕上有3个按钮（重新启动，上一个，下一个）和一个1 imageView。 android studio在java和xml中没有显示错误。然后为什么这个应用程序无法正常工作

XML：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">

   <ImageView android:id="@+id/i1"
       android:layout_width="fill_parent"
       android:layout_height="407dp"
       android:layout_alignParentTop="true"
       android:layout_alignParentLeft="true"
       android:src="@drawable/i1" />

   <Button android:id="@+id/previous"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Prev"
        android:layout_gravity="bottom"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true" />

    <Button android:id="@+id/next"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Next"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Res"
        android:id="@+id/restart"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true" />
        </RelativeLayout>

这是我的java代码：

public class MainActivity extends Activity implements View.OnClickListener { 

ImageView image = (ImageView) findViewById(R.id.i1);
Button next;

int a = 1;

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

    next = (Button) findViewById(R.id.next);
    next.setOnClickListener(this);

    Button restart = (Button) findViewById(R.id.restart);
    restart.setOnClickListener(this);

    Button previous = (Button) findViewById(R.id.previous);
    previous.setOnClickListener(this);
}

public void onClick(View view) {

    switch (view.getId())
    {
        case R.id.restart:
            image.setImageResource(R.drawable.i1);
            a = 0;
            break;

        case R.id.next:
            if (a == 0)
            {
                image.setImageResource(R.drawable.i2);
                a = 1;
            }
            else if (a == 1)
            {
                image.setImageResource(R.drawable.i3);
                a = 2;
            }
            else if (a == 2)
            {
                image.setImageResource(R.drawable.i4);
                a = 3;
            }
            else if (a == 3)
            {
                image.setImageResource(R.drawable.i5);
                a = 4;
            }
            else if (a == 4)
            {
                image.setImageResource(R.drawable.i6);
                a = 5;
            }
            else if (a == 5)
            {
                image.setImageResource(R.drawable.i7);
                a = 6;
            }
            else if (a == 6)
            {
                image.setImageResource(R.drawable.i8);
                a = 7;
            }
            else if (a == 7)
            {
                image.setImageResource(R.drawable.i9);
                image.setClickable(false);
            }
            break;
        case R.id.previous:
            a--;
            next.performClick();
            break;
       }
   }
}

Answer 1

ImageView image = (ImageView) findViewById(R.id.i1);

应该在

之后

setContentView(R.layout.activity_main);

onCreate方法中的

由于NullPointerException，应用程序崩溃。

Answer 2

为什么图像是从onCreate（）中初始化的？在setContentView（）

之后立即在onCreate（）方法中执行

Answer 3

          Button next; 
          int a = 1;
          ImageView image;

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

            image = (ImageView) findViewById(R.id.i1);
            next = (Button) findViewById(R.id.next);
            next.setOnClickListener(this);

            Button restart = (Button) findViewById(R.id.restart);
            restart.setOnClickListener(this);

            Button previous = (Button) findViewById(R.id.previous);
            previous.setOnClickListener(this);
        }

不幸停止这个应用程序

3 个答案: