android studio实训实例

### Android Studio 实训项目示例

#### BMI 计算器应用
此应用旨在帮助用户计算其身体质量指数 (BMI)，并提供健康建议。该应用展示了如何使用 `LinearLayout` 来安排 UI 组件的位置。

<!-- res/layout/activity_main.xml -->
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp">

    <!-- 输入身高 -->
    <EditText
        android:id="@+id/height_edit_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="输入您的身高（cm）"/>

    <!-- 输入体重 -->
    <EditText
        android:id="@+id/weight_edit_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="输入您的体重（kg）"/>

    <!-- 计算按钮 -->
    <Button
        android:id="@+id/calculate_button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="计算"/>
    
</LinearLayout>

// MainActivity.java
public class MainActivity extends AppCompatActivity {

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

        final EditText heightEditText = findViewById(R.id.height_edit_text);
        final EditText weightEditText = findViewById(R.id.weight_edit_text);
        Button calculateButton = findViewById(R.id.calculate_button);

        calculateButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String heightStr = heightEditText.getText().toString();
                String weightStr = weightEditText.getText().toString();

                if (!TextUtils.isEmpty(heightStr) && !TextUtils.isEmpty(weightStr)) {
                    float height = Float.parseFloat(heightStr) / 100;
                    float weight = Float.parseFloat(weightStr);
                    
                    // 计算BMI
                    float bmiValue = weight / (height * height);
                    Toast.makeText(MainActivity.this, "您的BMI：" + bmiValue, Toast.LENGTH_SHORT).show();
                } else {
                    Toast.makeText(MainActivity.this, "请输入有效的数值", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}

#### 类微信界面设计
此类实训项目专注于实现类似于微信的消息列表视图，重点在于理解 `RecyclerView` 的工作原理以及 `Activity` 生命周期管理。

<!-- res/layout/item_message.xml -->
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:padding="16dp">

    <!-- 头像 -->
    <ImageView
        android:id="@+id/avatar_image_view"
        android:layout_width="50dp"
        android:layout_height="50dp"
        android:src="@drawable/default_avatar"/>

    <!-- 昵称和消息内容 -->
    <LinearLayout
        android:layout_width="0dp"
        android:layout_weight="1"
        android:layout_height="wrap_content"
        android:orientation="vertical">
        
        <TextView
            android:id="@+id/name_text_view"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="18sp"
            android:textColor="#FF000000"
            android:text="昵称"/>
            
        <TextView
            android:id="@+id/message_text_view"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="14sp"
            android:textColor="#7F000000"
            android:text="最新一条消息"/>
    </LinearLayout>

    <!-- 时间戳 -->
    <TextView
        android:id="@+id/time_text_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="12sp"
        android:textColor="#BDBDBD"
        android:text="时间"/>
</LinearLayout>

// MessageAdapter.java
public class MessageAdapter extends RecyclerView.Adapter<MessageAdapter.MessageViewHolder> {

    private List<MessageItem> messageItems;

    static class MessageViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener{
        ImageView avatarImageView;
        TextView nameTextView;
        TextView messageTextView;
        TextView timeTextView;

        MessageViewHolder(@NonNull View itemView){
            super(itemView);
            avatarImageView = itemView.findViewById(R.id.avatar_image_view);
            nameTextView = itemView.findViewById(R.id.name_text_view);
            messageTextView = itemView.findViewById(R.id.message_text_view);
            timeTextView = itemView.findViewById(R.id.time_text_view);
            itemView.setOnClickListener(this);
        }

        @Override
        public void onClick(View view) {
            int position = getBindingAdapterPosition(); 
            if(position != RecyclerView.NO_POSITION){
                Intent intent = new Intent(view.getContext(), ChatDetailActivity.class);
                intent.putExtra("message_item", messageItems.get(position));
                view.getContext().startActivity(intent);
            }
        }
    }

    @NonNull
    @Override
    public MessageViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_message, parent, false);
        return new MessageViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull MessageViewHolder holder, int position) {
        MessageItem currentItem = messageItems.get(position);
        holder.nameTextView.setText(currentItem.getName());
        holder.messageTextView.setText(currentItem.getMessageContent());
        holder.timeTextView.setText(currentItem.getTimeStamp());

        Glide.with(holder.itemView.getContext())
             .